Red Hat Enterprise Linux 7
Using KVM performance features for host systems and virtualized guests on RHEL
Jiri Herrmann
Red Hat Customer Content Services
jherrman@redhat.com
Yehuda Zimmerman
Red Hat Customer Content Services
yzimmerm@redhat.com
Dayle Parker
Red Hat Customer Content Services
Scott Radvan
Red Hat Customer Content Services
RedHat Subject Matter Experts
Legal Notice
Abstract
The Red Hat Enterprise Linux Virtualization Tuning and Optimization Guide covers KVM and virtualization performance. Within this guide, you can find tips and suggestions for making full use of KVM performance features and options for your host systems and virtualized guests.
Chapter1.Introduction
1.1.Why Performance Optimization Matters in Virtualization
In KVM Virtualization, guests are represented by processes on the host machine. This means that processing power, memory, and other resources of the host are used to emulate the functions and capabilities of the guest's virtual hardware.
However, guest hardware can be less effective at using the resources than the host. Therefore, adjusting the amount of allocated host resources may be needed for the guest to perform its tasks at the expected speed. In addition, various types of virtual hardware may have different levels of overhead, so an appropriate virtual hardware configuration can have significant impact on guest performance. Finally, depending on the circ*mstances, specific configurations enable virtual machines to use host resources more efficiently.
1.2.KVM Performance Architecture Overview
The following points provide a brief overview of KVM as it pertains to system performance, as well as process and thread management:
When using KVM, guests run as a Linux processes on the host.
Virtual CPUs (vCPUs) are implemented as normal threads, handled by the Linux scheduler.
Guests do not automatically inherit features such as NUMA and huge pages from the kernel.
Disk and network I/O settings in the host have a significant performance impact.
Network traffic typically travels through a software-based bridge.
Depending on the devices and their models, there might be significant overhead due to emulation of that particular hardware.
1.3.Virtualization Performance Features and Improvements
Virtualization Performance Improvements in Red Hat Enterprise Linux7
The following features improve virtualization performance in Red Hat Enterprise Linux7:
- Automatic NUMA Balancing
Automatic NUMA balancing improves the performance of applications running on NUMA hardware systems, without any manual tuning required for Red Hat Enterprise Linux7 guests. Automatic NUMA balancing moves tasks, which can be threads or processes, closer to the memory they are accessing. This enables good performance with zero configuration. However, in some circ*mstances, providing more accurate guest configuration or setting up guest to host affinities for CPUs and memory may provide better results.
For more information on automatic NUMA balancing, see Section9.2, “Automatic NUMA Balancing”.
- VirtIO models
Any virtual hardware that has the virtio model does not have the overhead of emulating the hardware with all its particularities. VirtIO devices have low overhead thanks to the fact that they are designed specifically for use in Virtualization environments. However not all guest operating systems support such models.
- Multi-queue virtio-net
A networking approach that enables packet sending/receiving processing to scale with the number of available vCPUs of the guest.
For more information on multi-queue virtio-net, see Section5.4.2, “Multi-Queue virtio-net”.
- Bridge Zero Copy Transmit
Zero copy transmit mode reduces the host CPU overhead in transmitting large packets between a guest network and an external network by up to 15%, without affecting throughput. Bridge zero copy transmit is fully supported on Red Hat Enterprise Linux7 virtual machines, but disabled by default.
For more information on zero copy transmit, see Section5.4.1, “Bridge Zero Copy Transmit”.
- APIC Virtualization (APICv)
Newer Intel processors offer hardware virtualization of the Advanced Programmable Interrupt Controller (APICv). APICv improves virtualized AMD64 and Intel 64 guest performance by allowing the guest to directly access the APIC, dramatically cutting down interrupt latencies and the number of virtual machine exits caused by the APIC. This feature is used by default in newer Intel processors and improves I/O performance.
- EOI Acceleration
End-of-interrupt acceleration for high bandwidth I/O on older chipsets without virtual APIC capabilities.
- Multi-queue virtio-scsi
Improved storage performance and scalability provided by multi-queue support in the virtio-scsi driver. This enables each virtual CPU to have a separate queue and interrupt to use without affecting other vCPUs.
For more information on multi-queue virtio-scsi, see Section7.4.2, “Multi-Queue virtio-scsi”.
- Paravirtualized Ticketlocks
Paravirtualized ticketlocks (pvticketlocks) improve the performance of Red Hat Enterprise Linux 7 guest virtual machines running on Red Hat Enterprise Linux 7 hosts with oversubscribed CPUs.
- Paravirtualized Page Faults
Paravirtualized page faults are injected into a guest when it attempts to access a page swapped out by the host. This improves KVM guest performance when host memory is overcommitted and guest memory is swapped out.
- Paravirtualized Time
vsyscall
Optimization The
gettimeofday
andclock_gettime
system calls execute in the user space through thevsyscall
mechanism. Previously, issuing these system calls required the system to switch into kernel mode, and then back into the user space. This greatly improves performance for some applications.
Virtualization Performance Features in Red Hat Enterprise Linux
CPU/Kernel
NUMA - Non-Uniform Memory Access. See Chapter9, NUMA for details on NUMA.
CFS - Completely Fair Scheduler. A modern class-focused scheduler.
RCU - Read Copy Update. Better handling of shared thread data.
Up to 160 virtual CPUs (vCPUs).
Memory
huge pages and other optimizations for memory-intensive environments. See Chapter8, Memory for details.
Networking
vhost-net - A fast, kernel-based VirtIO solution.
SR-IOV - For near-native networking performance levels.
Block I/O
AIO - Support for a thread to overlap other I/O operations.
MSI - PCI bus device interrupt generation.
Disk I/O throttling - Controls on guest disk I/O requests to prevent over-utilizing host resources. See Section7.4.1, “Disk I/O Throttling” for details.
Note
For more details on virtualization support, limits, and features, see the Red Hat Enterprise Linux7 Virtualization Getting Started Guide and the following URLs:
https://access.redhat.com/certified-hypervisors
https://access.redhat.com/articles/rhel-kvm-limits
Chapter2.Performance Monitoring Tools
This chapter describes tools used to monitor guest virtual machine environments.
2.1.perf kvm
You can use the perf
command with the kvm
option to collect and analyze guest operating system statistics from the host. The perf package provides the perf
command. It is installed by running the following command:
# yum install perf
In order to use perf kvm
in the host, you must have access to the /proc/modules
and /proc/kallsyms
files from the guest. See Procedure2.1, “Copying /proc files from guest to host” to transfer the files into the host and run reports on the files.
Procedure2.1.Copying /proc files from guest to host
Important
If you directly copy the required files (for instance, using scp
) you will only copy files of zero length. This procedure describes how to first save the files in the guest to a temporary location (with the cat
command), and then copy them to the host for use by perf kvm
.
Log in to the guest and save files
Log in to the guest and save
/proc/modules
and/proc/kallsyms
to a temporary location,/tmp
:# cat /proc/modules > /tmp/modules# cat /proc/kallsyms > /tmp/kallsyms
Copy the temporary files to the host
Once you have logged off from the guest, run the following example
scp
commands to copy the saved files to the host. You should substitute your host name and TCP port if they are different:# scp root@GuestMachine:/tmp/kallsyms guest-kallsyms# scp root@GuestMachine:/tmp/modules guest-modules
You now have two files from the guest (
guest-kallsyms
andguest-modules
) on the host, ready for use byperf kvm
.Recording and reporting events with perf kvm
Using the files obtained in the previous steps, recording and reporting of events in the guest, the host, or both is now possible.
Run the following example command:
# perf kvm --host --guest --guestkallsyms=guest-kallsyms \--guestmodules=guest-modules record -a -o perf.data
Note
If both --host and --guest are used in the command, output will be stored in
perf.data.kvm
. If only --host is used, the file will be namedperf.data.host
. Similarly, if only --guest is used, the file will be namedperf.data.guest
.Pressing Ctrl-C stops recording.
Reporting events
The following example command uses the file obtained by the recording process, and redirects the output into a new file,
analyze
.perf kvm --host --guest --guestmodules=guest-modules report -i perf.data.kvm \--force > analyze
View the contents of the
analyze
file to examine the recorded events:# cat analyze
# Events: 7K cycles## Overhead Command Shared Object Symbol# ........ ............ ................. .........................# 95.06% vi vi [.] 0x48287 0.61% init [kernel.kallsyms] [k] intel_idle 0.36% vi libc-2.12.so [.] _wordcopy_fwd_aligned 0.32% vi libc-2.12.so [.] __strlen_sse42 0.14% swapper [kernel.kallsyms] [k] intel_idle 0.13% init [kernel.kallsyms] [k] uhci_irq 0.11% perf [kernel.kallsyms] [k] generic_exec_single 0.11% init [kernel.kallsyms] [k] tg_shares_up 0.10% qemu-kvm [kernel.kallsyms] [k] tg_shares_up[output truncated...]
2.2.Virtual Performance Monitoring Unit (vPMU)
The virtual performance monitoring unit (vPMU) displays statistics which indicate how a guest virtual machine is functioning.
The virtual performance monitoring unit allows users to identify sources of possible performance problems in their guest virtual machines. The vPMU is based on Intel's PMU (Performance Monitoring Units) and can only be used on Intel machines.
