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

    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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