Re: RT patch acceptance

Andi Kleen
2005-05-28T19:56:27Z

On Fri, May 27, 2005 at 03:53:10PM +0200, Ingo Molnar wrote:
>
> * Andi Kleen <ak@muc.de> wrote:
>
> > AFAIK the kernel has quite regressed recently, but that was not true
> > (for reasonable sound) at least for some earlier 2.6 kernels and some
> > of the low latency patchkit 2.4 kernels.
>
> (putting my scheduler maintainer hat on) was this under a stock !PREEMPT
> kernel?

Yes. I did not run the numbers personally, but I was told 2.6.11+
was already considerable worse for latency tests with jack than 2.6.8+
(this was with vendor kernels in SUSE releases); and apparently
2.6.8 was already worse than earlier 2.6.4/5 kernels or the later
and better 2.4s. CONFIG_PREEMPT in all cases did not change the
picture much. Sorry for being light on details; as I did
not run the tests personally.

BTW another reason I am pretty suspicious against the old style preempt
stuff and intrusive latency in general too is that it was broken forever
in x86-64 - I only fixed it after 2.6.11 which you may have noticed. Before
that it it would only preempt when the interrupts were off,not
on (pretty embarassing bug). And nobody complained; The problem was only found
during code review for a completely different project (thanks JanB!)
And x86-64 is quite widely used these days.

So in practice all these latencies cannot be that big a problem.

-Andi