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SubjectRe: [RFC PATCH v2 00/17] Core scheduling v2
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On 4/26/19 3:43 AM, Mel Gorman wrote:
> 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.
Doesn't select_idle_sibling already try to do that by calling
select_idle_core? For our OLTP workload we infact found the cost of
select_idle_core was actually hurting more than it helped to find a fully
idle core, so a net negative.

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