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    SubjectRe: [PATCH -mm 2/2] mm: do not reset mm->free_area_cache on every single munmap
    On Thu, 23 Feb 2012 15:00:34 -0500
    Rik van Riel <> 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.


    > 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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      Last update: 2012-02-23 22:59    [W:0.019 / U:16.368 seconds]
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