linux-kernel - Re: THP: AnonHugePages in /proc/[pid]/smaps is correct or not?

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Message-ID: <515A5329.6020805@cn.fujitsu.com>
Date:	Tue, 02 Apr 2013 11:40:25 +0800
From:	Lin Feng <linfeng@...fujitsu.com>
To:	Zhouping Liu <zliu@...hat.com>
CC:	David Rientjes <rientjes@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Amos Kong <akong@...hat.com>
Subject: Re: THP: AnonHugePages in /proc/[pid]/smaps is correct or not?

Hi Zhouping,

On 04/02/2013 11:09 AM, Zhouping Liu wrote:
> I don't understand clearly the last sentence 'you'll probably only get 100% hugepages only 1/512th of the time.'
> could you please explain more details about 'only 1/512th of the time'?

IIUC, thp size is 2M so it may be comprised of 512 normal page(size 4k).
Since your test code is not 2M aligned(not using  posix_memalign()) so 
the start address of the mapped vma will be random, such as 
2M*i+4k*1, 2M*i+4k*2...2M*i+k4*511, there is 512 possibilities.

The only chance you get thp happens when the first map just starts at 2M*i,
and the consequent maps also benefit from this.

-------- snip -------- 
> 
> so, again, if I understand correctly, thp should tune the naturally aligned maps, such as generated by mmap()/malloc(),
> make such maps 'hugepagesize' aligned if the maps or vma is equal and greater than 'hugepagesize', doesn't it?

We may gain performance improving from this.

thanks,
linfeng
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