Re: [PATCH] Preemption Latency Measurement Tool

Robert Love
2001-09-23T18:31:16Z

On Sun, 2001-09-23 at 08:03, Andre Pang wrote:
> I found out why, it's the driver. I'm an idiot for not trying
> this before ... I was previously using ALSA's snd-card-ymfpci
> driver; I switchted to the OSS ymfpci driver that comes with the
> kernel. My latencies used to be 15ms on average, with spikes >
> 30ms; they're now ~3ms with some occasional spikes up to 10ms.

You are the second or third person to report high latencies with ALSA
drivers. I wonder what braindead locking they do?

Good find.

> The graphs and numbers are up at
> http://www.algorithm.com.au/hacking/linux-lowlatency/

Nice results.

> > maybe the problem is in the "overruns" -- I don't know what that means
> > exactly. maybe someone else on the list can shed some light?
> > otheriwse, you can email the author perhaps.
>
> An over-run occurs when the latencytest program, which plays a
> continuous sound, doesn't get re-scheduled quickly enough. This
> results in a sound dropout because it can't re-fill its buffer.
> This is similar to what you would get in XMMS, except that
> latencytest simulates professional audio applications which must
> run with _very_ small buffers in order to get low latencies.
> (Imagine your computer being sync'ed in realtime with lots of
> other music equipment and have it drag behind by 30ms -- it
> doesn't sound good :).

Oh, ok -- its pretty much the number of times the scheduling latency was
greater than the audio buffer time.

> But I guess my problem's solved ... thanks so much to Andrew,
> yourself, MontaVista, Dietel and all the other guys who spend
> their hours benchmarking so this can be improved! If you still
> want me to run benchmarks, let me know. The 15 kernels I've
> compiled since starting testing have gotta be useful for
> something.

You are welcome :)

I should probably take a look at ALSA stuff and see if I can find what
exactly is the culprit.

Thank you for the feedback, now we know.

--
Robert M. Love
rml at ufl.edu
rml at tech9.net