Synnefo » snf-ganeti

Ganeti CPU Pinning

==================

Objective

---------

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

Validation

----------

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.

Failover

--------

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".

Migration

---------

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.

Data

----

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.

Xen

---

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:

vcpus

  controls the number of vCPUs

cpus

  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" ]``

KVM

---

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 :manpage:`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 (http://pypi.python.org/pypi/affinity). 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.

.. vim: set textwidth=72 :

.. Local Variables:

.. mode: rst

.. fill-column: 72

.. End: