Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for BALANCE_WAKE
From: Josef Bacik
Date: Wed Jun 03 2015 - 11:58:12 EST
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. 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,
Josef
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