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

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Message-ID: <20120126100429.GA3853@elte.hu>
Date:	Thu, 26 Jan 2012 11:04:29 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Michael Wang <wangyun@...ux.vnet.ibm.com>,
	Xiaotian Feng <xtfeng@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sched: Accelerate "pick_next_entity" under special
 condition


* 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
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