From d17b6bd3d53816a562c70ea113c8c7fb644072f7 Mon Sep 17 00:00:00 2001 From: Alexander Zeidler Date: Tue, 16 Jan 2024 14:22:38 +0100 Subject: [PATCH] qm: resource limits: revise section cpulimit * precise statements * increase compactness w/o complexity * improve section-formatting Signed-off-by: Alexander Zeidler --- qm.adoc | 40 +++++++++++++++++++++------------------- 1 file changed, 21 insertions(+), 19 deletions(-) diff --git a/qm.adoc b/qm.adoc index 3224715..12e5921 100644 --- a/qm.adoc +++ b/qm.adoc @@ -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 -- 2.39.2