From: Mike Galbraith <mgalbraith@suse.de>
To: Yuyang Du <yuyang.du@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Chris Mason <clm@fb.com>,
Ingo Molnar <mingo@kernel.org>,
Matt Fleming <matt@codeblueprint.co.uk>,
linux-kernel@vger.kernel.org
Subject: Re: sched: tweak select_idle_sibling to look for idle threads
Date: Mon, 09 May 2016 09:44:13 +0200 [thread overview]
Message-ID: <1462779853.3803.128.camel@suse.de> (raw)
In-Reply-To: <20160508202201.GM16093@intel.com>
On Mon, 2016-05-09 at 04:22 +0800, Yuyang Du wrote:
> On Mon, May 09, 2016 at 05:45:40AM +0200, Mike Galbraith wrote:
> > On Mon, 2016-05-09 at 02:57 +0800, Yuyang Du wrote:
> > > On Sun, May 08, 2016 at 10:08:55AM +0200, Mike Galbraith wrote:
> > > > > Maybe give the criteria a bit margin, not just wakees tend to equal llc_size,
> > > > > but the numbers are so wild to easily break the fragile condition, like:
> > > >
> > > > Seems lockless traversal and averages just lets multiple CPUs select
> > > > the same spot. An atomic reservation (feature) when looking for an
> > > > idle spot (also for fork) might fix it up. Run the thing as RT,
> > > > push/pull ensures that it reaches box saturation regardless of the
> > > > number of messaging threads, whereas with fair class, any number > 1
> > > > will certainly stack tasks before the box is saturated.
> > >
> > > Yes, good idea, bringing order to the race to grab idle CPU is absolutely
> > > helpful.
> >
> > Well, good ideas work, as yet this one helps jack diddly spit.
>
> Then a valid question is whether it is this selection screwed up in case
> like this, as it should necessarily always be asked.
That's a given, it's just a question of how to do a bit better cheaply.
> > > Regarding wake_wide(), it seems the M:N is 1:24, not 6:6*24, if so,
> > > the slave will be 0 forever (as last_wakee is never flipped).
> >
> > Yeah, it's irrelevant here, this load is all about instantaneous state.
> > I could use a bit more of that, reserving on the wakeup side won't
> > help this benchmark until everything else cares. One stack, and it's
> > game over. It could help generic utilization and latency some.. but it
> > seems kinda unlikely it'll be worth the cycle expenditure.
>
> Yes and no, it depends on how efficient work-stealing is, compared to
> selection, but remember, at the end of the day, the wakee CPU measures the
> latency, that CPU does not care it is selected or it steals.
In a perfect world, running only Chris' benchmark on an otherwise idle
box, there would never _be_ any work to steal. In the real world, we
smooth utilization, optimistically peek at this/that, and intentionally
throttle idle balancing (etc etc), which adds up to an imperfect world
for this (based on real world load) benchmark.
> En... should we try remove recording last_wakee?
The more the merrier, go for it! :)
-Mike
next prev parent reply other threads:[~2016-05-09 7:44 UTC|newest]
Thread overview: 80+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-05 18:08 [PATCH RFC] select_idle_sibling experiments Chris Mason
2016-04-05 18:43 ` Bastien Bastien Philbert
2016-04-05 19:28 ` Chris Mason
2016-04-05 20:03 ` Matt Fleming
2016-04-05 21:05 ` Bastien Philbert
2016-04-06 0:44 ` Chris Mason
2016-04-06 7:27 ` Mike Galbraith
2016-04-06 13:36 ` Chris Mason
2016-04-09 17:30 ` Chris Mason
2016-04-12 21:45 ` Matt Fleming
2016-04-13 3:40 ` Mike Galbraith
2016-04-13 15:54 ` Chris Mason
2016-04-28 12:00 ` Peter Zijlstra
2016-04-28 13:17 ` Mike Galbraith
2016-05-02 5:35 ` Mike Galbraith
2016-04-07 15:17 ` Chris Mason
2016-04-09 19:05 ` sched: tweak select_idle_sibling to look for idle threads Chris Mason
2016-04-10 10:04 ` Mike Galbraith
2016-04-10 12:35 ` Chris Mason
2016-04-10 12:46 ` Mike Galbraith
2016-04-10 19:55 ` Chris Mason
2016-04-11 4:54 ` Mike Galbraith
2016-04-12 0:30 ` Chris Mason
2016-04-12 4:44 ` Mike Galbraith
2016-04-12 13:27 ` Chris Mason
2016-04-12 18:16 ` Mike Galbraith
2016-04-12 20:07 ` Chris Mason
2016-04-13 3:18 ` Mike Galbraith
2016-04-13 13:44 ` Chris Mason
2016-04-13 14:22 ` Mike Galbraith
2016-04-13 14:36 ` Chris Mason
2016-04-13 15:05 ` Mike Galbraith
2016-04-13 15:34 ` Mike Galbraith
2016-04-30 12:47 ` Peter Zijlstra
2016-05-01 7:12 ` Mike Galbraith
2016-05-01 8:53 ` Peter Zijlstra
2016-05-01 9:20 ` Mike Galbraith
2016-05-07 1:24 ` Yuyang Du
2016-05-08 8:08 ` Mike Galbraith
2016-05-08 18:57 ` Yuyang Du
2016-05-09 3:45 ` Mike Galbraith
2016-05-08 20:22 ` Yuyang Du
2016-05-09 7:44 ` Mike Galbraith [this message]
2016-05-09 1:13 ` Yuyang Du
2016-05-09 9:39 ` Mike Galbraith
2016-05-09 23:26 ` Yuyang Du
2016-05-10 7:49 ` Mike Galbraith
2016-05-10 15:26 ` Mike Galbraith
2016-05-10 19:16 ` Yuyang Du
2016-05-11 4:17 ` Mike Galbraith
2016-05-11 1:23 ` Yuyang Du
2016-05-11 9:56 ` Mike Galbraith
2016-05-18 6:41 ` Mike Galbraith
2016-05-09 3:52 ` Mike Galbraith
2016-05-08 20:31 ` Yuyang Du
2016-05-02 8:46 ` Peter Zijlstra
2016-05-02 14:50 ` Mike Galbraith
2016-05-02 14:58 ` Peter Zijlstra
2016-05-02 15:47 ` Chris Mason
2016-05-03 14:32 ` Peter Zijlstra
2016-05-03 15:11 ` Chris Mason
2016-05-04 10:37 ` Peter Zijlstra
2016-05-04 15:31 ` Peter Zijlstra
2016-05-05 22:03 ` Matt Fleming
2016-05-06 18:54 ` Mike Galbraith
2016-05-09 8:33 ` Peter Zijlstra
2016-05-09 8:56 ` Mike Galbraith
2016-05-04 15:45 ` Peter Zijlstra
2016-05-04 17:46 ` Chris Mason
2016-05-05 9:33 ` Peter Zijlstra
2016-05-05 13:58 ` Chris Mason
2016-05-06 7:12 ` Peter Zijlstra
2016-05-06 17:27 ` Chris Mason
2016-05-06 7:25 ` Peter Zijlstra
2016-05-02 17:30 ` Mike Galbraith
2016-05-02 15:01 ` Peter Zijlstra
2016-05-02 16:04 ` Ingo Molnar
2016-05-03 11:31 ` Peter Zijlstra
2016-05-03 18:22 ` Peter Zijlstra
2016-05-02 15:10 ` Peter Zijlstra
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