linux-kernel - Re: [PATCH v2] sched: Accelerate "pick_next

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Message-ID: <4F21FC69.6040402@linux.vnet.ibm.com>
Date:	Fri, 27 Jan 2012 09:22:49 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Xiaotian Feng <xtfeng@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sched: Accelerate "pick_next_entity" under special
 condition

On 01/26/2012 06:04 PM, Ingo Molnar wrote:

> 
> * Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 
>> On Tue, 2012-01-17 at 11:04 +0800, Michael Wang wrote:
>>>
>>>> Then what if condition 1 is true now?
>>>
>>> We can see in original code, even condition 1 is true, we
>>> still will use value3 if condition3 is true, like this:
>>>
>>> original:
>>>
>>> condition1      condition3      result
>>> true            true            value3
>>> true            false           value1
>>>
>>> That means if condition3 is true, we don't care whether
>>> condition1 is true or not because we will finally use value3.
>>
>> Right, so from the original 8 possible states we used to evaluate 3*8 =
>> 24 conditionals. The new code will reduce this to 1*4 + 2*2 + 2*3 = 14.
>>
>> Now I guess the question is if it matters for the modal or average
>> state.
>>
>> I've applied the patch since it can't be worse, but I've no idea if it
>> matters or not in practice.
> 
> A before/after kernel/sched.o size comparison on 64-bit 
> defconfig typically gives a pretty good indication whether it's 
> a step forward or not.
> 
> Thanks,
> 
> 	Ingo
> 


Hi, Ingo

Thanks for your reply.

I have try use "ls -l" to see the size of sched.o, but after applied the
patch, it is still 1636.

I have not use this method before, may be I use the wrong command...

But I think the new code should be similar to the old one after compile,
because we still have 3 condition check here.

I suppose the new sched.o will be a little bigger, because one jump
command and a label need to be added.

Regards,
Michael Wang

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