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Subject: Re: 2.4.10-ac10-preempt lmbench output.
From: Robert Love <rml@ufl.edu>
To: Andrea Arcangeli <andrea@suse.de>
Cc: safemode <safemode@speakeasy.net>, linux-kernel@vger.kernel.org
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On Tue, 2001-10-09 at 23:06, Andrea Arcangeli wrote:
> [...]
> I think the issue you raise is that dbench gets a 10msec more of cpu
> time and xmms starts running 10msec later than expected (because of the
> scheduler latency peak worst case of 10msec).
> 
> But that doesn't matter. The scheduler isn't perfect anyways. The
> resolution of the scheduler is 10msec too, so you can easily lose 10msec
> anywhere else no matter of whatever scheduler latency of 10msec. [...]

I agree with generally everything you say.

I think, however,  you are making two assumptions:

(a) xmms has a very large leeway in the timing of its execution

(b) the maximum time a process sits in kernel space is 10ms.

While I agree (a) is true, it may not be so in all scenerios. 
Furthermore, the specified leeway does not exist for all timing-critical
tasks.  Not all of these tasks are specialized real-time applications,
either.

Most importantly, however, the maximum latency of the system is not
10ms.  Even _with_ preemption, we have observed greater latencies (due
to long held locks).

This is why I believe the a preemptible kernel benefits more than just
real-time signal processing.

	Robert Love

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