Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to 3.6-rc5 on AMD chipsets - bisected

Mike Galbraith
2012-09-28T04:13:22Z

On Thu, 2012-09-27 at 12:40 -0700, Linus Torvalds wrote:
> On Thu, Sep 27, 2012 at 11:29 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> 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?

At, that particular 'light' refers to se->load.weight.

> 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?

I think vruntime should be fine. We set take the delta between the
task's vruntime when it went to sleep and it's previous rq min_vruntime
to capture progress made while it slept, and apply the relative offset
in the task's new home so a task can migrate and still have a chance to
preempt on wakeup.

> 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?

The way I read it, Boris had wakeup preemption disabled.

-Mike