linux-kernel - Re: Scheduling tests on IPC methods, fc6, sd0.48, cfs12

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Message-ID: <20070521073016.GB3915@elte.hu>
Date:	Mon, 21 May 2007 09:30:16 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Bill Davidsen <davidsen@....com>
Cc:	Linux Kernel mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Scheduling tests on IPC methods, fc6, sd0.48, cfs12

* Bill Davidsen <davidsen@....com> wrote:

> I have posted the results of my initial testing, measuring IPC rates 
> using various schedulers under no load, limited nice load, and heavy 
> load at nice 0.
> 
> http://www.tmr.com/~davidsen/ctxbench_testing.html

nice! For this to become really representative though i'd like to ask 
for a real workload function to be used after the task gets the 
lock/message. The reason is that there is an inherent balancing conflict 
in this area: should the scheduler 'spread' tasks to other CPUs or not? 
In general, for all workloads that matter, the answer is almost always: 
'yes, it should'.

But in your ctxbench results the work a task performs after doing IPC is 
not reflected (the benchmark goes about to do the next IPC - hence 
penalizing scheduling strategies that move tasks to other CPUs) - hence 
the bonus of a scheduler properly spreading out tasks is not measured 
fairly. A real-life IPC workload is rarely just about messaging around 
(a single task could do that itself) - some real workload function is 
used. You can see this effect yourself: do a "taskset -p 01 $$" before 
running ctxbench and you'll see the numbers improve significantly on all 
of the schedulers.

As a solution i'd suggest to add a workload function with a 100 or 200 
usecs (or larger) cost (as a fixed-length loop or something like that) 
so that the 'spreading' effect/benefit gets measured fairly too.

	Ingo
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