Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to3.6-rc5 on AMD chipsets - bisected

[Linus Torvalds]

On Thu, Sep 27, 2012 at 11:29 AM, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
>
> Don't forget to run the desktop interactivity benchmarks after you're
> done wriggling with this knob... wakeup preemption is important for most
> those.

So I don't think we want to *just* wiggle that knob per se. We
definitely don't want to hurt latency on actual interactive asks. But
it's interesting that it helps psql so much, and that there seems to
be some interaction with the select_idle_sibling().

So I do have a few things I react to when looking at that wakeup granularity..

I wonder about this comment, for example:

         * By using 'se' instead of 'curr' we penalize light tasks, so
         * they get preempted easier. That is, if 'se' < 'curr' then
         * the resulting gran will be larger, therefore penalizing the
         * lighter, if otoh 'se' > 'curr' then the resulting gran will
         * be smaller, again penalizing the lighter task.

why would we want to preempt light tasks easier? It sounds backwards
to me. If they are light, we have *less* reason to preempt them, since
they are more likely to just go to sleep on their own, no?

Another question is whether the fact that this same load interacts
with select_idle_sibling() is perhaps a sign that maybe the preemption
logic is all fine, but it interacts badly with the "pick new cpu"
code. In particular, after having changed rq's, is the vruntime really
comparable? IOW, maybe this is an interaction between "place_entity()"
and then the immediately following (?) call to check wakeup
preemption?

The fact that *either* changing select_idle_sibling() *or* changing
the wakeup preemption granularity seems to have such a huge impact
does seem to tie them together somehow for this particular load. No?

             Linus
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