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    SubjectRe: MAP_SHARED bizarrely slow
    On Wed, Oct 27, 2004 at 01:06:59AM -0700, Andrew Morton wrote:
    > James Cloos <cloos@jhcloos.com> wrote:
    > >
    > > >>>>> "David" == David Gibson <david@gibson.dropbear.id.au> writes:
    > >
    > > David> http://www.ozlabs.org/people/dgibson/maptest.tar.gz
    > >
    > > David> On a number of machines I've tested - both ppc64 and x86 - the
    > > David> SHARED version is consistently and significantly (50-100%)
    > > David> slower than the PRIVATE version.
    > >
    > > Just gave it a test on my laptop and server. Both are p3. The
    > > laptop is under heavier mem pressure; the server has just under
    > > a gig with most free/cache/buff. Laptop is still running 2.6.7
    > > whereas the server is bk as of 2004-10-24.
    > >
    > > Buth took about 11 seconds for the private and around 30 seconds
    > > for the shared tests.
    > >
    >
    > I get the exact opposite, on a P4:
    >
    > vmm:/home/akpm/maptest> time ./mm-sharemmap
    > ./mm-sharemmap 10.81s user 0.05s system 100% cpu 10.855 total
    > vmm:/home/akpm/maptest> time ./mm-sharemmap
    > ./mm-sharemmap 11.04s user 0.05s system 100% cpu 11.086 total
    > vmm:/home/akpm/maptest> time ./mm-privmmap
    > ./mm-privmmap 26.91s user 0.02s system 100% cpu 26.903 total
    > vmm:/home/akpm/maptest> time ./mm-privmmap
    > ./mm-privmmap 26.89s user 0.02s system 100% cpu 26.894 total
    > vmm:/home/akpm/maptest> uname -a
    > Linux vmm 2.6.10-rc1-mm1 #14 SMP Tue Oct 26 23:23:23 PDT 2004 i686 i686 i386 GNU/Linux

    How very odd. I've now understood what was happening (see other
    post), but I'm not sure what could reverse the situation. Can you
    download the test tarball again - I've put up an updated version which
    pretouches the pages and gives some extra info. Running it both with
    and without pretouch would be interesting (#if 0/1 in matmul.h to
    change).

    > It's all user time so I can think of no reason apart from physical page
    > allocation order causing additional TLB reloads in one case. One is using
    > anonymous pages and the other is using shmem-backed pages, although I can't
    > think why that would make a difference.
    >
    >
    > Let's back out the no-buddy-bitmap patches:
    >
    > vmm:/home/akpm/maptest> time ./mm-sharemmap
    > ./mm-sharemmap 12.01s user 0.06s system 99% cpu 12.087 total
    > vmm:/home/akpm/maptest> time ./mm-sharemmap
    > ./mm-sharemmap 12.56s user 0.05s system 100% cpu 12.607 total
    > vmm:/home/akpm/maptest> time ./mm-privmmap
    > ./mm-privmmap 26.74s user 0.03s system 99% cpu 26.776 total
    > vmm:/home/akpm/maptest> time ./mm-privmmap
    > ./mm-privmmap 26.66s user 0.02s system 100% cpu 26.674 total
    >
    > much the same.
    >
    > Backing out "[PATCH] tweak the buddy allocator for better I/O merging" from
    > June 24 makes no difference.
    >

    --
    David Gibson | For every complex problem there is a
    david AT gibson.dropbear.id.au | solution which is simple, neat and
    | wrong.
    http://www.ozlabs.org/people/dgibson
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