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    SubjectRe: [2.4.17/18pre] VM and swap - it's really unusable
    On Sun, 6 Jan 2002, vda@port.imtp.ilyichevsk.odessa.ua wrote:

    > Like dd if=/dev/zero of=/tmp/file bs=... count=... ?
    >
    That would do it, but I was trying to give a real-world example from
    image processing, like copying a large image file.

    > > # perform a 2D in-place FFT of total size at least "MemTotal/2" but less
    > > # than "MemTotal"
    >
    > I'm willing to try. What program can I use for FFT?

    I use FFTW from http://www.fftw.org.

    > Can you describe FFT memory access pattern in more detail?
    > I'd like to write a simple testcase with similar 'bad' pattern.


    Imagine a 16384 by 16384 array of double complex values. That's a 4
    GByte image. Scale down to fit your machine, of course :). The first
    pass will do an FFT on every row (column) if your language is C
    (FORTRAN). The "stride" is 16 bytes (one complex value) in the inner
    loop. Each row (column) is 16384*16 = 262144 bytes long, which works out
    to 64 pages if the page size is 4096 bytes.

    Then the second pass will do an FFT on every column (row). The stride is
    16384*16 = 262144 bytes. This is a new page for each 16-byte complex
    value you process :-). That is, all 16384 pages have to be in memory, or
    swapped into memory if you've run out of real memory and the kernel has
    swapped them out.

    Please ... *don't* try to do this on a 512 MB machine and think that an
    efficient VM is gonna make it work :),
    --
    M. Edward Borasky

    znmeb@borasky-research.net
    http://www.borasky-research.net

    What phrase will you *never* hear Miss Piggy use?
    "You can't make a silk purse out of a sow's ear!"

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