On Wed, 2015-06-03 at 11:57 -0400, Josef Bacik wrote:
> On 06/03/2015 11:30 AM, Mike Galbraith wrote:
> > On Wed, 2015-06-03 at 16:24 +0200, Peter Zijlstra wrote:
> >> On Wed, 2015-06-03 at 10:12 -0400, Rik van Riel wrote:
> >>
> >>> There is a policy vs mechanism thing here. Ingo and Peter
> >>> are worried about the overhead in the mechanism of finding
> >>> an idle CPU. Your measurements show that the policy of
> >>> finding an idle CPU is the correct one.
> >>
> >> For his workload; I'm sure I can find a workload where it hurts.
> >>
> >> In fact, I'm fairly sure Mike knows one from the top of his head, seeing
> >> how he's the one playing about trying to shrink that idle search :-)
> >
> > Like anything where scheduling latency doesn't heavily dominate. Even
> > if searching were free, bounces aren't, even for the very light.
> >
>
> If scheduling latency doesn't hurt then making the search shouldn't
> matter should it? I get that migrations aren't free, but it seems like
> they can't hurt that much.
Nah, they don't hurt much :)
commit e0a79f529d5ba2507486d498b25da40911d95cf6
Author: Mike Galbraith <bitbuc...@online.de>
Date: Mon Jan 28 12:19:25 2013 +0100
sched: Fix select_idle_sibling() bouncing cow syndrome
If the previous CPU is cache affine and idle, select it.
The current implementation simply traverses the sd_llc domain,
taking the first idle CPU encountered, which walks buddy pairs
hand in hand over the package, inflicting excruciating pain.
1 tbench pair (worst case) in a 10 core + SMT package:
pre 15.22 MB/sec 1 procs
post 252.01 MB/sec 1 procs
> This application is huge, it's our
> webserver, we're doing like 400 requests per second on these things, and
> hands down moving stuff to idle cpus is beating the pants off of staying
> on the same cpu. Is there a specific workload I could build a test for
> that you think this approach would hurt? Thanks,
Search cost hurts fast movers, as does dragging even a small footprint
all over the place, as you can see above.
-Mike
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