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Message-Id: <1265458315.8803.79.camel@marge.simson.net> Date: Sat, 06 Feb 2010 13:11:55 +0100 From: Mike Galbraith <efault@....de> To: malc <av1474@...tv.ru> Cc: Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org Subject: Re: Scheduler oddity 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. -Mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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