Re: [linux-audio-user] alsa latency statistics

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Subject: Re: [linux-audio-user] alsa latency statistics
From: Takashi Iwai (tiwai_AT_suse.de)
Date: Wed Oct 24 2001 - 17:09:03 EEST


At Wed, 24 Oct 2001 03:14:17 -0500 (CDT),
dave willis wrote:
>
> On Wed, 24 Oct 2001, Jörn Nettingsmeier wrote:
>
> > dave willis wrote:
> > >
> > > i just finished testing several different patched and unpatched linux
> > > kernels, and was surprised by the results: all basically the same.
> > >
> > > alsa range = 33.4% to 33.8% 1.336 ms to 1.352 ms diff = 0.016 ms
> > > oss-emu range = 91.6% to 91.9% 3.664 ms to 3.676 ms diff = 0.012 ms
> > >
> >
> > did you generate any load on the system while testing ?
>
> yes. if you look below at the details of my post, i list cpu load as 80%
> which is created by latencytest (it varyies from about 76 to 85 when
> switching consoles and doing a 'ps ux', which of course increases
> latency). i just recompiled it with a load of 0.00, and the alsa stats
> were the same (kernel 2.4.12-ac3-pe), but the oss-emu went down to 2.66
> ms.

Hmm, the heaviest stress for low latency is usually disk i/o,
especially write and copy. CPU stress is usually no problem.
(And disk read is usual ok, too).

The latencytest program itself doesn't apply any stress except for CPU
delay loop. It is designed to run together with other stress program.
A typical example is "do_alsa_tests" script included in the package.

Anyway, thanks for testing! I'd suggest that it would be nicer if you
can show us the plot (or html page the script generated). It's more
intuitive than seeing the digits :)

And regarding the lowlatency test itself on 2.4.12 - I had no enough
good results without LL patch. At least, wait_for_buffer() in
fs/buffer.c holds too long spinlock. And on reiserfs, some additional
conditional_schedule()s seem necessary.
But one thing is sure: even vailla 2.4.12 is much better than 2.4.9
or older.

Takashi


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