linux-kernel - Re: [RFC patch 1/2] sched: dynamically adapt granularity with nr

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Date:	Sat, 11 Sep 2010 20:57:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tony Lindgren <tony@...mide.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [RFC patch 1/2] sched: dynamically adapt granularity with
 nr_running

On Sat, 2010-09-11 at 13:37 -0400, Mathieu Desnoyers wrote:

Its not at all clear what or why you're doing what exactly.

What we used to have is:

  period -- time in which each task gets scheduled once

This period was adaptive in that we had an ideal period
(sysctl_sched_latency), but since keeping to this means that each task
gets latency/nr_running time. This is undesired in that it means busy
systems will over-schedule due to tiny slices. Hence we also had a
minimum slice (sysctl_sched_min_granularity).

This yields:

  period := max(sched_latency, nr_running * sched_min_granularity)

[ where we introduce the intermediate: 
	nr_latency := sched_latency / sched_min_granularity
  in order to avoid the multiplication where possible ]

Now you introduce a separate preemption measure, sched_gran as:

		  sched_std_granularity; nr_running <= 8
  sched_gran := {
		  max(sched_min_granularity, sched_latency / nr_running)

Which doesn't make any sense at all, because it will either be larger or
as large as the current sched_min_granularity.

And you break the above definition of period by replacing nr_latency by
8.

Not at all charmed, this look like random changes without conceptual
integrity.
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