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    SubjectRe: large page patch (fwd) (fwd)
    On Mon, 12 Aug 2002, Bill Davidsen wrote:

    > Now tell us how someone who isn't a VM developer can tell if that's bad
    > or good. Is it good because it didn't swap more than it needed to, or
    > bad because there were more things it could have swapped to make more
    > buffer room?

    Good point, just looking at the swap usage doesn't mean
    much because we're interested in the _consequences_ of
    that number and not in the number itself.

    > Serious question, tuning the -aa VM sometimes makes the swap use higher,
    > even as the response to starting small jobs while doing kernel compiles
    > or mkisofs gets better. I don't normally tune -ac kernels much, so I
    > can't comment there.

    The key word here is "response", benchmarks really need
    to be able to measure responsiveness.

    Some benchmarks (eg. irman by Bob Matthews) do this
    already, but we're still focussing too much on throughput.


    In 1990 Margo Selzer wrote an excellent paper on disk IO
    sorting and its effects on throughput and latency. The
    end result was that in order to get decent throughput by
    doing just disk IO sorting you would need queues so deep
    that IO latency would grow to about 30 seconds. ;)

    Of course, if databases or online shops would increase
    their throughput by going to deep queueing and every
    request would get 30 second latencies ... they would
    immediately lose their users (or customers) !!!

    I'm pretty convinced that sysadmins aren't interested
    in throughput, at least not until throughput is so low
    that it starts affecting system response latency.


    regards,

    Rik
    --
    Bravely reimplemented by the knights who say "NIH".

    http://www.surriel.com/ http://distro.conectiva.com/

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