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

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Date:	Thu, 23 Feb 2012 22:57:23 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Mel Gorman <mel@....ul.ie>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>, hughd@...gle.com
Subject: Re: [PATCH -mm 1/2] mm: fix quadratic behaviour in
 get_unmapped_area_topdown

On Thu, Feb 23, 2012 at 02:56:36PM -0500, Rik van Riel 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.

Would it make sense to retain the restart for the case where we _know_
that the remaining address space can not fit the desired area?

	/* make sure it can fit in the remaining address space */
	if (addr > len) {
		vma = find_vma(mm, addr-len);
		if (!vma || addr <= vma->vm_start)
			/* remember the address as a hint for next time */
			return (mm->free_area_cache = addr-len);
	} else /* like this */
		addr = mm->mmap_base - len;

It would save one pointless find_vma() further down.  I don't feel too
strongly about it, though.  Either way:

> Signed-off-by: Rik van Riel <riel@...hat.com>

Acked-by: Johannes Weiner <hannes@...xchg.org>
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