On 06/03/2015 12:53 PM, Mike Galbraith wrote:
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 <[email protected]>
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.
Eesh ok, do you happen to remember how you ran tbench so I can add it to
my tests here? In addition to fixing this problem we're also interested
in tracking performance of new kernels so we don't have to do this "what
the hell went wrong in the last 6 releases" dance every year, so I'm
throwing every performance thing we find useful in our test
infrastructure. Thanks,
Josef
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