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    SubjectRe: why swap at all?
    Catalin BOIE wrote:
    > Hello!
    >
    >> But swap behaviour kills performance even when memory is more than
    >> adequate. Consider building a DVD image in a 4GB system. The i/o
    >> forces all of the unused programs out, in spite of the fact that an
    >> extra 100MB doesn't make a measurable difference in performance. But
    >> when I click Mozilla paging most of it in from disk make a big
    >> difference in performance to the user.
    >
    >
    > I think that kernel cannot know that you need some data once or more.
    > This is fadvise for.
    > With my wrapper (http://kernel.umbrella.ro) for fadvise you can do this:
    > NOCA_SIZE=128 NOCA_READ=1 NOCA_WRITE=1 NOCA_RA=1 \
    > noca mkisofs -R -o /tmp/1.iso /tmp/data
    >
    > This means:
    > NOCA_SIZE: Call fadvise only after 128KiB was read/wrote.
    > NOCA_RA: call fadvise with POSIX_FADV_SEQUENTIAL
    > NOCA_READ: use fadvise(POSIX_FADV_DONTNEED) for reads (because you don't
    > need anymore the source files)
    > NOCA_WRITE: use fadvise(POSIX_FADV_DONTNEED) for writes (because it's
    > useless to cache the end of the ISO)
    >
    > Do this program resolve your problem?

    It addresses one of the cases which trigger problems, certainly. Thank you.
    >
    >> The problems with small memory are different in kind, when not even
    >> the programs will fit in memory at the same time, or will leave next
    >> to nothing for i/o, swap is required for performance. But on a large
    >> memory system I believe the gain to pain ratio is way too low with the
    >> current VM. The solution at the moment is to turn off swap, which as
    >> you note has other problems (can't move between zones without swap?)
    >> which in theory could really hang a system.


    --
    -bill davidsen (davidsen@tmr.com)
    "The secret to procrastination is to put things off until the
    last possible moment - but no longer" -me
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