Messages in this thread |  | | Subject | Re: [PATCH] Preemption Latency Measurement Tool | From | Robert Love <> | Date | 23 Sep 2001 14:31:17 -0400 |
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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
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