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qm: resource limits: revise section cpulimit

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* precise statements
* 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 14:22:38 +01:00 committed by Thomas Lamprecht
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1 changed files with 21 additions and 19 deletions
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@ -336,31 +336,33 @@ context switches.
Resource Limits
^^^^^^^^^^^^^^^
In addition to the number of virtual cores, you can configure how much resources
a VM can get in relation to the host CPU time and also in relation to other
VMs.
With the *cpulimit* (``Host CPU Time'') option you can limit how much CPU time
the whole VM can use on the host. It is a floating point value representing CPU
time in percent, so `1.0` is equal to `100%`, `2.5` to `250%` and so on. If a
single process would fully use one single core it would have `100%` CPU Time
usage. If a VM with four cores utilizes all its cores fully it would
theoretically use `400%`. In reality the usage may be even a bit higher as QEMU
can have additional threads for VM peripherals besides the vCPU core ones.
*cpulimit*
In addition to the number of virtual cores, the total available ``Host CPU
Time'' for the VM can be set with the *cpulimit* option. It is a floating point
value representing CPU time in percent, so `1.0` is equal to `100%`, `2.5` to
`250%` and so on. If a single process would fully use one single core it would
have `100%` CPU Time usage. If a VM with four cores utilizes all its cores
fully it would theoretically use `400%`. In reality the usage may be even a bit
higher as QEMU can have additional threads for VM peripherals besides the vCPU
core ones.
This setting can be useful if a VM should have multiple vCPUs, as it runs a few
processes in parallel, but the VM as a whole should not be able to run all
vCPUs at 100% at the same time. Using a specific example: lets say we have a VM
which would profit from having 8 vCPUs, but at no time all of those 8 cores
should run at full load - as this would make the server so overloaded that
other VMs and CTs would get to less CPU. So, we set the *cpulimit* limit to
`4.0` (=400%). If all cores do the same heavy work they would all get 50% of a
real host cores CPU time. But, if only 4 would do work they could still get
almost 100% of a real core each.
vCPUs at 100% at the same time.
Using a specific example: lets say we have a VM which would profit from having
8 vCPUs, but at no time all of those 8 cores should run at full load - as this
would make the server so overloaded that other VMs and CTs would get too less
CPU. So, we set the *cpulimit* limit to `4.0` (=400%). If we now fully utilize
all 8 vCPUs, they will receive maximum 50% CPU time of the physical cores. But
with only 4 vCPUs fully utilized, they could still get up to 100% CPU time.
NOTE: VMs can, depending on their configuration, use additional threads, such
as for networking or IO operations but also live migration. Thus a VM can show
up to use more CPU time than just its virtual CPUs could use. To ensure that a
VM never uses more CPU time than virtual CPUs assigned set the *cpulimit*
setting to the same value as the total core count.
VM never uses more CPU time than vCPUs assigned, set the *cpulimit* to
the same value as the total core count.
The second CPU resource limiting setting, *cpuunits* (nowadays often called CPU
shares or CPU weight), controls how much CPU time a VM gets compared to other