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

[Sean McNamara]

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 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
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