linux-kernel - Re: Performance changes between 2.6.13 and 2.6.23

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Message-ID: <20080328092856.GC30863@elte.hu>
Date:	Fri, 28 Mar 2008 10:28:56 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Sanders, Rob M." <sanders-rob@....com>
Cc:	Ray Lee <ray-lk@...rabbit.org>, linux-kernel@...r.kernel.org,
	bart.vanassche@...il.com
Subject: Re: Performance changes between 2.6.13 and 2.6.23


* Sanders, Rob M. <sanders-rob@....com> wrote:

> Sorry to disappear for a few days, I just hadn't had time to do 
> anything at home.  Finally had some time to run some test, and at this 
> point I'm thinking that the problem is not with the kernel.  Not quite 
> sure where to look next, but I've done the following tests:
> 
> YDL4.0.91 with 2.6.13 kernel - normal speed
> YDL6.0    with 2.6.23 kenrel - 4-5x slowdown
> YDL4.1    with 2.6.15 kernel - 4-5x slowdown
> YDL4.0.91 with 2.6.15 kernel - *normal speed* - kernel config pulled from YDL4.1 install
> YDL4.0.91 with 2.6.23 kernel - failed boot

if you suspect the scheduler then please try the following suggestions i 
made in another thread:

---------->
could you run this script while such a slowdown is really prominent:

  http://people.redhat.com/mingo/cfs-scheduler/tools/cfs-debug-info.sh

and send me the output it generates? The output is the most useful if
you do this on a kernel that has CONFIG_SCHED_DEBUG=y and
CONFIG_SCHEDSTATS=y enabled.

on the off chance that this issue has been fixed in the soon-to-be
2.6.25 kernel, you might also want to try x86.git/latest, which is based
on the latest Linus tree and has all relevant x86 fixes and improvements
added as well:
  http://people.redhat.com/mingo/x86.git/README

several of the changes can affect performance.

a third (and most comprehensive) way to debug this would be to send me a
scheduler trace of such a slowdown, you can generate a scheduler trace
the following way:

  http://people.redhat.com/mingo/sched-devel.git/readme-tracer.txt

but we can probably give a first estimation based on the cfs-debug-info
output already. Btw., you can combine the scheduler and the x86 git tree
into a temporary unified tree by doing these two commands:

  git-checkout -b tmp x86/latest
  git-merge sched-devel/latest

(run "make oldconfig" to pick up the new config options.)

	Ingo
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