This feature is only supported with guest virtual machines running Red Hat Enterprise Linux6 or Red Hat Enterprise Linux7 and is disabled by default.
To verify if the vPMU is supported on your system, check for the arch_perfmon
flag on the host CPU by running:
# cat /proc/cpuinfo|grep arch_perfmon
To enable the vPMU, specify the cpu mode
in the guest XML as host-passthrough
:
# virsh dumpxml guest_name |grep "cpu mode"
<cpu mode='host-passthrough'>
After the vPMU is enabled, display a virtual machine's performance statistics by running the perf
command from the guest virtual machine.
2.3.Monitoring Performance in Virtual Machine Manager
You can use the Virtual Machine Monitor to view performance information for any virtual machine on your system. You can also configure the performance information displayed in Virtual Machine Manager.
2.3.1.Viewing a Performance Overview in Virtual Machine Manager
To view a performance overview for a virtual machine using Virtual Machine Manager:
In the Virtual Machine Manager main window, highlight the virtual machine that you want to view.
Figure2.1.Selecting a virtual machine to display
From the Virtual Machine Manager Edit menu, select Virtual Machine Details.
When the Virtual Machine details window opens, there may be a console displayed. Should this happen, click View and then select Details. The Overview window opens first by default.
Select Performance from the navigation pane on the left hand side.
The Performance view shows a summary of guest performance, including CPU and Memory usage and Disk and Network input and output.
Figure2.2.Displaying guest performance details
2.3.2.Performance Monitoring
Performance monitoring preferences can be modified with virt-manager
's preferences window.
To configure performance monitoring:
From the Edit menu, select Preferences.
The Preferences window appears.
From the Polling tab specify the time in seconds or stats polling options.
Figure2.3.Configuring performance monitoring
2.3.3.Displaying CPU Usage for Guests
To view the CPU usage for all guests on your system:
From the View menu, select Graph, then the Guest CPU Usage check box.
The Virtual Machine Manager shows a graph of CPU usage for all virtual machines on your system.
Figure2.4.Guest CPU usage graph
2.3.4.Displaying CPU Usage for Hosts
To view the CPU usage for all hosts on your system:
From the View menu, select Graph, then the Host CPU Usage check box.
The Virtual Machine Manager shows a graph of host CPU usage on your system.
Figure2.5.Host CPU usage graph
2.3.5.Displaying Disk I/O
To view the disk I/O for all virtual machines on your system:
Make sure that the Disk I/O statistics collection is enabled. To do this, from the Edit menu, select Preferences and click the Polling tab.
Select the Disk I/O check box.
Figure2.6.Enabling Disk I/O
To enable the Disk I/O display, from the View menu, select Graph, then the Disk I/O check box.
The Virtual Machine Manager shows a graph of Disk I/O for all virtual machines on your system.
Figure2.7.Displaying Disk I/O
2.3.6.Displaying Network I/O
To view the network I/O for all virtual machines on your system:
Make sure that the Network I/O statistics collection is enabled. To do this, from the Edit menu, select Preferences and click the Pollingtab.
Select the Network I/O check box.
Figure2.8.Enabling Network I/O
To display the Network I/O statistics, from the View menu, select Graph, then the Network I/O check box.
The Virtual Machine Manager shows a graph of Network I/O for all virtual machines on your system.
Figure2.9.Displaying Network I/O
2.3.7.Displaying Memory Usage
To view the memory usage for all virtual machines on your system:
Make sure that the memory usage statistics collection is enabled. To do this, from the Edit menu, select Preferences and click the Pollingtab.
Select the Poll Memory stats check box.
To display the memory usage, from the View menu, select Graph, then the Memory Usage check box.
The Virtual Machine Manager lists the percentage of memory in use (in megabytes) for all virtual machines on your system.
Figure2.11.Displaying memory usage
Chapter3.Optimizing Virtualization Performance with virt-manager
This chapter covers performance tuning options available in virt-manager, a desktop tool for managing guest virtual machines.
3.1.Operating System Details and Devices
3.1.1.Specifying Guest Virtual Machine Details
The virt-manager tool provides different profiles depending on what operating system type and version are selected for a new guest virtual machine. When creating a guest, you should provide as many details as possible; this can improve performance by enabling features available for your specific type of guest.
See the following example screen capture of the virt-manager tool. When creating a new guest virtual machine, always specify your intended OS type and Version:
Figure3.1.Provide the OS type and Version
3.1.2.Remove Unused Devices
Removing unused or unnecessary devices can improve performance. For instance, a guest tasked as a web server is unlikely to require audio features or an attached tablet.
See the following example screen capture of the virt-manager tool. Click the Remove button to remove unnecessary devices:
Figure3.2.Remove unused devices
3.2.CPU Performance Options
Several CPU related options are available to your guest virtual machines. Configured correctly, these options can have a large impact on performance. The following image shows the CPU options available to your guests. The remainder of this section shows and explains the impact of these options.
Figure3.3.CPU Performance Options
3.2.1.Option: Available CPUs
Use this option to adjust the amount of virtual CPUs (vCPUs) available to the guest. If you allocate more than is available on the host (known as overcommitting), a warning is displayed, as shown in the following image:
Figure3.4.CPU overcommit
CPUs are overcommitted when the sum of vCPUs for all guests on the system is greater than the number of host CPUs on the system. You can overcommit CPUs with one or multiple guests if the total number of vCPUs is greater than the number of host CPUs.
Important
As with memory overcommitting, CPU overcommitting can have a negative impact on performance, for example in situations with a heavy or unpredictable guest workload. See the Virtualization Deployment and Administration Guide for more details on overcommitting.
3.2.2.Option: CPU Configuration
Use this option to select the CPU configuration type, based on the intended CPU model. Click the Copy host CPU configuration check box to detect and apply the physical host's CPU model and configuration, or expand the list to see available options. Once you select a CPU configuration, its available CPU features/instructions are displayed and can be individually enabled/disabled in the CPU Features list.
Figure3.5.CPU Configuration Options
Note
Copying the host CPU configuration is recommended over manual configuration.
Note
Alternately, run the virsh capabilities
command on your host machine to view the virtualization capabilities of your system, including CPU types and NUMA capabilities.
3.2.3.Option: CPU Topology
Use this option to apply a particular CPU topology (Sockets, Cores, Threads) to the virtual CPUs for your guest virtual machine.
Figure3.6.CPU Topology Options
Note
Although your environment may dictate other requirements, selecting any intended number of sockets, but with only a single core and a single thread usually gives the best performance results.
3.3.Virtual Disk Performance Options
Several virtual disk related options are available to your guest virtual machines during installation that can impact performance. The following image shows the virtual disk options available to your guests.
The cache mode, IO mode, and IO tuning can be selected in the Virtual Disk section in virt-manager. Set these parameters in the fields under Performance options, as shown in the following image:
Figure3.7.Virtual Disk Performance Options
Important
When setting the virtual disk performance options in virt-manager, the virtual machine must be restarted for the settings to take effect.
See Section7.2, “Caching” and Section7.3, “I/O Mode” for descriptions of these settings and instructions for editing these settings in the guest XML configuration.
Chapter4.tuned and tuned-adm
This chapter covers using the tuned daemon for tuning system settings in virtualized environments.
tuned is a tuning profile delivery mechanism that adapts Red Hat Enterprise Linux for certain workload characteristics, such as requirements for CPU-intensive tasks, or storage/network throughput responsiveness. It provides a number of tuning profiles that are pre-configured to enhance performance and reduce power consumption in a number of specific use cases. Edit these profiles or create new profiles to create performance solutions tailored to your environment.
The virtualization-related profiles provided as part of tuned include:
virtual-guest
Based on the
throughput-performance
profile,virtual-guest
also decreases the swappiness of virtual memory.The
virtual-guest
profile is automatically selected when creating a Red Hat Enterprise Linux7 guest virtual machine. It is the recommended profile for virtual machines.This profile is available in Red Hat Enterprise Linux6.3 and later, but must be manually selected when installing a virtual machine.
virtual-host
Based on the
throughput-performance
profile,virtual-host
also enables more aggressive writeback of dirty pages. This profile is the recommended profile for virtualization hosts, including both KVM and Red Hat Virtualization (RHV) hosts.
By default in a Red Hat Enterprise Linux 7 installation, the tuned package is installed and the tuned
service is enabled.
To list all available profiles and identify the current active profile, run:
# tuned-adm list
Available profiles:- balanced- desktop- latency-performance- network-latency- network-throughput- powersave- sap- throughput-performance- virtual-guest- virtual-hostCurrent active profile: throughput-performance
It is also possible to create custom tuned profiles to encapsulate a set of tuning parameters. For instructions on creating custom tuned profiles, see the tuned.conf
man page.
To only display the currently active profile, run:
tuned-adm active
To switch to one of the available profiles, run:
tuned-adm profile profile_name
For example, to switch to the virtual-host
profile, run:
tuned-adm profile virtual-host
Important
After setting a tuned profile in Red Hat Enterprise Linux 7.1 and above, make sure the tuned service is enabled for the configured profile to be applied after a reboot:
# systemctl enable tuned
In some cases, it is preferable to disable tuned to use parameters set manually. To disable all tuning for the current session, run:
# tuned-adm off
To permanently disable tuned and revert all changes it performed, run:
# tuned-adm off; systemctl disable tuned
Note
For further information about tuned, see the Red Hat Enterprise Linux7 Performance Tuning Guide.
