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

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Date:	Thu, 01 Nov 2007 11:02:59 +0100
From:	Cyrus Massoumi <cyrusm@....net>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
CC:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: aim7 -30% regression in 2.6.24-rc1

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 
would be nice if you could provide some numbers for 2.6.22, 2.6.23 and 
2.6.24-rc1.

> -yanmin

greetings
Cyrus

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