linux-kernel - Re: [PATCH] sched/fair: Do not decay new task load on first enqueue

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Message-ID: <20161018101119.GH16071@codeblueprint.co.uk>
Date:   Tue, 18 Oct 2016 11:11:19 +0100
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Wanpeng Li <kernellwp@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mike Galbraith <umgwanakikbuti@...il.com>,
        Yuyang Du <yuyang.du@...el.com>
Subject: Re: [PATCH] sched/fair: Do not decay new task load on first enqueue

On Tue, 11 Oct, at 11:39:57AM, Matt Fleming wrote:
> On Tue, 11 Oct, at 10:44:25AM, Dietmar Eggemann wrote:
> > 
> > [...]
> > 
> > Yeah, you're right. But I can't see any significant difference. IMHO,
> > it's all in the noise.
> > 
> > (A) Performance counter stats for 'perf bench sched messaging -g 100 -l
> >     1 -t'
> >     # 20 sender and receiver threads per group
> >     # 100 groups == 4000 threads run
> > 
> 
> FWIW, our tests run with 1000 loops, not 1, and we don't use 100
> groups for low cpu machines. We tend to test upto $((nproc * 4)).

It's worth pointing out that using less than ~655 loops allows the
'sender' tasks to write down the pipes (when using -p) without
blocking due to the pipe being full, since the default number of pipe
buffers is 16 (PIPE_DEF_BUFFERS) and each buffer is one page.

This has implications for the "optimal" schedule which in that case
is: schedule all sender tasks first, and run to exit, then schedule
all readers to read from the pipes.

Just something I noticed to watch out for when picking a loops count.
There's probably some similar issue for socket buffers.