Re: [RFC] Scheduler issue 1, RT tasks ...

Victor Yodaiken
2001-12-27T03:08:32Z

On Mon, Dec 24, 2001 at 10:52:46AM -0800, Davide Libenzi wrote:
> I know what you're saying but my goal now is to fix the scheduler not the
> overall RT latency ( at least not the one that does not depend on the

my bias is to fix the cause of the problem, but go ahead.

> scheduler ). Just take for example your 17us for your 800MHz machine, in
> my dual PIII 733 MHz with an rqlen of 4 the scheduler latency ( with that
> std scheduler ) is about 0.9us ( real one, not lat_ctx ). That means the
> the scheduler responsibility in your 17us is about 5%, and the remaining
> 95% is due "external" kernel paths. With an rqlen of 16 ( std scheduler )

No: we've measured. The time in our system, which does not follow any
Linux kernel paths, is dominated by motherboard bus delays.

> the latency peaks up to ~2.4us going to ~14-15% of scheduler responsibility.
> I've coded this simple app :
>
> http://www.xmailserver.org/linux-patches/lnxsched.html#RtLats
>
> and i use it with the cpuhog ( hi-tech software that is available inside
> the same link ) to load the run queue. I'm going to plot the measured
> latency versus the runqueue length. Thanks to OSDLAB i'll have an 8 way
> machine to make some test on these big SMPs. I'll code even the simple
> app you're proposing but the real problem is how to load the system. The
> cpuhog load is a runqueue load and is "neutral", that means that is the
> same on all the systems. Loading the system with other kind of loads can
> introduce a device-driver/hw dependency on the measure ( much or less run
> time with irq disabled for example ).

Try
ping -f localhost&
ping -f onsamelocalnet &
dd if=/dev/hda1 of=/dev/null &
make clean; make bzImage;

as a simple start

>
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>
> - Davide
>
>
>
> -
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