On Mon, 02 Aug 2004 14:40:37 -0700 Florin Andrei <florin@xxxxxxxxxxxxxxx> wrote: > I've not been able to follow LKML too closely in the last few weeks, > so i'm out of loop with the recent developments. If some of you folks > did watch it, do you care to bring the rest of us up to speed WRT the > latest 2.6 developments that are related to "audio" kernels? The > issues i'm interested in are: > > 1. "Classic" pre-emptability. Why Con Kolivas recommends against > enabling preempt in vanilla 2.6, and (if it was a valid > recommendation) whether that is still an issue > > 2. The Ingo Molnar voluntary preempt patches. Are they ready yet, > yes/no? With or without -mm? > > 3. Staircase scheduler, either as a standalone thing or incorporated > in the latest -mm patch. Better than vanilla for audio, yes/no? > > 4. Combinations of the above. Is the classic preemt compatible with > the rest? Anyone using the "vanilla preempt + Ingo Molnar patches + > Andrew Morton series + staircase" supercombo? Hi, while i cannot give detailed answers to your questions i might chime in with some general observations i made.. There are still unsolved issues with NPTL and jackd. Since 2.6.x has the infrastructure to use NPTL [as opposed to unpatched 2.4.x] you will stumble over these issues. The "fix" to export LD_ASSUME_KERNEL=2.4.19 [which makes libc use linuxthreads and not NPTL] seems to help with these issues though. But as long as the problem isn't fully understood, noone can say if there aren't any other potential issues with this. 2.6.x vanilla, with preempt enabled yields usable performance for hd recording and other work. It is not sufficicent for real low latency work, since although most of the kernel is preemptable, several very important parts are not.. I can use 2*512 frames with jack alright in a vanilla 2.6.x with RT jackd and the LD_ASSUME_KERNEL fix. But i don't get rocksolid-compile-a-kernel-at-the-same-time-and-still-get-no-xruns-perf ormance as i did in 2.4.x+preempt+ll.. I once tried the staircase scheduler on a vanilla 2.6.x with preempt [i think i applied the ck-patch directly to it, but it's been a while]. Since the staircase scheduler is only used for SCHED_OTHER threads, an RT'ed jackd's performance will not differ substantially to a kernel with the default O(1) scheduler. SCHED_FIFO threads are in a completely different scheduling class which always gets cpu before any SCHED_OTHER stuff.. Ingos volountary preemption stuff on the other hand does improve the situation somewhat. I can use jackd and jackd-rack at 2*256 pretty reliably.. 2*128 is ok, too, if you don't care about the occasional xrun. Right now i use 2.6.8-rc2-M5 and it looks good. The threaded IRQ handler stuff by Ingo looks exciting, too. Afaik this makes all threaded irq handlers fully preemptible, but i'm not sure about this. I'm also not not sure whether i see real improvements. The irq handlers all run at priority -51. I have jackd running with prio -75 [so it should be scheduled before the prio=-51 irq threads] and i have my soundcard irq handler set to non threaded.. I will watch this over the next days or will try O2 maybe tonight.. I think that this is the right mooment to grab a vanilla 2.6.8-rc2 and just try one of Ingos new volountary preemption patches. You might be positively surprised.. If not, send a report to Ingo. Lee Revel is also busy testing Ingos patches and reporting all latencies he found. But more testers are needed. -- Palimm Palimm! http://affenbande.org/~tapas/
