linux-kernel - Re: + mm-memblock-reduce-overhead-in-binary-search.patch added to -mm tree

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Date:	Mon, 8 Oct 2012 12:42:34 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	kamezawa.hiroyu@...fujitsu.com, minchan@...nel.org,
	shangw@...ux.vnet.ibm.com, yinghai@...nel.org, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: + mm-memblock-reduce-overhead-in-binary-search.patch added to
 -mm tree

On Mon, 10 Sep 2012 13:55:15 +0200
Michal Hocko <mhocko@...e.cz> wrote:

> > >OK. Thanks for the clarification. The main question remains, though. Is
> > >this worth for memblock_is_memory?
> > 
> > There are many call sites need to call pfn_valid, how can you guarantee all
> > the addrs are between memblock_start_of_DRAM() and memblock_end_of_DRAM(), 
> > if not can this reduce possible overhead ? 
> 
> That was my question. I hoped for an answer in the patch description. I
> am really not familiar with unicore32 which is the only user now.
> 
> > I add unlikely which means that this will not happen frequently. :-)
> 
> unlikely doesn't help much in this case. You would be doing the test for
> every pfn_valid invocation anyway. So the main question is. Do you want
> to optimize for something that doesn't happen often when it adds a cost
> (not a big one but still) for the more probable cases?
> I would say yes if we clearly see that the exceptional case really pays
> off. Nothing in the changelog convinces me about that.

I don't believe Michal's questions have been resolved yet, so I'll keep
this patch on hold for now.

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