linux-kernel - Re: Scheduler oddity

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Message-ID: <alpine.LNX.2.00.1002061819330.18643@linmac>
Date:	Sat, 6 Feb 2010 18:20:40 +0300 (MSK)
From:	malc <av1474@...tv.ru>
To:	Mike Galbraith <efault@....de>
cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Scheduler oddity

On Sat, 6 Feb 2010, Mike Galbraith wrote:

> On Fri, 2010-02-05 at 19:33 +0300, malc wrote:
> > Following test exhibits somewhat odd behaviour on, at least, 2.6.32.3 
> > (ppc) and 2.6.29.1 (x86_64), perhaps someone could explain why.
> 
> Expected behavior.
> 
> SCHED_BATCH tasks do not wakeup preempt, preemption is tick driven.  The
> writer therefore has time to fill the pipe/block, so reader can then
> drain the pipe, leading to efficient data transfer.
> 
> SCHED_NORMAL tasks do preempt.  Every write wakes a reader who's
> vruntime (CPU utilization fairness yardstick) lags the writer enough to
> warrant preemption, so one write translates to one preemption followed
> by a read.  The reader can't possibly catch up to the writer (being
> synchronous) in either case, but the scheduler doesn't know that or
> care, it simply tries to equalize the two.  Since the writer's CPU
> utilization stems entirely from tiny writes, that time is what goes
> toward equalizing the reader.  Result is the tiny I/Os the programmer
> asked for, extreme low latency, and utterly horrid throughput.
> 

Thank you.

-- 
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