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SubjectRe: [RFC PATCH v2 00/17] Core scheduling v2

* Mel Gorman <mgorman@techsingularity.net> wrote:

> > > Same -- performance is better until the machine gets saturated and
> > > disabling HT hits scaling limits earlier.
> >
> > Interesting. This strongly suggests sub-optimal SMT-scheduling in the
> > non-saturated HT case, i.e. a scheduler balancing bug.
> >
>
> Yeah, it does but mpstat didn't appear to indicate that SMT siblings are
> being used prematurely so it's a bit of a curiousity.
>
> > As long as loads are clearly below the physical cores count (which they
> > are in the early phases of your table) the scheduler should spread tasks
> > without overlapping two tasks on the same core.
> >
>
> 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.

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.

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.

Thanks,

Ingo

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