From: Andrew Morton <akpm@linux-foundation.org>
To: Rik van Riel <riel@redhat.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Mel Gorman <mel@csn.ul.ie>, Johannes Weiner <hannes@cmpxchg.org>,
KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
Andrea Arcangeli <aarcange@redhat.com>,
hughd@google.com
Subject: Re: [PATCH -mm 2/2] mm: do not reset mm->free_area_cache on every single munmap
Date: Thu, 23 Feb 2012 13:56:14 -0800 [thread overview]
Message-ID: <20120223135614.7c4e02db.akpm@linux-foundation.org> (raw)
In-Reply-To: <20120223150034.2c757b3a@cuia.bos.redhat.com>
On Thu, 23 Feb 2012 15:00:34 -0500
Rik van Riel <riel@redhat.com> wrote:
> Some programs have a large number of VMAs, and make frequent calls
> to mmap and munmap. Having munmap constantly cause the search
> pointer for get_unmapped_area to get reset can cause a significant
> slowdown for such programs.
>
> Likewise, starting all the way from the top any time we mmap a small
> VMA can greatly increase the amount of time spent in
> arch_get_unmapped_area_topdown.
>
> For programs with many VMAs, a next-fit algorithm would be fastest,
> however that could waste a lot of virtual address space, and potentially
> page table memory.
>
> A compromise is to reset the search pointer for get_unmapped_area
> after we have unmapped 1/8th of the normal memory in a process.
ick!
> For
> a process with 1000 similar sized VMAs, that means the search pointer
> will only be reset once every 125 or so munmaps. The cost is that
> the program may use about 1/8th more virtual space for these VMAs,
> and up to 1/8th more page tables.
>
> We do not count special mappings, since there are programs that
> use a large fraction of their address space mapping device memory,
> etc.
>
> The benefit is that things scale a lot better, and we remove about
> 200 lines of code.
We've been playing whack-a-mole with this search for many years. What
about developing a proper data structure with which to locate a
suitable-sized hole in O(log(N)) time?
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next prev parent reply other threads:[~2012-02-23 21:56 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
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 [this message]
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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