Chapter5.Networking
This chapter covers network optimization topics for virtualized environments.
5.1.Networking Tuning Tips
Use multiple networks to avoid congestion on a single network. For example, have dedicated networks for management, backups, or live migration.
Red Hat recommends not using multiple interfaces in the same network segment. However, if this is unavoidable, you can use
arp_filter
to prevent ARP Flux, an undesirable condition that can occur in both hosts and guests and is caused by the machine responding to ARP requests from more than one network interface:echo 1 > /proc/sys/net/ipv4/conf/all/arp_filter
or edit/etc/sysctl.conf
to make this setting persistent.
Note
For more information on ARP Flux, see http://linux-ip.net/html/ether-arp.html#ether-arp-flux
5.2.Virtio and vhost_net
The following diagram demonstrates the involvement of the kernel in the Virtio and vhost_net architectures.
Figure5.1.Virtio and vhost_net architectures
vhost_net moves part of the Virtio driver from the user space into the kernel. This reduces copy operations, lowers latency and CPU usage.
5.3.Device Assignment and SR-IOV
The following diagram demonstrates the involvement of the kernel in the Device Assignment and SR-IOV architectures.
Figure5.2.Device assignment and SR-IOV
Device assignment presents the entire device to the guest. SR-IOV needs support in drivers and hardware, including the NIC and the system board and allows multiple virtual devices to be created and passed into different guests. A vendor-specific driver is required in the guest, however, SR-IOV offers the lowest latency of any network option.
5.4.Network Tuning Techniques
This section describes techniques for tuning network performance in virtualized environments.
Important
The following features are supported on Red Hat Enterprise Linux 7 hypervisors and virtual machines, but also on virtual machines running Red Hat Enterprise Linux 6.6 and later.
5.4.1.Bridge Zero Copy Transmit
Zero copy transmit mode is effective on large packet sizes. It typically reduces the host CPU overhead by up to 15% when transmitting large packets between a guest network and an external network, without affecting throughput.
It does not affect performance for guest-to-guest, guest-to-host, or small packet workloads.
Bridge zero copy transmit is fully supported on Red Hat Enterprise Linux7 virtual machines, but disabled by default. To enable zero copy transmit mode, set the experimental_zcopytx
kernel module parameter for the vhost_net module to 1. For detailed instructions, see the Virtualization Deployment and Administration Guide.
Note
An additional data copy is normally created during transmit as a threat mitigation technique against denial of service and information leak attacks. Enabling zero copy transmit disables this threat mitigation technique.
If performance regression is observed, or if host CPU utilization is not a concern, zero copy transmit mode can be disabled by setting experimental_zcopytx
to 0.
5.4.2.Multi-Queue virtio-net
Multi-queue virtio-net provides an approach that scales the network performance as the number of vCPUs increases, by allowing them to transfer packets through more than one virtqueue pair at a time.
Today's high-end servers have more processors, and guests running on them often have an increasing number of vCPUs. In single queue virtio-net, the scale of the protocol stack in a guest is restricted, as the network performance does not scale as the number of vCPUs increases. Guests cannot transmit or retrieve packets in parallel, as virtio-net has only one TX and RX queue.
Multi-queue support removes these bottlenecks by allowing paralleled packet processing.
Multi-queue virtio-net provides the greatest performance benefit when:
Traffic packets are relatively large.
The guest is active on many connections at the same time, with traffic running between guests, guest to host, or guest to an external system.
The number of queues is equal to the number of vCPUs. This is because multi-queue support optimizes RX interrupt affinity and TX queue selection in order to make a specific queue private to a specific vCPU.
Note
Currently, setting up a multi-queue virtio-net connection can have a negative effect on the performance of outgoing traffic. Specifically, this may occur when sending packets under 1,500 bytes over the Transmission Control Protocol (TCP) stream. For more information, see the Red Hat Knowledgebase.
5.4.2.1.Configuring Multi-Queue virtio-net
To use multi-queue virtio-net, enable support in the guest by adding the following to the guest XML configuration (where the value of N is from 1 to 256, as the kernel supports up to 256 queues for a multi-queue tap device):
<interface type='network'> <source network='default'/> <model type='virtio'/> <driver name='vhost' queues='N'/></interface>
When running a virtual machine with N virtio-net queues in the guest, enable the multi-queue support with the following command (where the value of M is from 1 to N):
# ethtool -L eth0 combined M
5.5.Batching Network Packets
In configurations with a long transmission path, batching packets before submitting them to the kernel may improve cache utilization.
To configure the maximum number of packets that can be batched where N is the maximum number of packets to batch:
# ethtool -C $tap rx-frames
N
To provide support for tun
/tap
rx batching for type='bridge' or type='network' interfaces, add a snippet similar to the following to the domain XML file.
...<devices> <interface type='network'> <source network='default'/> <target dev='vnet0'/> <coalesce> <rx> <frames max='7'/> </rx> </coalesce> </interface></devices>
Chapter6.I/O Scheduling
You can use an input/output (I/O) scheduler to improve disk performance both when Red Hat Enterprise Linux 7 is a virtualization host as well as when it is a virtualization guest.
6.1.I/O Scheduling with Red Hat Enterprise Linux as a Virtualization Host
When using Red Hat Enterprise Linux 7 as a host for virtualized guests, the default deadline
scheduler is usually ideal. This scheduler performs well on nearly all workloads.
However, if maximizing I/O throughput is more important than minimizing I/O latency on the guest workloads, it may be beneficial to use the cfq
scheduler instead.
6.2.I/O Scheduling with Red Hat Enterprise Linux as a Virtualization Guest
You can use I/O scheduling on a Red Hat Enterprise Linux guest virtual machine, regardless of the hypervisor on which the guest is running. The following is a list of benefits and issues that should be considered:
Red Hat Enterprise Linux guests often benefit greatly from using the
noop
scheduler. The scheduler merges small requests from the guest operating system into larger requests before sending the I/O to the hypervisor. This enables the hypervisor to process the I/O requests more efficiently, which can significantly improve the guest's I/O performance.Depending on the workload I/O and how storage devices are attached, schedulers like
deadline
can be more beneficial thannoop
. Red Hat recommends performance testing to verify which scheduler offers the best performance impact.Guests that use storage accessed by iSCSI, SR-IOV, or physical device passthrough should not use the
noop
scheduler. These methods do not allow the host to optimize I/O requests to the underlying physical device.
Note
In virtualized environments, it is sometimes not beneficial to schedule I/O on both the host and guest layers. If multiple guests use storage on a file system or block device managed by the host operating system, the host may be able to schedule I/O more efficiently because it is aware of requests from all guests. In addition, the host knows the physical layout of storage, which may not map linearly to the guests' virtual storage.
All scheduler tuning should be tested under normal operating conditions, as synthetic benchmarks typically do not accurately compare performance of systems using shared resources in virtual environments.
6.2.1.Configuring the I/O Scheduler for Red Hat Enterprise Linux 7
The default scheduler used on a Red Hat Enterprise Linux 7 system is deadline
. However, on a Red Hat Enterprise Linux 7 guest machine, it may be beneficial to change the scheduler to noop
, by doing the following:
In the
/etc/default/grub
file, change theelevator=deadline
string on theGRUB_CMDLINE_LINUX
line toelevator=noop
. If there is noelevator=
string, addelevator=noop
at the end of the line.The following shows the
/etc/default/grub
file after a successful change.#
cat /etc/default/grub
[...]GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=vg00/lvroot rhgb quiet elevator=noop"[...]Rebuild the
/boot/grub2/grub.cfg
file.On a BIOS-based machine:
#
grub2-mkconfig -o /boot/grub2/grub.cfg
On an UEFI-based machine:
#
grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
Chapter7.Block I/O
This chapter covers optimizing I/O settings in virtualized environments.
7.1.Block I/O Tuning
The virsh blkiotune
command allows administrators to set or display a guest virtual machine's block I/O parameters manually in the <blkio>
element in the guest XML configuration.
To display current <blkio>
parameters for a virtual machine:
# virsh blkiotune virtual_machine
To set a virtual machine's <blkio>
parameters, use the virsh blkiotune
command and replace option values according to your environment:
# virsh blkiotune virtual_machine [--weight number] [--device-weights string] [--config] [--live] [--current]
Parameters include:
weight
The I/O weight, within the range 100 to 1000.
Increasing the I/O weight of a device increases its priority for I/O bandwidth, and therefore provides it with more host resources. Similarly, reducing a device’s weight makes it consume less host resources.
device-weights
A single string listing one or more device/weight pairs, in the format of
. Each weight must be within the range 100-1000, or the value 0 to remove that device from per-device listings. Only the devices listed in the string are modified; any existing per-device weights for other devices remain unchanged./path/to/device
,weight,/path/to/device
,weightconfig
Add the
option for changes to take effect at next boot.--config
live
Add the
option to apply the changes to the running virtual machine.--live
Note
The
option requires the hypervisor to support this action. Not all hypervisors allow live changes of the maximum memory limit.--live
current
Add the
option to apply the changes to the current virtual machine.--current
For example, the following changes the weight of the /dev/sda
device in the liftbrul VM to 500.
