linux-kernel - Re: [PATCH 2/2] mm/percpu.c: don't bother to re-walk the pcpu_slot list if nobody free space since we last drop pcpu

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Date:	Sat, 29 Mar 2014 09:14:34 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Jianyu Zhan <nasa4836@...il.com>
Cc:	linux-mm@...ck.org, akpm@...ux-foundation.org,
	cl@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm/percpu.c: don't bother to re-walk the pcpu_slot
 list if nobody free space since we last drop pcpu_lock

On Fri, Mar 28, 2014 at 08:55:57PM +0800, Jianyu Zhan wrote:
> Quoted tj: 
> >Hmmm... I'm not sure whether the added complexity is worthwhile.  It's
> >a fairly cold path.  Can you show how helpful this optimization is?
> 
> The patch is quite less intrusive in the normal path
> and if we fall on the cold path, it means after satifying this allocation 
> the chunk may be moved to lower slot, and the follow-up allocation 
> of same or larger size(though rare) is likely to fail to cold path again. So
> this patch could be based on to do some heuristic later.

The above really doesn't show how helpful it is.  This adds complexity
to optimize what seemingly is a quite cold path, which often is a
pretty bad idea as they tend to trade off readability and long term
maintainability for almost non-existing actual gain.  If you think
this is a worthwhile optimization, please give justifications - use
scenarios, performance numbers and so on.

Thanks.

-- 
tejun
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