[PVE-User] linux idle cpu overhead in kvm - old issue, but still there in 2020...

[Atila Vasconcelos]

Wow, I just tried this at my servers (very old Dell PowerEdge 2950);

The results are impressive!

8o


ABV


On 2020-07-13 3:15 a.m., Roland wrote:
> hello,
>
> i have found that there is an old bug still around in linux, which is
> causing quite an amount of unnecessary cpu consumption in kvm/proxmox,
> and thus, wasting precious power.
>
> i run some proxmox installations on older systems and on those, it's
> quite significant difference.
>
> on the slowest system, a single debian 10 VM , kvm process is at 20% cpu
> (VM is 100% idle) when this issue is present.
>
> if i change VMs machine type from i440fx(default) to q35 the problem
> goes away.
>
> the same applies when running "powertop --auto-tune" inside the guest
> (with i440fx type - enable autosuspend for usb-controller + tablet 
> device).
>
> on some L5630 machine, in proxmox summary i see "CPU usage" drop from
> 10% to <1%.
>
> see:
> https://bugzilla.redhat.com/show_bug.cgi?id=478317
> https://bugzilla.redhat.com/show_bug.cgi?id=949547
>
> i guess this information could make a difference for people who run a
> large amount of virtual machines or use older systems/cpu's.
>
> on most recent cpu's, i think the difference is not that big.
>
> anyway, i really wonder how linux bugs have such great survival
> capability....
>
> regards
> roland
>
> more references:
> https://lists.gnu.org/archive/html/qemu-devel/2010-04/msg00149.html
> https://www.redhat.com/archives/vfio-users/2015-November/msg00159.html
>
>
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