# virsh blkiotune liftbrul --device-weights /dev/sda, 500
Note
Use the virsh help blkiotune
command for more information on using the virsh blkiotune
command.
7.2.Caching
Caching options can be configured with virt-manager during guest installation, or on an existing guest virtual machine by editing the guest XML configuration.
Caching Option | Description |
---|---|
Cache=none | I/O from the guest is not cached on the host, but may be kept in a writeback disk cache. Use this option for guests with large I/O requirements. This option is generally the best choice, and is the only option to support migration. |
Cache=writethrough | I/O from the guest is cached on the host but written through to the physical medium. This mode is slower and prone to scaling problems. Best used for small number of guests with lower I/O requirements. Suggested for guests that do not support a writeback cache (such as Red Hat Enterprise Linux 5.5 and earlier), where migration is not needed. |
Cache=writeback | I/O from the guest is cached on the host. |
Cache=directsync | Similar to writethrough , but I/O from the guest bypasses the host page cache. |
Cache=unsafe | The host may cache all disk I/O, and sync requests from guest are ignored. |
Cache=default | If no cache mode is specified, the system's default settings are chosen. |
In virt-manager, the caching mode can be specified under Virtual Disk. For information on using virt-manager to change the cache mode, see Section3.3, “Virtual Disk Performance Options”
To configure the cache mode in the guest XML, edit the cache
setting inside the driver
tag to specify a caching option. For example, to set the cache as writeback
:
<disk type='file' device='disk'> <driver name='qemu' type='raw' cache='writeback'/>
7.3.I/O Mode
I/O mode options can be configured with virt-manager during guest installation, or on an existing guest virtual machine by editing the guest XML configuration.
IO Mode Option | Description |
---|---|
IO=native | The default for Red Hat Virtualization (RHV) environments. This mode refers to kernel asynchronous I/O with direct I/O options. |
IO=threads | The default are host user-mode based threads. |
IO=default | The default in Red Hat Enterprise Linux7 is threads mode. |
In virt-manager, the I/O mode can be specified under Virtual Disk. For information on using virt-manager to change the I/O mode, see Section3.3, “Virtual Disk Performance Options”
To configure the I/O mode in the guest XML, edit the io
setting inside the driver
tag, specifying native
, threads
, or default
. For example, to set the I/O mode to threads
:
<disk type='file' device='disk'> <driver name='qemu' type='raw' io='threads'/>
7.4.Block I/O Tuning Techniques
This section describes more techniques for tuning block I/O performance in virtualized environments.
7.4.1.Disk I/O Throttling
When several virtual machines are running simultaneously, they can interfere with system performance by using excessive disk I/O. Disk I/O throttling in KVM provides the ability to set a limit on disk I/O requests sent from virtual machines to the host machine. This can prevent a virtual machine from over-utilizing shared resources and impacting the performance of other virtual machines.
Disk I/O throttling can be useful in various situations, for example when guest virtual machines belonging to different customers are running on the same host, or when quality of service guarantees are given for different guests. Disk I/O throttling can also be used to simulate slower disks.
I/O throttling can be applied independently to each block device attached to a guest and supports limits on throughput and I/O operations. Use the virsh blkdeviotune
command to set I/O limits for a virtual machine:
# virsh blkdeviotune virtual_machine device --parameter limit
Device specifies a unique target name (<target dev='name'/>
) or source file (<source file='name'/>
) for one of the disk devices attached to the virtual machine. Use the virsh domblklist
command for a list of disk device names.
Optional parameters include:
total-bytes-sec
The total throughput limit in bytes per second.
read-bytes-sec
The read throughput limit in bytes per second.
write-bytes-sec
The write throughput limit in bytes per second.
total-iops-sec
The total I/O operations limit per second.
read-iops-sec
The read I/O operations limit per second.
write-iops-sec
The write I/O operations limit per second.
For example, to throttle vda
on virtual_machine
to 1000 I/O operations per second and 50 MB per second throughput, run this command:
# virsh blkdeviotune virtual_machine vda --total-iops-sec 1000 --total-bytes-sec 52428800
7.4.2.Multi-Queue virtio-scsi
Multi-queue virtio-scsi provides improved storage performance and scalability in the virtio-scsi driver. It enables each virtual CPU to have a separate queue and interrupt to use without affecting other vCPUs.
7.4.2.1.Configuring Multi-Queue virtio-scsi
Multi-queue virtio-scsi is disabled by default on Red Hat Enterprise Linux7.
To enable multi-queue virtio-scsi support in the guest, add the following to the guest XML configuration, where N is the total number of vCPU queues:
<controller type='scsi' index='0' model='virtio-scsi'> <driver queues='N' /> </controller>
Chapter8.Memory
This chapter covers memory optimization options for virtualized environments.
8.1.Memory Tuning Tips
To optimize memory performance in a virtualized environment, consider the following:
Do not allocate more resources to guest than it will use.
If possible, assign a guest to a single NUMA node, providing that resources are sufficient on that NUMA node. For more information on using NUMA, see Chapter9, NUMA.
8.2.Memory Tuning on Virtual Machines
8.2.1.Memory Monitoring Tools
Memory usage can be monitored in virtual machines using tools used in bare metal environments. Tools useful for monitoring memory usage and diagnosing memory-related problems include:
top
vmstat
numastat
/proc/
Note
For details on using these performance tools, see the Red Hat Enterprise Linux7 Performance Tuning Guide and the man pages for these commands.
8.2.2.Memory Tuning with virsh
The optional <memtune>
element in the guest XML configuration allows administrators to configure guest virtual machine memory settings manually. If <memtune>
is omitted, the VM uses memory based on how it was allocated and assigned during the VM creation.
Display or set memory parameters in the <memtune>
element in a virtual machine with the virsh memtune
command, replacing values according to your environment:
# virsh memtune virtual_machine --parameter size
Optional parameters include:
hard_limit
The maximum memory the virtual machine can use, in kibibytes (blocks of 1024 bytes).
Warning
Setting this limit too low can result in the virtual machine being killed by the kernel.
soft_limit
The memory limit to enforce during memory contention, in kibibytes (blocks of 1024 bytes).
swap_hard_limit
The maximum memory plus swap the virtual machine can use, in kibibytes (blocks of 1024 bytes). The
swap_hard_limit
value must be more than thehard_limit
value.min_guarantee
The guaranteed minimum memory allocation for the virtual machine, in kibibytes (blocks of 1024 bytes).
Note
See # virsh help memtune
for more information on using the virsh memtune
command.
The optional <memoryBacking>
element may contain several elements that influence how virtual memory pages are backed by host pages.
Setting locked
prevents the host from swapping out memory pages belonging to the guest. Add the following to the guest XML to lock the virtual memory pages in the host's memory:
<memoryBacking> <locked/></memoryBacking>
Important
When setting locked
, a hard_limit
must be set in the <memtune>
element to the maximum memory configured for the guest, plus any memory consumed by the process itself.
Setting nosharepages
prevents the host from merging the same memory used among guests. To instruct the hypervisor to disable share pages for a guest, add the following to the guest's XML:
<memoryBacking> <nosharepages/></memoryBacking>
8.2.3.Huge Pages and Transparent Huge Pages
AMD64 and Intel 64 CPUs usually address memory in 4kB pages, but they are capable of using larger 2MB or 1GB pages known as huge pages. KVM guests can be deployed with huge page memory support in order to improve performance by increasing CPU cache hits against the Transaction Lookaside Buffer (TLB).
A kernel feature enabled by default in Red Hat Enterprise Linux7, huge pages can significantly increase performance, particularly for large memory and memory-intensive workloads. Red Hat Enterprise Linux7 is able to manage large amounts of memory more effectively by increasing the page size through the use of huge pages. To increase the effectiveness and convenience of managing huge pages, Red Hat Enterprise Linux7 uses Transparent Huge Pages (THP) by default. For more information on huge pages and THP, see the Performance Tuning Guide.
Red Hat Enterprise Linux7 systems support 2MB and 1GB huge pages, which can be allocated at boot or at runtime. See Section8.2.3.3, “Enabling 1 GB huge pages for guests at boot or runtime” for instructions on enabling multiple huge page sizes.
8.2.3.1.Configuring Transparent Huge Pages
Transparent huge pages (THP) are an abstraction layer that automates most aspects of creating, managing, and using huge pages. By default, they automatically optimize system settings for performance.
Note
Using KSM can reduce the occurrence of transparent huge pages, so it is recommended to disable KSM before enabling THP. For more information, see Section8.3.4, “Deactivating KSM”.
Transparent huge pages are enabled by default. To check the current status, run:
# cat /sys/kernel/mm/transparent_hugepage/enabled
To enable transparent huge pages to be used by default, run:
# echo always > /sys/kernel/mm/transparent_hugepage/enabled
This will set /sys/kernel/mm/transparent_hugepage/enabled
to always
.
