Re: single aio thread is migrated crazily by scheduler

Ming Lei
2019-12-04T13:50:56Z

On Tue, Dec 03, 2019 at 10:45:38AM +0100, Vincent Guittot wrote:
> On Mon, 2 Dec 2019 at 22:22, Phil Auld <pauld@redhat.com> wrote:
> >
> > Hi Vincent,
> >
> > On Mon, Dec 02, 2019 at 02:45:42PM +0100 Vincent Guittot wrote:
> > > On Mon, 2 Dec 2019 at 05:02, Dave Chinner <david@fromorbit.com> wrote:
> >
> > ...
> >
> > > > So, we can fiddle with workqueues, but it doesn't address the
> > > > underlying issue that the scheduler appears to be migrating
> > > > non-bound tasks off a busy CPU too easily....
> > >
> > > The root cause of the problem is that the sched_wakeup_granularity_ns
> > > is in the same range or higher than load balance period. As Peter
> > > explained, This make the kworker waiting for the CPU for several load
> > > period and a transient unbalanced state becomes a stable one that the
> > > scheduler to fix. With default value, the scheduler doesn't try to
> > > migrate any task.
> >
> > There are actually two issues here. With the high wakeup granularity
> > we get the user task actively migrated. This causes the significant
> > performance hit Ming was showing. With the fast wakeup_granularity
> > (or smaller IOs - 512 instead of 4k) we get, instead, the user task
> > migrated at wakeup to a new CPU for every IO completion.
>
> Ok, I haven't noticed that this one was a problem too. Do we have perf
> regression ?

Follows the test result on one server(Dell, R630: Haswell-E):

kernel.sched_wakeup_granularity_ns = 4000000
kernel.sched_min_granularity_ns = 3000000

---------------------------------------
test | IOPS
---------------------------------------
./xfs_complete 512 | 7.8K
---------------------------------------
taskset -c 8 ./xfs_complete 512 | 9.8K
---------------------------------------

Thanks,
Ming