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SubjectRe: [RFC PATCH v2 00/17] Core scheduling v2
On Fri, Apr 26, 2019 at 10:42:22AM +0200, Ingo Molnar wrote:
> > It should, but it's not perfect. For example, wake_affine_idle does not
> > take sibling activity into account even though select_idle_sibling *may*
> > take it into account. Even select_idle_sibling in its fast path may use
> > an SMT sibling instead of searching.
> >
> > There are also potential side-effects with cpuidle. Some workloads
> > migration around the socket as they are communicating because of how the
> > search for an idle CPU works. With SMT on, there is potentially a longer
> > opportunity for a core to reach a deep c-state and incur a bigger wakeup
> > latency. This is a very weak theory but I've seen cases where latency
> > sensitive workloads with only two communicating tasks are affected by
> > CPUs reaching low c-states due to migrations.
> >
> > > Clearly it doesn't.
> > >
> >
> > It's more that it's best effort to wakeup quickly instead of being perfect
> > by using an expensive search every time.
>
> Yeah, but your numbers suggest that for *most* not heavily interacting
> under-utilized CPU bound workloads we hurt in the 5-10% range compared to
> no-SMT - more in some cases.
>

Indeed, it was higher than expected and we can't even use the excuse that
more resources are available to a single logical CPU as the scheduler is
meant to keep them apart.

> So we avoid a maybe 0.1% scheduler placement overhead but inflict 5-10%
> harm on the workload, and also blow up stddev by randomly co-scheduling
> two tasks on the same physical core? Not a good trade-off.
>
> I really think we should implement a relatively strict physical core
> placement policy in the under-utilized case, and resist any attempts to
> weaken this for special workloads that ping-pong quickly and benefit from
> sharing the same physical core.
>

It's worth a shot at least. Changes should mostly be in the wake_affine
path for most loads of interest.

> I.e. as long as load is kept below ~50% the SMT and !SMT benchmark
> results and stddev numbers should match up. (With a bit of a leewy if the
> workload gets near to 50% or occasionally goes above it.)
>
> There's absolutely no excluse for these numbers at 30-40% load levels I
> think.
>

Agreed. I'll put it on the todo list but there is no way I'll get to it
in the short term due to LSF/MM. Minimally I'll put some thought into
tooling to track how often siblings are used with some reporting on when a
sibling was used when there was an idle core available. That'll at least
quantify the problem and verify the hypothesis.

--
Mel Gorman
SUSE Labs

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