To disable transparent huge pages:
# echo never > /sys/kernel/mm/transparent_hugepage/enabled
Transparent Huge Page support does not prevent the use of static huge pages. However, when static huge pages are not used, KVM will use transparent huge pages instead of the regular 4kB page size.
8.2.3.2.Configuring Static Huge Pages
In some cases, greater control of huge pages is preferable. To use static huge pages on guests, add the following to the guest XML configuration using virsh edit
:
<memoryBacking> <hugepages/></memoryBacking>
This instructs the host to allocate memory to the guest using huge pages, instead of using the default page size.
View the current huge pages value by running the following command:
cat /proc/sys/vm/nr_hugepages
Procedure8.1.Setting huge pages
The following example procedure shows the commands to set huge pages.
View the current huge pages value:
# cat /proc/meminfo | grep Huge
AnonHugePages: 2048 kBHugePages_Total: 0HugePages_Free: 0HugePages_Rsvd: 0HugePages_Surp: 0Hugepagesize: 2048 kBHuge pages are set in increments of 2MB. To set the number of huge pages to 25000, use the following command:
echo 25000 > /proc/sys/vm/nr_hugepages
Note
To make the setting persistent, add the following lines to the
/etc/sysctl.conf
file on the guest machine, with X being the intended number of huge pages:# echo 'vm.nr_hugepages = X' >> /etc/sysctl.conf# sysctl -p
Afterwards, add
transparent_hugepage=never
to the kernel boot parameters by appending it to the end of the/kernel
line in the/etc/grub2.cfg
file on the guest.Mount the huge pages:
# mount -t hugetlbfs hugetlbfs /dev/hugepages
Add the following lines to the memoryBacking section in the virtual machine's XML configuration:
<hugepages> <page size='1' unit='GiB'/></hugepages>
Restart libvirtd:
#
systemctl restart libvirtd
Start the VM:
#
virsh start virtual_machine
Restart the VM if it is already running:
#
virsh reset virtual_machine
Verify the changes in
/proc/meminfo
:# cat /proc/meminfo | grep Huge
AnonHugePages: 0 kBHugePages_Total: 25000HugePages_Free: 23425HugePages_Rsvd: 0HugePages_Surp: 0Hugepagesize: 2048 kB
Huge pages can benefit not only the host but also guests, however, their total huge pages value must be less than what is available in the host.
8.2.3.3.Enabling 1 GB huge pages for guests at boot or runtime
Red Hat Enterprise Linux7 systems support 2MB and 1GB huge pages, which can be allocated at boot or at runtime.
Procedure8.2.Allocating 1GB huge pages at boot time
To allocate different sizes of huge pages at boot time, use the following command, specifying the number of huge pages. This example allocates four 1GB huge pages and 1024 2MB huge pages:
'default_hugepagesz=1G hugepagesz=1G hugepages=4 hugepagesz=2M hugepages=1024'
Change this command line to specify a different number of huge pages to be allocated at boot.
Note
The next two steps must also be completed the first time you allocate 1GB huge pages at boot time.
Mount the 2MB and 1GB huge pages on the host:
# mkdir /dev/hugepages1G# mount -t hugetlbfs -o pagesize=1G none /dev/hugepages1G# mkdir /dev/hugepages2M# mount -t hugetlbfs -o pagesize=2M none /dev/hugepages2M
Add the following lines to the memoryBacking section in the virtual machine's XML configuration:
<hugepages> <page size='1' unit='GiB'/></hugepages>
Restart libvirtd to enable the use of 1GB huge pages on guests:
# systemctl restart libvirtd
Procedure8.3.Allocating 1GB huge pages at runtime
1GB huge pages can also be allocated at runtime. Runtime allocation allows the system administrator to choose which NUMA node to allocate those pages from. However, runtime page allocation can be more prone to allocation failure than boot time allocation due to memory fragmentation.
To allocate different sizes of huge pages at runtime, use the following command, replacing values for the number of huge pages, the NUMA node to allocate them from, and the huge page size:
# echo 4 > /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/nr_hugepages# echo 1024 > /sys/devices/system/node/node3/hugepages/hugepages-2048kB/nr_hugepages
This example command allocates four 1GB huge pages from
node1
and 1024 2MB huge pages fromnode3
.These huge page settings can be changed at any time with the above command, depending on the amount of free memory on the host system.
Note
The next two steps must also be completed the first time you allocate 1GB huge pages at runtime.
Mount the 2MB and 1GB huge pages on the host:
# mkdir /dev/hugepages1G# mount -t hugetlbfs -o pagesize=1G none /dev/hugepages1G# mkdir /dev/hugepages2M# mount -t hugetlbfs -o pagesize=2M none /dev/hugepages2M
Add the following lines to the memoryBacking section in the virtual machine's XML configuration:
<hugepages> <page size='1' unit='GiB'/></hugepages>
Restart libvirtd to enable the use of 1GB huge pages on guests:
# systemctl restart libvirtd
8.3.Kernel Same-page Merging (KSM)
Kernel same-page Merging (KSM), used by the KVM hypervisor, allows KVM guests to share identical memory pages. These shared pages are usually common libraries or other identical, high-use data. KSM allows for greater guest density of identical or similar guest operating systems by avoiding memory duplication.
The concept of shared memory is common in modern operating systems. For example, when a program is first started, it shares all of its memory with the parent program. When either the child or parent program tries to modify this memory, the kernel allocates a new memory region, copies the original contents and allows the program to modify this new region. This is known as copy on write.
KSM is a Linux feature which uses this concept in reverse. KSM enables the kernel to examine two or more already running programs and compare their memory. If any memory regions or pages are identical, KSM reduces multiple identical memory pages to a single page. This page is then marked copy on write. If the contents of the page is modified by a guest virtual machine, a new page is created for that guest.
This is useful for virtualization with KVM. When a guest virtual machine is started, it only inherits the memory from the host qemu-kvm
process. Once the guest is running, the contents of the guest operating system image can be shared when guests are running the same operating system or applications. KSM allows KVM to request that these identical guest memory regions be shared.
KSM provides enhanced memory speed and utilization. With KSM, common process data is stored in cache or in main memory. This reduces cache misses for the KVM guests, which can improve performance for some applications and operating systems. Secondly, sharing memory reduces the overall memory usage of guests, which allows for higher densities and greater utilization of resources.
Note
In Red Hat Enterprise Linux 7, KSM is NUMA aware. This allows it to take NUMA locality into account while coalescing pages, thus preventing performance drops related to pages being moved to a remote node. Red Hat recommends avoiding cross-node memory merging when KSM is in use. If KSM is in use, change the /sys/kernel/mm/ksm/merge_across_nodes
tunable to 0
to avoid merging pages across NUMA nodes. This can be done with the virsh node-memory-tune --shm-merge-across-nodes 0
command. Kernel memory accounting statistics can eventually contradict each other after large amounts of cross-node merging. As such, numad can become confused after the KSM daemon merges large amounts of memory. If your system has a large amount of free memory, you may achieve higher performance by turning off and disabling the KSM daemon. See Chapter9, NUMA" for more information on NUMA.
Important
Ensure the swap size is sufficient for the committed RAM even without taking KSM into account. KSM reduces the RAM usage of identical or similar guests. Overcommitting guests with KSM without sufficient swap space may be possible, but is not recommended because guest virtual machine memory use can result in pages becoming unshared.
Red Hat Enterprise Linux uses two separate methods for controlling KSM:
The ksm service starts and stops the KSM kernel thread.
The ksmtuned service controls and tunes the
ksm
service, dynamically managing same-page merging.ksmtuned
starts theksm
service and stops theksm
service if memory sharing is not necessary. When new guests are created or destroyed,ksmtuned
must be instructed with theretune
parameter to run.
Both of these services are controlled with the standard service management tools.
Note
KSM is off by default on Red Hat Enterprise Linux 6.7.
8.3.1.The KSM Service
The
ksm
service is included in the qemu-kvm package.When the
ksm
service is not started, Kernel same-page merging (KSM) shares only 2000 pages. This default value provides limited memory-saving benefits.When the
ksm
service is started, KSM will share up to half of the host system's main memory. Start theksm
service to enable KSM to share more memory.
# systemctl start ksm
Starting ksm: [ OK ]
The ksm
service can be added to the default startup sequence. Make the ksm
service persistent with the systemctl command.
# systemctl enable ksm
8.3.2.The KSM Tuning Service
The ksmtuned
service fine-tunes the kernel same-page merging (KSM) configuration by looping and adjusting ksm
. In addition, the ksmtuned
service is notified by libvirt when a guest virtual machine is created or destroyed. The ksmtuned
service has no options.
# systemctl start ksmtuned
Starting ksmtuned: [ OK ]
The ksmtuned
service can be tuned with the retune
parameter, which instructs ksmtuned
to run tuning functions manually.
