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qm: resource limits: revise section affinity

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* mention side effects
* increase compactness w/o complexity
* improve section-formatting

Signed-off-by: Alexander Zeidler <a.zeidler@proxmox.com>
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Alexander Zeidler 2024-01-16 14:22:40 +01:00 committed by Thomas Lamprecht
parent 6a31c01b5a
commit b3848f246c
1 changed files with 16 additions and 12 deletions
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@ -380,20 +380,24 @@ For more information see `man systemd.resource-control`, here `CPUQuota`
corresponds to `cpulimit` and `CPUWeight` to our `cpuunits` setting. Visit its
Notes section for references and implementation details.
The third CPU resource limiting setting, *affinity*, controls what host cores
the virtual machine will be permitted to execute on. E.g., if an affinity value
of `0-3,8-11` is provided, the virtual machine will be restricted to using the
host cores `0,1,2,3,8,9,10,` and `11`. Valid *affinity* values are written in
cpuset `List Format`. List Format is a comma-separated list of CPU numbers and
ranges of numbers, in ASCII decimal.
*affinity*
NOTE: CPU *affinity* uses the `taskset` command to restrict virtual machines to
a given set of cores. This restriction will not take effect for some types of
processes that may be created for IO. *CPU affinity is not a security feature.*
With the *affinity* option you can specify the physical CPU cores which are
used to run the VM's vCPUs. Periphal VM processes, such as those for I/O, are
not affected by this setting. Note that the *CPU affinity is not a security
feature*.
For more information regarding *affinity* see `man cpuset`. Here the
`List Format` corresponds to valid *affinity* values. Visit its `Formats`
section for more examples.
Forcing a CPU *affinity* can make sense in certain cases, but is accompanied by
an increase in complexity and maintenance effort. For example, if you want to
add more VMs later or migrate VMs to nodes with fewer CPU cores. It can also
easily lead to asynchronous and therefore limited system performance if some
CPUs are fully utilized while others are almost idle.
The *affinity* is set by calling `taskset`. It accepts the host CPU numbers
(see `lscpu`) in the `List Format` from `man cpuset`. This ASCII decimal list
can contain numbers but also number ranges. E.g., the *affinity* `0-1,8-11` (or
alternatively `0,1,8,9,10,11`) only allow the VM to run on these six host
cores.
CPU Type
^^^^^^^^