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SubjectRe: Low Latency
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>Robert, Paul: test cases, please.  Can you please tell me what system
>activity causes the most problems? Is there a particular system
>activity or application which will reliably kill scheduling latency? If

There is no specific thing in the experiences that many of us have
collected. However, the closest to a "common" source of problems is
anything involving disk i/o. Not necessarily disk i/o from the
application in question, but anything (e.g. syslog). Also, this is
true of both IDE and SCSI (though SCSI is better). For the most part,
I can be running a SCHED_FIFO/mlockall process as the only user-driven
runnable process on the system, and the combination of xosview, the X
server (matrox g{2,4}00) and bits of disk i/o from various system
activities will cause scheduling "misses". Turning off xosview can
help, but it doesn't get rid of the problems.

Benno will have better clues on this front for you.

However, I imagine that Ingo and/or his previous patches will have
some really good clues on where to look. Don't forget that the
empirical evidence is that his patches fixed more or less every
problem for 2.2-UP. So before you go running off to write a lot of
code, it would be worth checking those out to see what specific
problem areas he fixed. OK, Larry, "fixed" :)

>I can't reproduce it here will you be able to apply a truckload of
>patches (against 2.4.0) so we can get to the bottom of it?

To the extent that I will be around this summer, I will be happy to
try any patches if we can agree on a baseline kernel to start from
(e.g. 2.4.0-testNN). However, I am away a *lot* between July 4th and
September 5th, so it will be unwise to use me as the primary rapid
response target.

I really appreciate your offer to help try to identify the trouble
spots. Thanks.

I know that the computer music group at the Libre Software Meeting in
Bordeaux next week extended an invitation to several linux-kernel
members to talk about this stuff. I'd like to second that invitation,
both formally (we plan to have a 30-60min session discussing
strategies for handling the current low latency situation) and
informally (as in: i'll be around).

--p



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