The /etc/ksmtuned.conf
file is the configuration file for the ksmtuned
service. The file output below is the default ksmtuned.conf
file:
# Configuration file for ksmtuned.# How long ksmtuned should sleep between tuning adjustments# KSM_MONITOR_INTERVAL=60# Millisecond sleep between ksm scans for 16Gb server.# Smaller servers sleep more, bigger sleep less.# KSM_SLEEP_MSEC=10# KSM_NPAGES_BOOST - is added to the `npages` value, when `free memory` is less than `thres`.# KSM_NPAGES_BOOST=300# KSM_NPAGES_DECAY - is the value given is subtracted to the `npages` value, when `free memory` is greater than `thres`.# KSM_NPAGES_DECAY=-50# KSM_NPAGES_MIN - is the lower limit for the `npages` value.# KSM_NPAGES_MIN=64# KSM_NPAGES_MAX - is the upper limit for the `npages` value.# KSM_NPAGES_MAX=1250# KSM_THRES_COEF - is the RAM percentage to be calculated in parameter `thres`.# KSM_THRES_COEF=20# KSM_THRES_CONST - If this is a low memory system, and the `thres` value is less than `KSM_THRES_CONST`, then reset `thres` value to `KSM_THRES_CONST` value.# KSM_THRES_CONST=2048# uncomment the following to enable ksmtuned debug information# LOGFILE=/var/log/ksmtuned# DEBUG=1
Within the /etc/ksmtuned.conf
file, npages
sets how many pages ksm
will scan before the ksmd
daemon becomes inactive. This value will also be set in the /sys/kernel/mm/ksm/pages_to_scan
file.
The KSM_THRES_CONST
value represents the amount of available memory used as a threshold to activate ksm
. ksmd
is activated if either of the following occurs:
The amount of free memory drops below the threshold, set in
KSM_THRES_CONST
.The amount of committed memory plus the threshold,
KSM_THRES_CONST
, exceeds the total amount of memory.
8.3.3.KSM Variables and Monitoring
Kernel same-page merging (KSM) stores monitoring data in the /sys/kernel/mm/ksm/
directory. Files in this directory are updated by the kernel and are an accurate record of KSM usage and statistics.
The variables in the list below are also configurable variables in the /etc/ksmtuned.conf
file, as noted above.
Files in /sys/kernel/mm/ksm/
:
- full_scans
Full scans run.
- merge_across_nodes
Whether pages from different NUMA nodes can be merged.
Total pages shared.
- pages_sharing
Pages currently shared.
- pages_to_scan
Pages not scanned.
Pages no longer shared.
- pages_volatile
Number of volatile pages.
- run
Whether the KSM process is running.
- sleep_millisecs
Sleep milliseconds.
These variables can be manually tuned using the virsh node-memory-tune
command. For example, the following specifies the number of pages to scan before the shared memory service goes to sleep:
# virsh node-memory-tune --shm-pages-to-scan number
KSM tuning activity is stored in the /var/log/ksmtuned
log file if the DEBUG=1
line is added to the /etc/ksmtuned.conf
file. The log file location can be changed with the LOGFILE
parameter. Changing the log file location is not advised and may require special configuration of SELinux settings.
8.3.4.Deactivating KSM
Kernel same-page merging (KSM) has a performance overhead which may be too large for certain environments or host systems. KSM may also introduce side channels that could be potentially used to leak information across guests. If this is a concern, KSM can be disabled on per-guest basis.
KSM can be deactivated by stopping the ksmtuned
and the ksm
services. However, this action does not persist after restarting. To deactivate KSM, run the following in a terminal as root:
# systemctl stop ksmtunedStopping ksmtuned: [ OK ]# systemctl stop ksmStopping ksm: [ OK ]
Stopping the ksmtuned
and the ksm
deactivates KSM, but this action does not persist after restarting. Persistently deactivate KSM with the systemctl
commands:
# systemctl disable ksm
# systemctl disable ksmtuned
When KSM is disabled, any memory pages that were shared prior to deactivating KSM are still shared. To delete all of the PageKSM in the system, use the following command:
# echo 2 >/sys/kernel/mm/ksm/run
After this is performed, the khugepaged
daemon can rebuild transparent hugepages on the KVM guest physical memory. Using # echo 0 >/sys/kernel/mm/ksm/run
stops KSM, but does not unshare all the previously created KSM pages (this is the same as the # systemctl stop ksmtuned
command).
Chapter9.NUMA
Historically, all memory on AMD64 and Intel 64 systems is equally accessible by all CPUs. Known as Uniform Memory Access (UMA), access times are the same no matter which CPU performs the operation.
This behavior is no longer the case with recent AMD64 and Intel 64 processors. In Non-Uniform Memory Access (NUMA), system memory is divided across NUMA nodes, which correspond to sockets or to a particular set of CPUs that have identical access latency to the local subset of system memory.
This chapter describes memory allocation and NUMA tuning configurations in virtualized environments.
9.1.NUMA Memory Allocation Policies
The following policies define how memory is allocated from the nodes in a system:
Strict
Strict policy means that the allocation will fail if the memory cannot be allocated on the target node.
Specifying a NUMA nodeset list without defining a memory mode attribute defaults to
strict
mode.Interleave
Memory pages are allocated across nodes specified by a nodeset, but are allocated in a round-robin fashion.
Preferred
Memory is allocated from a single preferred memory node. If sufficient memory is not available, memory can be allocated from other nodes.
To enable the intended policy, set it as the value of the <memory mode>
element of the domain XML file:
<numatune> <memory mode='preferred' nodeset='0'></numatune>
Important
If memory is overcommitted in strict
mode and the guest does not have sufficient swap space, the kernel will kill some guest processes to retrieve additional memory. Red Hat recommends using preferred
allocation and specifying a single nodeset (for example, nodeset='0') to prevent this situation.
9.2.Automatic NUMA Balancing
Automatic NUMA balancing improves the performance of applications running on NUMA hardware systems. It is enabled by default on Red Hat Enterprise Linux7 systems.
An application will generally perform best when the threads of its processes are accessing memory on the same NUMA node as the threads are scheduled. Automatic NUMA balancing moves tasks (which can be threads or processes) closer to the memory they are accessing. It also moves application data to memory closer to the tasks that reference it. This is all done automatically by the kernel when automatic NUMA balancing is active.
Automatic NUMA balancing uses a number of algorithms and data structures, which are only active and allocated if automatic NUMA balancing is active on the system:
Periodic NUMA unmapping of process memory
NUMA hinting fault
Migrate-on-Fault (MoF) - moves memory to where the program using it runs
task_numa_placement - moves running programs closer to their memory
9.2.1.Configuring Automatic NUMA Balancing
Automatic NUMA balancing is enabled by default in Red Hat Enterprise Linux7, and will automatically activate when booted on hardware with NUMA properties.
Automatic NUMA balancing is enabled when both of the following conditions are met:
# numactl --hardware
shows multiple nodes# cat /proc/sys/kernel/numa_balancing
shows1
Manual NUMA tuning of applications will override automatic NUMA balancing, disabling periodic unmapping of memory, NUMA faults, migration, and automatic NUMA placement of those applications.
In some cases, system-wide manual NUMA tuning is preferred.
To disable automatic NUMA balancing, use the following command:
# echo 0 > /proc/sys/kernel/numa_balancing
To enable automatic NUMA balancing, use the following command:
# echo 1 > /proc/sys/kernel/numa_balancing
9.3.libvirt NUMA Tuning
Generally, best performance on NUMA systems is achieved by limiting guest size to the amount of resources on a single NUMA node. Avoid unnecessarily splitting resources across NUMA nodes.
Use the numastat
tool to view per-NUMA-node memory statistics for processes and the operating system.
In the following example, the numastat
tool shows four virtual machines with suboptimal memory alignment across NUMA nodes:
# numastat -c qemu-kvm
Per-node process memory usage (in MBs)PID Node 0 Node 1 Node 2 Node 3 Node 4 Node 5 Node 6 Node 7 Total--------------- ------ ------ ------ ------ ------ ------ ------ ------ -----51722 (qemu-kvm) 68 16 357 6936 2 3 147 598 812851747 (qemu-kvm) 245 11 5 18 5172 2532 1 92 807653736 (qemu-kvm) 62 432 1661 506 4851 136 22 445 811653773 (qemu-kvm) 1393 3 1 2 12 0 0 6702 8114--------------- ------ ------ ------ ------ ------ ------ ------ ------ -----Total 1769 463 2024 7462 10037 2672 169 7837 32434
Run numad
to align the guests' CPUs and memory resources automatically.
Then run numastat -c qemu-kvm
again to view the results of running numad
. The following output shows that resources have been aligned:
# numastat -c qemu-kvm
Per-node process memory usage (in MBs)PID Node 0 Node 1 Node 2 Node 3 Node 4 Node 5 Node 6 Node 7 Total--------------- ------ ------ ------ ------ ------ ------ ------ ------ -----51747 (qemu-kvm) 0 0 7 0 8072 0 1 0 808053736 (qemu-kvm) 0 0 7 0 0 0 8113 0 812053773 (qemu-kvm) 0 0 7 0 0 0 1 8110 811859065 (qemu-kvm) 0 0 8050 0 0 0 0 0 8051--------------- ------ ------ ------ ------ ------ ------ ------ ------ -----Total 0 0 8072 0 8072 0 8114 8110 32368
Note
Running numastat
with -c
provides compact output; adding the -m
option adds system-wide memory information on a per-node basis to the output. See the numastat
man page for more information.
