Re: Help on tuning the Linux kernel for soft real-time requiremen ts

[Eugene Surovegin <ebs@ebshome.net>]

On Mon, Oct 18, 2004 at 05:44:52PM -0500, Zajac Adam-AAZ004 wrote:
> It sounds like you're doing exactly what we've been trying to achieve.
> We're trying to meet soft real-time deadlines that are in order of seconds
> (2sec).

Hmm, 2s is a lot. You shouldn't have any problems with getting 2 
second accuracy.

> The only differences between our systems are:
> - kernel 2.4.20
> - HZ = 100
> - Compact Flash

And O(1) scheduler + SCHED_FIFO instead of round-robin (I don't like 
this scheduling policy :), SCHED_FIFO is true priority based 
scheduling.

BTW, we have systems with CF card, writing to it doesn't seem to 
affect real-time threads either.

> We tried to do a similar thing to the scenario you described where we were
> running all other threads with SCHED_OTHER, and the thread with real-time
> priority with RoundRobin scheduler policy. We keep experiencing the greatest
> slips for such file system intensive operations like gzip and tar.
> Also, for an experiment, we tried to turn disk caching off, which helped
> meeting the deadlines a little but slowed down all file transfers
> unacceptably.
> To eliminate the question on the compact flash performance we tried using
> NFS. Using NFS  yielded similar results (missed deadlines) to performing the
> transfers with the cflash card.
> 
> Did you have to set anything special at your kernel config file (.config)?

Nothing special I can think of.

> Did you need to apply low-latency patch separately or the MV kernel you have
> came pre-patched?

I applied _all_ patches myself, although I used MV kernel as a 
reference in some cases, e.g. where rml PPC preempt patches were 
buggy.

--
Eugene