Re: [PATCH v7 00/13] fold per-CPU vmstats remotely

Michal Hocko
2023-03-23T07:51:24Z

On Wed 22-03-23 11:20:55, Marcelo Tosatti wrote:
> On Wed, Mar 22, 2023 at 02:35:20PM +0100, Michal Hocko wrote:
[...]
> > > "Performance details for the kworker interruption:
> > >
> > > oslat 1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000)
> > > oslat 1094.456971: workqueue_queue_work: ... function=vmstat_update ...
> > > oslat 1094.456974: sched_switch: prev_comm=oslat ... ==> next_comm=kworker/5:1 ...
> > > kworker 1094.456978: sched_switch: prev_comm=kworker/5:1 ==> next_comm=oslat ...
> > >
> > > The example above shows an additional 7us for the
> > >
> > > oslat -> kworker -> oslat
> > >
> > > switches. In the case of a virtualized CPU, and the vmstat_update
> > > interruption in the host (of a qemu-kvm vcpu), the latency penalty
> > > observed in the guest is higher than 50us, violating the acceptable
> > > latency threshold for certain applications."
> >
> > Yes, I have seen that but it doesn't really give a wider context to
> > understand why those numbers matter.
>
> OK.
>
> "In the case of RAN, a MAC scheduler with TTI=1ms, this causes >100us
> interruption observed in a guest (which is above the safety
> threshold for this application)."
>
> Is that OK?

This might be a sufficient information for somebody familiar with the
matter (not me). So no, not enough. We need to hear a more complete
story.

--
Michal Hocko
SUSE Labs