linux-kernel - Re: aim7 -30% regression in 2.6.24-rc1

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Message-Id: <1194413459.20251.13.camel@ymzhang>
Date:	Wed, 07 Nov 2007 13:30:59 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Cyrus Massoumi <cyrusm@....net>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl
Subject: Re: aim7 -30% regression in 2.6.24-rc1

On Mon, 2007-11-05 at 10:37 +0100, Cyrus Massoumi wrote:
> Zhang, Yanmin wrote:
> > On Thu, 2007-11-01 at 11:02 +0100, Cyrus Massoumi wrote:
> >> Zhang, Yanmin wrote:
> >>> On Wed, 2007-10-31 at 17:57 +0800, Zhang, Yanmin wrote:
> >>>> On Tue, 2007-10-30 at 16:36 +0800, Zhang, Yanmin wrote:
> >>>>> On Tue, 2007-10-30 at 08:26 +0100, Ingo Molnar wrote:
> >>>>>> * Zhang, Yanmin <yanmin_zhang@...ux.intel.com> wrote:
> >>>>>>
> >>>>>>> sub-bisecting captured patch 
> >>>>>>> 38ad464d410dadceda1563f36bdb0be7fe4c8938(sched: uniform tunings) 
> >>>>>>> caused 20% regression of aim7.
> >>>>>>>
> >>>>>>> The last 10% should be also related to sched parameters, such like 
> >>>>>>> sysctl_sched_min_granularity.
> >>>>>> ah, interesting. Since you have CONFIG_SCHED_DEBUG enabled, could you 
> >>>>>> please try to figure out what the best value for 
> >>>>>> /proc/sys/kernel_sched_latency, /proc/sys/kernel_sched_nr_latency and 
> >>>>>> /proc/sys/kernel_sched_min_granularity is?
> >>>>>>
> >>>>>> there's a tuning constraint for kernel_sched_nr_latency: 
> >>>>>>
> >>>>>> - kernel_sched_nr_latency should always be set to 
> >>>>>>   kernel_sched_latency/kernel_sched_min_granularity. (it's not a free 
> >>>>>>   tunable)
> >>>>>>
> >>>>>> i suspect a good approach would be to double the value of 
> >>>>>> kernel_sched_latency and kernel_sched_nr_latency in each tuning 
> >>>>>> iteration, while keeping kernel_sched_min_granularity unchanged. That 
> >>>>>> will excercise the tuning values of the 2.6.23 kernel as well.
> >>>>> I followed your idea to test 2.6.24-rc1. The improvement is slow.
> >>>>> When sched_nr_latency=2560 and sched_latency_ns=640000000, the performance
> >>>>> is still about 15% less than 2.6.23.
> >>>> I got the aim7 30% regression on my new upgraded stoakley machine. I found
> >>>> this mahcine is slower than the old one. Maybe BIOS has issues, or memeory(Might not
> >>>> be dual-channel?) is slow. So I retested it on the old machine and found on the old
> >>>> stoakley machine, the regression is about 6%, quite similiar to the regression on tigerton
> >>>> machine.
> >>>>
> >>>> By sched_nr_latency=640 and sched_latency_ns=640000000 on the old stoakley machine,
> >>>> the regression becomes about 2%. Other latency has more regression.
> >>>>
> >>>> On my tulsa machine, by sched_nr_latency=640 and sched_latency_ns=640000000,
> >>>> the regression becomes less than 1% (The original regression is about 20%).
> >>> I rerun SPECjbb by ched_nr_latency=640 and sched_latency_ns=640000000. On tigerton,
> >>> the regression is still more than 40%. On stoakley machine, it becomes worse (26%,
> >>> original is 9%). I will do more investigation to make sure SPECjbb regression is
> >>> also casued by the bad default values.
> >>>
> >>> We need a smarter method to calculate the best default values for the key tuning
> >>> parameters.
> >>>
> >>> One interesting is sysbench+mysql(readonly) got the same result like 2.6.22 (no
> >>> regression). Good job!
> >> Do you mean you couldn't reproduce the regression which was reported 
> >> with 2.6.23 (http://lkml.org/lkml/2007/10/30/53) with 2.6.24-rc1?
> > It looks like you missed my emails.
> 
> Yeah :(
> 
> > Firstly, I reproduced (or just find the same myself :) ) the issue with kernel 2.6.22,
> > 2.6.23-rc and 2.6.23.
> > 
> > Ingo wrote a big patch to fix it and the new patch is in 2.6.24-rc1 now.
> 
> That's nice, could you please point me to the commit?
The patch is very big. 
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=b5869ce7f68b233ceb81465a7644be0d9a5f3dbb
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