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    SubjectRe: Interesting analysis of linux kernel threading by IBM
    On Fri, Jan 21, 2000 at 06:14:09PM +0100, Davide Libenzi wrote:
    > Hi James,
    >
    > Friday, January 21, 2000 3:02 PM
    > James A Simmons <jsimmons@acsu.buffalo.edu> wrote :
    > > I know the guy form IBM is watching this thread. I see alot of boo and hah
    > > about this patch.
    >
    > sure ? anyway everyone has its own opinions, but remember that You can
    > optimize the fast path and CPU cacheline
    > as You want, the algorithm still remain an O( N ) where N is the number of
    > processes in RQ.

    Now, since we're talking Big-O notation, lets get some facts in with
    the theory. O(log n) or even O(1) is not a benefit if the overhead of
    the implementation is high and n is low. Low-overhead O(n^2) may be
    faster than high-overhead O(log n) for small values of n. Now, since
    n is the number of runnable processes (usually low in all the
    real-world non-benchmark cases people have stepped up with), and the
    scheduler is very sensitive to overhead (being called hundereds or
    even thousands of times a second), its very likely that low-overhead
    O(n) will beat even a pretty good O(log n) in the real world.

    That's why everyone is talking about how the performance is "only x%
    worse in the case of less than y runnable tasks". In other words,
    only a little bit worse for all the real people doing real things, but
    a whole lot better at the specARSE3000 benchmark.

    >
    > > Before we do anything the patch really needs to be
    > > tested and studied under all types of conditions. When we have really
    > > numbers then we can chose what to do with the patch.
    >
    > This is the thing we all want.

    What I want is a scheduler that does _better_ not worse, at real
    loads, then we're on to a winner.

    Sean

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