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

Peter Zijlstra
2012-09-28T08:37:46Z

On Thu, 2012-09-27 at 12:40 -0700, Linus Torvalds wrote:
> 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?

No, weight is nice, you nicing a task doesn't make it want to run less.
So preempting them sooner means they disturb the heavier less, which is
I think what you want with nice.

> 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?

No, the vruntime comparison between cpus is dubious, its not complete
nonsense but its not 'correct' either. PJT has patches to improve that
based on his per-entity tracking stuff.