Messages in this thread | | | From | Roger Larsson <> | Subject | Re: Scheduler ( was: Just a second ) ... | Date | Tue, 18 Dec 2001 17:16:50 +0100 |
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This might be of interest on linux-audio-dev too...
On Tuesday den 18 December 2001 07.09, Linus Torvalds wrote: > On Mon, 17 Dec 2001, William Lee Irwin III wrote: > > 5: 46490271 XT-PIC soundblaster > > > > Approximately 4 times more often than the timer interrupt. > > That's not nice... > > Yeah. > > Well, looking at the issue, the problem is probably not just in the sb > driver: the soundblaster driver shares the output buffer code with a > number of other drivers (there's some horrible "dmabuf.c" code in common). > > And yes, the dmabuf code will wake up the writer on every single DMA > complete interrupt. Considering that you seem to have them at least 400 > times a second (and probably more, unless you've literally had sound going > since the machine was booted), I think we know why your setup spends time > in the scheduler. > > > On Mon, Dec 17, 2001 at 08:27:18PM -0800, Linus Torvalds wrote: > > > Which sound driver are you using, just in case this _is_ the reason? > > > > SoundBlaster 16 > > A change of hardware should help verify this. > > A number of sound drivers will use the same logic. > > You may be able to change this more easily some other way, by using a > larger fragment size for example. That's up to the sw that actually feeds > the sound stream, so it might be your decoder that selects a small > fragment size. > > Quite frankly I don't know the sound infrastructure well enough to make > any more intelligent suggestions about other decoders or similar to try, > at this point I just start blathering. > > But yes, I bet you'll also see much less impact of this if you were to > switch to more modern hardware. > > grep grep grep.. Oh, before you do that, how about changing "min_fragment" > in sb_audio.c from 5 to something bigger like 9 or 10? > > That > > audio_devs[devc->dev]->min_fragment = 5; > > literally means that your minimum fragment size seems to be a rather > pathetic 32 bytes (which doesn't mean that your sound will be set to that, > but it _might_ be). That sounds totally ridiculous, but maybe I've > misunderstood the code.
I think it really is 32 samples, yes that is little - but too small? It depends on the used sample frequency...
Paul Davis wrote this on linux-audio-dev 2001-12-05 "in doing lots of testing on JACK, i've noticed that although the trident driver now works (there were some patches from jaroslav and myself), in general i still get xruns with the lowest possible latency setting for that card (1.3msec per interrupt, 2.6msec buffer). with the same settings on my hammerfall, i don't get xruns, even with substantial system load."
> > Jeff, you've worked on the sb code at some point - does it really do > 32-byte sound fragments? Why? That sounds truly insane if I really parsed > that code correctly. That's thousands of separate DMA transfers > and interrupts per second.. >
Lets see: we have >1 GHz CPU and interrupts at >1000 Hz => 1 Mcycle / interrupt - is that insane?
If the hardware can support it? Why not let it? It is really up to the applications/user to decide...
> Raising that min_fragment thing from 5 to 10 would make the minimum DMA > buffer go from 32 bytes to 1kB, which is a _lot_ more reasonable (what, > at 2*2 bytes per sample and 44kHz would mean that a 1kB DMA buffer empties > in less than 1/100th of a second, but at least it should be < 200 irqs/sec > rather than >400). >
Yes, it is probably more reasonable - but if the soundcard can support it? (I have a vision of lots of linux-audio-dev folks pulling out their new soundcard and replacing it with their since long forgotten SB16...)
/RogerL
-- Roger Larsson Skellefteå Sweden - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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