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    SubjectRe: [patch] Latency Tracer, voluntary-preempt-2.6.8-rc4-O6
    Andi Kleen wrote:
    >>The final algorithm pre-calculates markers on the compressed symbols so
    >>that the search time is almost divided by the number of markers.
    >
    >
    > You could do that at compile time (in scripts/kallsyms.c)

    You're right! I'll see if I can come up with a patch.

    If this is done in scripts/kallsyms.c, there is less code needed in the
    actual kernel, and there is no time penalty on the first lookup. This is
    a true win-win scenario :)

    Thanks a lot for your suggestion.

    >>There are still a few issues with this approach. The biggest issue is
    >>that this is clearly a speed/space trade-off, and maybe we don't want to
    >>waste the space on a code path that is not supposed to be "hot". If this
    >>is the case, I can make a smaller patch, that fixes just the name
    >>"decompression" strcpy's.
    >
    >
    > I'm surprised that using 8 markers helps anything. There should
    > be many many more 0 stems than that in a not so big kernel.
    > Did you actually measure the hit rate of the cache? I bet it is pretty low.

    Well, I only left in the resulting source code the pieces that I
    actually measured to be a significant improvment.

    The thing is, this is not exactly a cache.

    The problem that the markers address is that the decompression must be
    done sequentially.

    If I have 20000 symbols and already figured out from the binary search
    that I need symbol 11201 (for instance), I would have to go through
    11201 symbols to get to the symbol I need.

    The algorithm I wrote keeps markers in the middle of the stream, so that
    when I try to fetch symbol 11201, I already know where symbol 10000 is
    because of the marker and only have to go through 1201 symbols.

    With N markers you get N times the speed on the name search. (actually
    this is not exactly right for high values of N, because you can only use
    a marker that is a 0 stem, but for N<~32 this is a good approximation)

    --
    Paulo Marques - www.grupopie.com
    "In a world without walls and fences who needs windows and gates?"
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