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SubjectRe: Performance changes between 2.6.13 and 2.6.23
On Tue, Mar 25, 2008 at 5:34 AM, Sanders, Rob M. <sanders-rob@zai.com> wrote:
> Hello all,
> I've been lurking on the digest for some time (don't want to receive full lkml traffic at work) and saw the
> posts about Wine performance regressions in 2.6.24. Some of what I saw there, particularly Andi Kleen's
> responses, mirror something that I see on my box at home. I had emailed Andi directly since I only read the
> digest, and I'm posting this here at his suggestion. Please CC: rms@zai.com with any replies, as I'm only
> getting the lkml digest.
> I'm running on a dual 2GHZ G5 Powermac w/2GB ram. I recently upgraded from YDL4.0.91 (2.6.13 kernel) to
> YDL6 (2.6.23 kernel) and noticed that the overall performance of the box seems more sluggish after the upgrade.
> Of more particular concern, the main application that I build has seen a 4-5X slowdown in performance. Under
> YDL4.0.91 I could process roughly 1e8 data points in ~2 seconds, and under YDL6.0 I now process 1e8 data points

So, two processors, and multiple processes passing data back and
forth. Key point seems to be:

> in ~8 seconds. Total CPU loading (from top) is about 5%, under both systems. The application spawns multiple
> processes and uses semaphores and shared memory to move data between the processes. When I use 'vmstat 3' the
> single biggest difference I see between YDL4.0.91 and YDL6 is that the YDL4.0.91 system is the YDL4.0.91 system is
> context switching about 7000 per interval, whereas the YDl6 system is context switching about 1200 times per interval.

Many more context switches per second, so a lot less work is getting
done each time.

> I've been talking somewhat with Owen Stampflee (works for the distro maker, TSS) and have rebuilt the kernel on
> the 2.6 box removing some of the things that I don't need on my box (cell support, etc). There did seem (subjectively)
> to be a slight improvement after that. When I get some more time to play at home I'm going to take the 2.6.13 .config
> I use and build a 2.6.23 kernal using 'make oldconfig' and try to figure if it is a config issues.
> I realize that going from 2.6.13 to 2.6.23 is a *huge* change, and that the problems may not be tied to the kernel
> but to other things, but are there any other suggestions folks have for finding the cause of the performance slowdow

A lot has changed between 2.6.23 and current mainline as well.
Particularly in the scheduler, which I suspect is the issue for your
test. If possible, could you try a 2.6.25-rc-latest kernel, both
before and after an "echo 5 > /proc/sys/kernel/sched_features" and see
if that makes any difference?

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