9.3.1.Monitoring Memory per host NUMA Node
You can use the nodestats.py
script to report the total memory and free memory for each NUMA node on a host. This script also reports how much memory is strictly bound to certain host nodes for each running domain. For example:
# /usr/share/doc/libvirt-python-2.0.0/examples/nodestats.py
NUMA statsNUMA nodes: 0 1 2 3MemTotal: 3950 3967 3937 3943MemFree: 66 56 42 41Domain 'rhel7-0': Overall memory: 1536 MiBDomain 'rhel7-1': Overall memory: 2048 MiBDomain 'rhel6': Overall memory: 1024 MiB nodes 0-1 Node 0: 1024 MiB nodes 0-1Domain 'rhel7-2': Overall memory: 4096 MiB nodes 0-3 Node 0: 1024 MiB nodes 0 Node 1: 1024 MiB nodes 1 Node 2: 1024 MiB nodes 2 Node 3: 1024 MiB nodes 3
This example shows four host NUMA nodes, each containing approximately 4GB of RAM in total (MemTotal
). Nearly all memory is consumed on each domain (MemFree
). There are four domains (virtual machines) running: domain 'rhel7-0' has 1.5GB memory which is not pinned onto any specific host NUMA node. Domain 'rhel7-2' however, has 4GB memory and 4 NUMA nodes which are pinned 1:1 to host nodes.
To print host NUMA node statistics, create a nodestats.py
script for your environment. An example script can be found the libvirt-python package files in /usr/share/doc/libvirt-python-version/examples/nodestats.py
. The specific path to the script can be displayed by using the rpm -ql libvirt-python
command.
9.3.2.NUMA vCPU Pinning
vCPU pinning provides similar advantages to task pinning on bare metal systems. Since vCPUs run as user-space tasks on the host operating system, pinning increases cache efficiency. One example of this is an environment where all vCPU threads are running on the same physical socket, therefore sharing a L3 cache domain.
Note
In Red Hat Enterprise Linux versions 7.0 to 7.2, it is only possible to pin active vCPUs. However, with Red Hat Enterprise Linux 7.3, pinning inactive vCPUs is available as well.
Combining vCPU pinning with numatune
can avoid NUMA misses. The performance impacts of NUMA misses are significant, generally starting at a 10% performance hit or higher. vCPU pinning and numatune
should be configured together.
If the virtual machine is performing storage or network I/O tasks, it can be beneficial to pin all vCPUs and memory to the same physical socket that is physically connected to the I/O adapter.
Note
The lstopo tool can be used to visualize NUMA topology. It can also help verify that vCPUs are binding to cores on the same physical socket. See the following Knowledgebase article for more information on lstopo: https://access.redhat.com/site/solutions/62879.
Important
Pinning causes increased complexity where there are many more vCPUs than physical cores.
The following example XML configuration has a domain process pinned to physical CPUs 0-7. The vCPU thread is pinned to its own cpuset. For example, vCPU0 is pinned to physical CPU 0, vCPU1 is pinned to physical CPU 1, and so on:
<vcpu cpuset='0-7'>8</vcpu> <cputune> <vcpupin vcpu='0' cpuset='0'/> <vcpupin vcpu='1' cpuset='1'/> <vcpupin vcpu='2' cpuset='2'/> <vcpupin vcpu='3' cpuset='3'/> <vcpupin vcpu='4' cpuset='4'/> <vcpupin vcpu='5' cpuset='5'/> <vcpupin vcpu='6' cpuset='6'/> <vcpupin vcpu='7' cpuset='7'/> </cputune>
There is a direct relationship between the vcpu and vcpupin tags. If a vcpupin option is not specified, the value will be automatically determined and inherited from the parent vcpu tag option. The following configuration shows <vcpupin>
for vcpu 5 missing. Hence, vCPU5 would be pinned to physical CPUs 0-7, as specified in the parent tag <vcpu>
:
<vcpu cpuset='0-7'>8</vcpu> <cputune> <vcpupin vcpu='0' cpuset='0'/> <vcpupin vcpu='1' cpuset='1'/> <vcpupin vcpu='2' cpuset='2'/> <vcpupin vcpu='3' cpuset='3'/> <vcpupin vcpu='4' cpuset='4'/> <vcpupin vcpu='6' cpuset='6'/> <vcpupin vcpu='7' cpuset='7'/> </cputune>
Important
<vcpupin>
, <numatune>
, and <emulatorpin>
should be configured together to achieve optimal, deterministic performance. For more information on the <numatune>
tag, see Section9.3.3, “Domain Processes”. For more information on the <emulatorpin>
tag, see Section9.3.6, “Using emulatorpin”.
9.3.3.Domain Processes
As provided in Red Hat Enterprise Linux, libvirt uses libnuma to set memory binding policies for domain processes. The nodeset for these policies can be configured either as static (specified in the domain XML) or auto (configured by querying numad). See the following XML configuration for examples on how to configure these inside the <numatune>
tag:
<numatune> <memory mode='strict' placement='auto'/></numatune>
<numatune> <memory mode='strict' nodeset='0,2-3'/></numatune>
libvirt uses sched_setaffinity(2) to set CPU binding policies for domain processes. The cpuset option can either be static (specified in the domain XML) or auto (configured by querying numad). See the following XML configuration for examples on how to configure these inside the <vcpu>
tag:
<vcpu placement='auto'>8</vcpu>
<vcpu placement='static' cpuset='0-10,ˆ5'>8</vcpu>
There are implicit inheritance rules between the placement mode you use for <vcpu>
and <numatune>
:
The placement mode for
<numatune>
defaults to the same placement mode of<vcpu>
, or to static if a<nodeset>
is specified.Similarly, the placement mode for
<vcpu>
defaults to the same placement mode of<numatune>
, or to static if<cpuset>
is specified.
This means that CPU tuning and memory tuning for domain processes can be specified and defined separately, but they can also be configured to be dependent on the other's placement mode.
It is also possible to configure your system with numad to boot a selected number of vCPUs without pinning all vCPUs at startup.
For example, to enable only 8 vCPUs at boot on a system with 32 vCPUs, configure the XML similar to the following:
<vcpu placement='auto' current='8'>32</vcpu>
Note
See the following URLs for more information on vcpu and numatune: http://libvirt.org/formatdomain.html#elementsCPUAllocation and http://libvirt.org/formatdomain.html#elementsNUMATuning
9.3.4.Domain vCPU Threads
In addition to tuning domain processes, libvirt also permits the setting of the pinning policy for each vcpu thread in the XML configuration. Set the pinning policy for each vcpu thread inside the <cputune>
tags:
<cputune> <vcpupin vcpu="0" cpuset="1-4,ˆ2"/> <vcpupin vcpu="1" cpuset="0,1"/> <vcpupin vcpu="2" cpuset="2,3"/> <vcpupin vcpu="3" cpuset="0,4"/></cputune>
In this tag, libvirt uses either cgroup or sched_setaffinity(2) to pin the vcpu thread to the specified cpuset.
Note
For more details on <cputune>
, see the following URL: http://libvirt.org/formatdomain.html#elementsCPUTuning
In addition, if you need to set up a virtual machines with more vCPU than a single NUMA node, configure the host so that the guest detects a NUMA topology on the host. This allows for 1:1 mapping of CPUs, memory, and NUMA nodes. For example, this can be applied with a guest with 4 vCPUs and 6 GB memory, and a host with the following NUMA settings:
4 available nodes (0-3)Node 0:CPUs 0 4, size 4000 MiBNode 1: CPUs 1 5, size 3999 MiBNode 2: CPUs 2 6, size 4001 MiBNode 3: CPUs 0 4, size 4005 MiB
In this scenario, use the following Domain XML setting:
<cputune><vcpupin vcpu="0" cpuset="1"/><vcpupin vcpu="1" cpuset="5"/><vcpupin vcpu="2" cpuset="2"/><vcpupin vcpu="3" cpuset="6"/></cputune><numatune> <memory mode="strict" nodeset="1-2"/> </numatune><cpu><numa><cell id="0" cpus="0-1" memory="3" unit="GiB"/><cell id="1" cpus="2-3" memory="3" unit="GiB"/></numa></cpu>
9.3.5.Using Cache Allocation Technology to Improve Performance
You can make use of Cache Allocation Technology (CAT) provided by the kernel on specific CPU models. This enables allocation of part of the host CPU's cache for vCPU threads, which improves real-time performance.
See the following XML configuration for an example on how to configure vCPU cache allocation inside the cachetune
tag:
<domain> <cputune> <cachetune vcpus='0-1'> <cache id='0' level='3' type='code' size='3' unit='MiB'/> <cache id='0' level='3' type='data' size='3' unit='MiB'/> </cachetune> </cputune></domain>
The XML file above configures the thread for vCPUs 0 and 1 to have 3 MiB from the first L3 cache (level='3' id='0') allocated, once for the L3CODE and once for L3DATA.
Note
A single virtual machine can have multiple <cachetune>
elements.
For more information see cachetune
in the upstream libvirt
documentation.
9.3.6.Using emulatorpin
Another way of tuning the domain process pinning policy is to use the <emulatorpin>
tag inside of <cputune>
.
