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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
parent 0554c751fd
commit d17b6bd3d5
1 changed files with 21 additions and 19 deletions
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@ -336,31 +336,33 @@ context switches.
Resource Limits Resource Limits
^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^
In addition to the number of virtual cores, you can configure how much resources *cpulimit*
a VM can get in relation to the host CPU time and also in relation to other
VMs. In addition to the number of virtual cores, the total available ``Host CPU
With the *cpulimit* (``Host CPU Time'') option you can limit how much CPU time Time'' for the VM can be set with the *cpulimit* option. It is a floating point
the whole VM can use on the host. It is a floating point value representing CPU value representing CPU time in percent, so `1.0` is equal to `100%`, `2.5` to
time in percent, so `1.0` is equal to `100%`, `2.5` to `250%` and so on. If a `250%` and so on. If a single process would fully use one single core it would
single process would fully use one single core it would have `100%` CPU Time have `100%` CPU Time usage. If a VM with four cores utilizes all its cores
usage. If a VM with four cores utilizes all its cores fully it would fully it would theoretically use `400%`. In reality the usage may be even a bit
theoretically use `400%`. In reality the usage may be even a bit higher as QEMU higher as QEMU can have additional threads for VM peripherals besides the vCPU
can have additional threads for VM peripherals besides the vCPU core ones. core ones.
This setting can be useful if a VM should have multiple vCPUs, as it runs a few 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 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 vCPUs at 100% at the same time.
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 Using a specific example: lets say we have a VM which would profit from having
other VMs and CTs would get to less CPU. So, we set the *cpulimit* limit to 8 vCPUs, but at no time all of those 8 cores should run at full load - as this
`4.0` (=400%). If all cores do the same heavy work they would all get 50% of a would make the server so overloaded that other VMs and CTs would get too less
real host cores CPU time. But, if only 4 would do work they could still get CPU. So, we set the *cpulimit* limit to `4.0` (=400%). If we now fully utilize
almost 100% of a real core each. 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 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 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 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* VM never uses more CPU time than vCPUs assigned, set the *cpulimit* to
setting to the same value as the total core count. the same value as the total core count.
The second CPU resource limiting setting, *cpuunits* (nowadays often called CPU 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 shares or CPU weight), controls how much CPU time a VM gets compared to other