From: Johannes Weiner <hannes@cmpxchg.org>
To: Rik van Riel <riel@redhat.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
akpm@linux-foundation.org, Mel Gorman <mel@csn.ul.ie>,
KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
Andrea Arcangeli <aarcange@redhat.com>,
hughd@google.com
Subject: Re: [PATCH -mm 1/2] mm: fix quadratic behaviour in get_unmapped_area_topdown
Date: Thu, 23 Feb 2012 22:57:23 +0100 [thread overview]
Message-ID: <20120223215723.GB1701@cmpxchg.org> (raw)
In-Reply-To: <20120223145636.616bef1c@cuia.bos.redhat.com>
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@redhat.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
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next prev parent reply other threads:[~2012-02-23 21:57 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-02-23 19:54 [PATCH -mm 0/2] speed up arch_get_unmapped_area Rik van Riel
2012-02-23 19:56 ` [PATCH -mm 1/2] mm: fix quadratic behaviour in get_unmapped_area_topdown Rik van Riel
2012-02-23 21:50 ` Andrew Morton
2012-02-23 21:57 ` Johannes Weiner [this message]
2012-02-23 20:00 ` [PATCH -mm 2/2] mm: do not reset mm->free_area_cache on every single munmap Rik van Riel
2012-02-23 21:56 ` Andrew Morton
2012-02-27 16:12 ` Rik van Riel
2012-03-20 18:32 ` Rik van Riel
2012-03-20 19:00 ` Andrea Arcangeli
2012-03-20 19:06 ` Rik van Riel
2012-02-23 21:57 ` Andi Kleen
2012-02-27 16:13 ` Rik van Riel
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