Re: [linux-audio-dev] huge latency peaks with 2.4.10-smp kernel

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Subject: Re: [linux-audio-dev] huge latency peaks with 2.4.10-smp kernel
From: Takashi Iwai (tiwai_AT_suse.de)
Date: Fri Oct 19 2001 - 12:58:19 EEST


At Thu, 18 Oct 2001 14:21:37 -0400,
Paul Davis wrote:
>
> in the meantime, a small but focused group has provided ways to avoid
> the behaviour that Kai reported. linus has indicated that he's happy
> with the goal of low latency, but he doesn't want "hacks" to fix
> it. despite the fact that it was "hacks" that created it (code that
> steals the CPU for too long), one can understand his point. there's no
> point fixing a mistake with another mistake.
>
> the preemptive kernel patch seems to have generated a great deal of
> interest in this area, and mr. love has been a very effective
> spokesperson for the work. its hard to tell right now which is better,
> andrew's lowish patches or the PE patch.

Through Benno's latency tests, I find that basically PE patch gives
quicker responce than LL patch, _usually_ within 0.5 msec.
(Paul, I didn't write "latency", at this time :)

However even PE patched kernel can cause unacceptably high latency.
PE patch checks context switching with help of spinlock/unlock hacks.
Thus it cannot overcome the long spinlock itself, which can be seen in
VM and fs. Consequently PE patch requires also the LL'ish
preemption/scheduling checks inside the long locked loops to reduce
the high peaks.

Another drawback of PE patch is SMP support. But I'm not sure about
this point.

Well, yes, it's difficult to tell which patch is better.. My
understanding is like this:

- Unless you need <1msec (scheduling-)latency, the LL patch would be
  enough.
- When you need a stable SMP system, LL would be the choice, again.
- If on UP and really good responce is necessary, get PE patch.

Other notes:
- LL patch includes sysctl so that you can turn off on-the-fly.
- Besides, at this moment, PE seems being developed more actively.

Takashi


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