The <emulatorpin>
tag specifies which host physical CPUs the emulator (a subset of a domain, not including vCPUs) will be pinned to. The <emulatorpin>
tag provides a method of setting a precise affinity to emulator thread processes. As a result, vhost threads run on the same subset of physical CPUs and memory, and therefore benefit from cache locality. For example:
<cputune> <emulatorpin cpuset="1-3"/></cputune>
Note
In Red Hat Enterprise Linux7, automatic NUMA balancing is enabled by default. Automatic NUMA balancing reduces the need for manually tuning <emulatorpin>
, since the vhost-net emulator thread follows the vCPU tasks more reliably. For more information about automatic NUMA balancing, see Section9.2, “Automatic NUMA Balancing”.
9.3.7.Tuning vCPU Pinning with virsh
Important
These are example commands only. You will need to substitute values according to your environment.
The following example virsh
command will pin the vcpu thread rhel7 which has an ID of 1 to the physical CPU 2:
% virsh vcpupin rhel7 1 2
You can also obtain the current vcpu pinning configuration with the virsh
command. For example:
% virsh vcpupin rhel7
9.3.8.Tuning Domain Process CPU Pinning with virsh
Important
These are example commands only. You will need to substitute values according to your environment.
The emulatorpin
option applies CPU affinity settings to threads that are associated with each domain process. For complete pinning, you must use both virsh vcpupin
(as shown previously) and virsh emulatorpin
for each guest. For example:
% virsh emulatorpin rhel7 3-4
9.3.9.Tuning Domain Process Memory Policy with virsh
Domain process memory can be dynamically tuned. See the following example command:
% virsh numatune rhel7 --nodeset 0-10
More examples of these commands can be found in the virsh
man page.
9.3.10.Guest NUMA Topology
Guest NUMA topology can be specified using the <numa>
tag inside the <cpu>
tag in the guest virtual machine's XML. See the following example, and replace values accordingly:
<cpu> ... <numa> <cell cpus='0-3' memory='512000'/> <cell cpus='4-7' memory='512000'/> </numa> ...</cpu>
Each <cell>
element specifies a NUMA cell or a NUMA node. cpus
specifies the CPU or range of CPUs that are part of the node, and memory
specifies the node memory in kibibytes (blocks of 1024 bytes). Each cell or node is assigned a cellid
or nodeid
in increasing order starting from 0.
Important
When modifying the NUMA topology of a guest virtual machine with a configured topology of CPU sockets, cores, and threads, make sure that cores and threads belonging to a single socket are assigned to the same NUMA node. If threads or cores from the same socket are assigned to different NUMA nodes, the guest may fail to boot.
Warning
Using guest NUMA topology simultaneously with huge pages is not supported on Red Hat Enterprise Linux 7 and is only available in layered products such as Red Hat Virtualization or Red Hat OpenStack Platform.
9.3.11.NUMA Node Locality for PCI Devices
When starting a new virtual machine, it is important to know both the host NUMA topology and the PCI device affiliation to NUMA nodes, so that when PCI passthrough is requested, the guest is pinned onto the correct NUMA nodes for optimal memory performance.
For example, if a guest is pinned to NUMA nodes 0-1, but one of its PCI devices is affiliated with node 2, data transfer between nodes will take some time.
In Red Hat Enterprise Linux7.1 and above, libvirt reports the NUMA node locality for PCI devices in the guest XML, enabling management applications to make better performance decisions.
This information is visible in the sysfs
files in /sys/devices/pci*/*/numa_node
. One way to verify these settings is to use the lstopo tool to report sysfs
data:
# lstopo-no-graphics
Machine (126GB) NUMANode L#0 (P#0 63GB) Socket L#0 + L3 L#0 (20MB) L2 L#0 (256KB) + L1d L#0 (32KB) + L1i L#0 (32KB) + Core L#0 + PU L#0 (P#0) L2 L#1 (256KB) + L1d L#1 (32KB) + L1i L#1 (32KB) + Core L#1 + PU L#1 (P#2) L2 L#2 (256KB) + L1d L#2 (32KB) + L1i L#2 (32KB) + Core L#2 + PU L#2 (P#4) L2 L#3 (256KB) + L1d L#3 (32KB) + L1i L#3 (32KB) + Core L#3 + PU L#3 (P#6) L2 L#4 (256KB) + L1d L#4 (32KB) + L1i L#4 (32KB) + Core L#4 + PU L#4 (P#8) L2 L#5 (256KB) + L1d L#5 (32KB) + L1i L#5 (32KB) + Core L#5 + PU L#5 (P#10) L2 L#6 (256KB) + L1d L#6 (32KB) + L1i L#6 (32KB) + Core L#6 + PU L#6 (P#12) L2 L#7 (256KB) + L1d L#7 (32KB) + L1i L#7 (32KB) + Core L#7 + PU L#7 (P#14) HostBridge L#0 PCIBridge PCI 8086:1521 Net L#0 "em1" PCI 8086:1521 Net L#1 "em2" PCI 8086:1521 Net L#2 "em3" PCI 8086:1521 Net L#3 "em4" PCIBridge PCI 1000:005b Block L#4 "sda" Block L#5 "sdb" Block L#6 "sdc" Block L#7 "sdd" PCIBridge PCI 8086:154d Net L#8 "p3p1" PCI 8086:154d Net L#9 "p3p2" PCIBridge PCIBridge PCIBridge PCIBridge PCI 102b:0534 GPU L#10 "card0" GPU L#11 "controlD64" PCI 8086:1d02 NUMANode L#1 (P#1 63GB) Socket L#1 + L3 L#1 (20MB) L2 L#8 (256KB) + L1d L#8 (32KB) + L1i L#8 (32KB) + Core L#8 + PU L#8 (P#1) L2 L#9 (256KB) + L1d L#9 (32KB) + L1i L#9 (32KB) + Core L#9 + PU L#9 (P#3) L2 L#10 (256KB) + L1d L#10 (32KB) + L1i L#10 (32KB) + Core L#10 + PU L#10 (P#5) L2 L#11 (256KB) + L1d L#11 (32KB) + L1i L#11 (32KB) + Core L#11 + PU L#11 (P#7) L2 L#12 (256KB) + L1d L#12 (32KB) + L1i L#12 (32KB) + Core L#12 + PU L#12 (P#9) L2 L#13 (256KB) + L1d L#13 (32KB) + L1i L#13 (32KB) + Core L#13 + PU L#13 (P#11) L2 L#14 (256KB) + L1d L#14 (32KB) + L1i L#14 (32KB) + Core L#14 + PU L#14 (P#13) L2 L#15 (256KB) + L1d L#15 (32KB) + L1i L#15 (32KB) + Core L#15 + PU L#15 (P#15) HostBridge L#8 PCIBridge PCI 1924:0903 Net L#12 "p1p1" PCI 1924:0903 Net L#13 "p1p2" PCIBridge PCI 15b3:1003 Net L#14 "ib0" Net L#15 "ib1" OpenFabrics L#16 "mlx4_0"
This output shows:
NICs
em*
and diskssd*
are connected to NUMA node 0 and cores 0,2,4,6,8,10,12,14.NICs
p1*
andib*
are connected to NUMA node 1 and cores 1,3,5,7,9,11,13,15.
9.4.NUMA-Aware Kernel SamePage Merging (KSM)
Kernel SamePage Merging (KSM) allows virtual machines to share identical memory pages. KSM can detect that a system is using NUMA memory and control merging pages across different NUMA nodes.
Use the sysfs /sys/kernel/mm/ksm/merge_across_nodes
parameter to control merging of pages across different NUMA nodes. By default, pages from all nodes can be merged together. When this parameter is set to zero, only pages from the same node are merged.
Generally, unless you are oversubscribing the system memory, you will get better runtime performance by disabling KSM sharing.
Important
When KSM merges across nodes on a NUMA host with multiple guest virtual machines, guests and CPUs from more distant nodes can suffer a significant increase of access latency to the merged KSM page.
To instruct the hypervisor to disable share pages for a guest, add the following to the guest's XML:
<memoryBacking> <nosharepages/></memoryBacking>
For more information about tuning memory settings with the <memoryBacking>
element, see Section8.2.2, “Memory Tuning with virsh”.
AppendixA.Revision History
Revision History | ||
---|---|---|
Revision 1.0-35 | Thus May 23 2019 | Jiri Herrmann |
Version for 7.7 Beta publication | ||
Revision 1.0-34 | Tue Oct 25 2018 | Jiri Herrmann |
Version for 7.6 GA publication | ||
Revision 1.0-32 | Tue Aug 14 2018 | Jiri Herrmann |
Version for 7.6 Beta publication | ||
Revision 1.0-31 | Wed Apr 4 2018 | Jiri Herrmann |
Version for 7.5 GA publication | ||
Revision 1.0-27 | Mon Jul 27 2017 | Jiri Herrmann |
Version for 7.4 GA publication | ||
Revision 1.0-24 | Mon Oct 17 2016 | Jiri Herrmann |
Version for 7.3 GA publication | ||
Revision 1.0-22 | Mon Dec 21 2015 | Laura Novich |
Republished Guide to fix several bugs | ||
Revision 1.0-19 | Thu Oct 08 2015 | Jiri Herrmann |
Cleaned up Revision History |
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