[linux-audio-dev] 2.4.0test5-pre4-lowlat latencies benchmarked, 2.2.16+Ingo's LL patch = strange behaviour

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Subject: [linux-audio-dev] 2.4.0test5-pre4-lowlat latencies benchmarked, 2.2.16+Ingo's LL patch = strange behaviour
From: Benno Senoner (sbenno_AT_gardena.net)
Date: Wed Jul 26 2000 - 01:41:33 EEST

Hi,

here the tests of Andrew's lowlatency patch on 2.4.0-test5-pre4:

Unfortunately during disk writes it performs still quite bad , about 50msec
during disk writes.

first test with CPU usage=80% , 2nd CPU usage=10%
(cpu usage = latencytest which artificially wastes CPU in an empty loop)

http://www.gardena.net/benno/linux/audio/2.4.0test5-cpu80/3x256.html
http://www.gardena.net/benno/linux/audio/2.4.0test5-cpu10/3x256.html

Although better than standard kernels we are still about 20times worse
than Ingo's 2.2.10/2.2.15 patches, and speaking in other OSes terms
still 5-7 times worse than Windows/MacOs.
So far for the 2.4 kernels.

Additionally,
I tested Redhat's kernel 2.2.16-3 + Ingo's 2.2.15 patch, and we
made rpms and src.rpms available here:
http://www.gardena.net/benno/linux/lowlatency-rpm.html

The very strange thing is that 2.2.16 although the latency behaviour is good
it shows high spikes (around 2msec) at regular intervals, and you see that these
are in green, which means that when our thread is in the cpu wasting loop,
something pre-empts us and chews away about 2msec.

Notice the box is my reference machine which produced excellent behaviour with
2.2.10. I tested 2.2.15+lowlat (original 2.2.15 kernel, not Redhat's one) too
and I do not see the green spikes.
So it must be either something which happened
during the 2.2.15 -> 2.2.16 transition , or the cause are the Redhat patches

http://www.gardena.net/benno/linux/audio/2.2.16cpu80/3x128.html
http://www.gardena.net/benno/linux/audio/2.2.16cpu10/3x128.html

Ingo, Andrea any guess ?

I will retry with a fresh 2.2.16 to see if the problems go away.

BTW: there is now a version of latencytest available which outputs PNG instead
of GIF, because some people reported that their libgd does not support GIF
anymore. So if you have problems with the GIF version grab the PNG version here:
http://www.gardena.net/benno/linux/latencytest-0.42-png.tar.gz

cheers,
Benno.

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