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

Roland devzero at web.de
Tue Jul 14 19:16:40 CEST 2020


nice :)

what guest OS do you use and which showed the problem ?

as i think that q35 will not be default in kvm or proxmox anytime soon,
shouldn't we perhaps file a bug report for every distro affected ?

is there someone who likes to work with this (help testing,
writing/tracking bug reports...) ?

could perhaps save some tons of CO2 ....

roland


Am 14.07.20 um 18:05 schrieb 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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>>
>
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