[pulseaudio-discuss] Distribution kernels and glitch-free (Packagers, read this!)

[Mark Greenwood]

On Thursday 12 March 2009 14:38:18 Sean McNamara wrote:
> Hi,
> 
> On Tue, Feb 24, 2009 at 3:36 PM, Lennart Poettering
> <lennart at poettering.net> wrote:
> > On Tue, 24.02.09 12:20, Jan Claeys (lists at janc.be) wrote:
> >
> >>
> >> Op maandag 23-02-2009 om 15:18 uur [tijdzone -0500], schreef Sean
> >> McNamara:
> >> > * Once upon a time (old releases, e.g. 7.04 and below?) there was the
> >> > -lowlatency kernel flavor. This one went all the way and gave us
> >> > CONFIG_HZ=1000 and a fully preemptible kernel (not only voluntary, but
> >> > forced preemption). I'm not sure of the rationale for discontinuing
> >> > this kernel flavor, but it would be silly to say it's only because of
> >> > mirror disk space or something; their disk space consumption has gone
> >> > way up since then.
> >>
> >> Maybe have a look at the -rt kernel?  :)
> >>
> >> (There is a fourth kernel too: -virtual, which is optimized for running
> >> in a virtual machine, where the number of hardware to support is much
> >> lower than in real machines.)
> >
> > I cannot say I believe in the -rt kernel. We don't need *that*
> > reliable latencies for PA. And given that -rt in real live breaks more
> > things than it fixes I don't think it is really worth the effort if
> > all you want to do is run PA.
> >
> > The vanilla kernel is mostly fine for doing multimedia work, even for
> > audio production. There's no need to bother with -rt.
> 
> Indeed! And with the Ubuntu -generic kernel, 0.9.15-test* is terribly
> glitchy with glitch-free.  By rebuilding the same kernel sources using
> CONFIG_HZ=1000 and full preemption, rather than CONFIG_HZ=250 and
> voluntary preemption, I get *zero* dropouts under a normal desktop
> load with the same userspace.
> 
> And the Intel GEM/UXA stuff is fairly CPU intensive at times,
> especially scrolling Firefox. That used to glitch like crazy. A simple
> tuning of kernel parameters erased this problem.
> 
> Now I can run PA 0.9.15-test without touching the default
> configuration at all, and it's very, very good in terms of (lack of)
> glitchiness.

I've noticed that the latest ubuntu update (on jaunty) has disabled glitch-free mode (tsched=0 passed to module-hal-detect). Are you sure that isn't the cause of the lack of glitches? It certainly is for me, wrong though it sounds.

Mark

> 
> I wish I could come up with some quantitative data that demonstrates
> clearly how the same kernel sources and userspace stack have wildly
> different user experience results with PA. It would be nice to make a
> case to the Ubuntu kernel folks to re-introduce the "lowlatency"
> kernel flavor.
> 
> BTW, Lennart, last time I ran Fedora I didn't see a "lowlatency"
> flavor (or similar) either. Is your default kernel fully preemptible,
> or do you have an alternate kernel flavor that is? If not -- it may be
> worth considering. At least on my system, full preemption (and
> possibly to a lesser degree, CONFIG_HZ=1000) does just the right
> things to get latency down to make glitch-free happy.
> 
> Thanks,
> 
> Sean
> 
> >
> > Let's keep things in perspective. PA is not a super-ultra-low-latency
> > sound server. It's a desktop sound server ... that is all. All I am
> > asking for is for latencies not as bad as 210 ms!
> >
> > Lennart
> >
> > --
> > Lennart Poettering                        Red Hat, Inc.
> > lennart [at] poettering [dot] net         ICQ# 11060553
> > http://0pointer.net/lennart/           GnuPG 0x1A015CC4
> > _______________________________________________
> > pulseaudio-discuss mailing list
> > pulseaudio-discuss at mail.0pointer.de
> > https://tango.0pointer.de/mailman/listinfo/pulseaudio-discuss
> >
> _______________________________________________
> pulseaudio-discuss mailing list
> pulseaudio-discuss at mail.0pointer.de
> https://tango.0pointer.de/mailman/listinfo/pulseaudio-discuss
>