Ganeti CPU Pinning


This document defines Ganeti’s support for CPU pinning (aka CPU affinity).

CPU pinning enables mapping and unmapping entire virtual machines or a specific virtual CPU (vCPU), to a physical CPU or a range of CPUs.

At this stage Pinning will be implemented for Xen and KVM.

Command Line

Suggested command line parameters for controlling CPU pinning are as follows:

gnt-instance modify -H cpu_mask=<cpu-pinning-info> <instance>

cpu-pinning-info can be any of the following:

  • One vCPU mapping, which can be the word “all” or a combination of CPU numbers and ranges separated by comma. In this case, all vCPUs will be mapped to the indicated list.

  • A list of vCPU mappings, separated by a colon ‘:’. In this case each vCPU is mapped to an entry in the list, and the size of the list must match the number of vCPUs defined for the instance. This is enforced when setting CPU pinning or when setting the number of vCPUs using -B vcpus=#.

    The mapping list is matched to consecutive virtual CPUs, so the first entry would be the CPU pinning information for vCPU 0, the second entry for vCPU 1, etc.

The default setting for new instances is “all”, which maps the entire instance to all CPUs, thus effectively turning off CPU pinning.

Here are some usage examples:

# Map vCPU 0 to physical CPU 1 and vCPU 1 to CPU 3 (assuming 2 vCPUs)
gnt-instance modify -H cpu_mask=1:3 my-inst

# Pin vCPU 0 to CPUs 1 or 2, and vCPU 1 to any CPU
gnt-instance modify -H cpu_mask=1-2:all my-inst

# Pin vCPU 0 to any CPU, vCPU 1 to CPUs 1, 3, 4 or 5, and CPU 2 to
# CPU 0
gnt-instance modify -H cpu_mask=all:1\\,3-5:0 my-inst

# Pin entire VM to CPU 0
gnt-instance modify -H cpu_mask=0 my-inst

# Turn off CPU pinning (default setting)
gnt-instance modify -H cpu_mask=all my-inst

Assuming an instance has 3 vCPUs, the following commands will fail:

# not enough mappings
gnt-instance modify -H cpu_mask=0:1 my-inst

# too many
gnt-instance modify -H cpu_mask=2:1:1:all my-inst


CPU pinning information is validated by making sure it matches the number of vCPUs. This validation happens when changing either the cpu_mask or vcpus parameters. Changing either parameter in a way that conflicts with the other will fail with a proper error message. To make such a change, both parameters should be modified at the same time. For example: gnt-instance modify -B vcpus=4 -H cpu_mask=1:1:2-3:4\\,6 my-inst

Besides validating CPU configuration, i.e. the number of vCPUs matches the requested CPU pinning, Ganeti will also verify the number of physical CPUs is enough to support the required configuration. For example, trying to run a configuration of vcpus=2,cpu_mask=0:4 on a node with 4 cores will fail (Note: CPU numbers are 0-based).

This validation should repeat every time an instance is started or migrated live. See more details under Migration below.

Cluster verification should also test the compatibility of other nodes in the cluster to required configuration and alert if a minimum requirement is not met.


CPU pinning configuration can be transferred from node to node, unless the number of physical CPUs is smaller than what the configuration calls for. It is suggested that unless this is the case, all transfers and migrations will succeed.

In case the number of physical CPUs is smaller than the numbers indicated by CPU pinning information, instance failover will fail.

In case of emergency, to force failover to ignore mismatching CPU information, the following switch can be used: gnt-instance failover --fix-cpu-mismatch my-inst. This command will try to failover the instance with the current cpu mask, but if that fails, it will change the mask to be “all”.


In case of live migration, and in addition to failover considerations, it is required to remap CPU pinning after migration. This can be done in realtime for instances for both Xen and KVM, and only depends on the number of physical CPUs being sufficient to support the migrated instance.


Pinning information will be kept as a list of integers per vCPU. To mark a mapping of any CPU, we will use (-1). A single entry, no matter what the number of vCPUs is, will always mean that all vCPUs have the same mapping.

Configuration file

The pinning information is kept for each instance’s hypervisor params section of the configuration file as the original string.


There are 2 ways to control pinning in Xen, either via the command line or through the configuration file.

The commands to make direct pinning changes are the following:

# To pin a vCPU to a specific CPU
xm vcpu-pin <domain> <vcpu> <cpu>

# To unpin a vCPU
xm vcpu-pin <domain> <vcpu> all

# To get the current pinning status
xm vcpu-list <domain>

Since currently controlling Xen in Ganeti is done in the configuration file, it is straight forward to use the same method for CPU pinning. There are 2 different parameters that control Xen’s CPU pinning and configuration:

controls the number of vCPUs
maps vCPUs to physical CPUs

When no pinning is required (pinning information is “all”), the “cpus” entry is removed from the configuration file.

For all other cases, the configuration is “translated” to Xen, which expects either cpus = "a" or cpus = [ "a", "b", "c", ...], where each a, b or c are a physical CPU number, CPU range, or a combination, and the number of entries (if a list is used) must match the number of vCPUs, and are mapped in order.

For example, CPU pinning information of 1:2,4-7:0-1 is translated to this entry in Xen’s configuration cpus = [ "1", "2,4-7", "0-1" ]


Controlling pinning in KVM is a little more complicated as there is no configuration to control pinning before instances are started.

The way to change or assign CPU pinning under KVM is to use taskset or its underlying system call sched_setaffinity. Setting the affinity for the VM process will change CPU pinning for the entire VM, and setting it for specific vCPU threads will control specific vCPUs.

The sequence of commands to control pinning is this: start the instance with the -S switch, so it halts before starting execution, get the process ID or identify thread IDs of each vCPU by sending info cpus to the monitor, map vCPUs as required by the cpu-pinning information, and issue a cont command on the KVM monitor to allow the instance to start execution.

For example, a sequence of commands to control CPU affinity under KVM may be:

  • Start KVM: /usr/bin/kvm <kvm-command-line-options> -S
  • Use socat to connect to monitor
  • send info cpus to monitor to get thread/vCPU information
  • call sched_setaffinity for each thread with the CPU mask
  • send cont to KVM’s monitor

A CPU mask is a hexadecimal bit mask where each bit represents one physical CPU. See man page for sched_setaffinity(2) for more details.

For example, to run a specific thread-id on CPUs 1 or 3 the mask is 0x0000000A.

We will control process and thread affinity using the python affinity package ( This package is a Python wrapper around the two affinity system calls, and has no other requirements.

Alternative Design Options

  1. There’s an option to ignore the limitations of the underlying hypervisor and instead of requiring explicit pinning information for all vCPUs, assume a mapping of “all” to vCPUs not mentioned. This can lead to inadvertent missing information, but either way, since using cpu-pinning options is probably not going to be frequent, there’s no real advantage.

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