linux-kernel - Re: [PATCH -mm 1/2] mm: fix quadratic behaviour in get_unmapped_area

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Date:	Thu, 23 Feb 2012 13:50:07 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Mel Gorman <mel@....ul.ie>,
	Johannes Weiner <hannes@...xchg.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>, hughd@...gle.com,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Subject: Re: [PATCH -mm 1/2] mm: fix quadratic behaviour in
 get_unmapped_area_topdown

On Thu, 23 Feb 2012 14:56:36 -0500
Rik van Riel <riel@...hat.com> wrote:

> When we look for a VMA smaller than the cached_hole_size, we set the
> starting search address to mm->mmap_base, to try and find our hole.
> 
> However, even in the case where we fall through and found nothing at
> the mm->free_area_cache, we still reset the search address to mm->mmap_base.
> This bug results in quadratic behaviour, with observed mmap times of 0.4
> seconds for processes that have very fragmented memory.
> 
> If there is no hole small enough for us to fit the VMA, and we have
> no good spot for us right at mm->free_area_cache, we are much better
> off continuing the search down from mm->free_area_cache, instead of
> all the way from the top.

This has been at least partially addressed in recent patches from Xiao
Guangrong.  Please review his five-patch series starting with "[PATCH
1/5] hugetlbfs: fix hugetlb_get_unmapped_area".

I've already merged those patches and we need to work out what way to
go.

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