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

[Roland]

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