Re: 2.4.10-ac10-preempt lmbench output.

From: Robert Love (
Date: Tue Oct 09 2001 - 21:37:56 EST

On Tue, 2001-10-09 at 22:30, Andrea Arcangeli wrote:
> As said it's very very unlikely that preemption points can fix xmms
> skips anyways, the worst scheduler latency is always of the order of the
> msecs, to generate skips you need a latency of seconds.
> [...]
> There's nothing magic in the software, if you divide the cpu in 10 parts
> and you give 1/10 of the cpu to xmms, but xmms needs 1/2 of the cpu to
> play your .mp3 then there's nothing you can do to fix it but to tell
> the scheduler to give more cpu to xmms (renicing to -20 gives more cpu

What if the CPU does divide its time into two 1/2 parts, and gives one
each to xmms and dbench. Everything runs fine, since xmms needs 1/2 cpu
to play without skips.

Now dbench (or any task) is in kernel space for too long. The CPU time
xmms needs will of course still be given, but _too late_. Its just not
a cpu resource problem, its a timing problem. xmms needs x units of CPU
every y units of time. Just getting the x whenever is not enough.

With preempt-kernel patch, the long-lasting kernel space activity dbench
is engaged in won't hog the CPU until it completes. When xmms is ready
(time y arrives), the scheduler will yield the CPU.

        Robert Love

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