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    SubjectRe: [PATCH] dcache: better name hash function

    On Tue, 27 Oct 2009, Stephen Hemminger wrote:
    > Agreed. Here is the reduced version of the program.
    > To run:
    > find /home -printf '%f\n' 2>/dev/null | ./htest -n 100

    The timings are very sensitive to random I$ layout at least on Nehalem.
    The reason seems to be that the inner loop is _so_ tight that just
    depending on exactly where the loop ends up, you can get subtle
    interactions with the loop cache.

    Look here:

    [torvalds@nehalem ~]$ find /home -printf '%f\n' 2>/dev/null | ./htest -n 100
    Algorithm Time Ratio Max StdDev
    full_name_hash 1.141899 1.03 4868 263.37
    djb2 0.980200 1.03 4835 266.05
    string10 0.909175 1.03 4850 262.67
    string10a 0.673915 1.03 4850 262.67
    string10b 0.909374 1.03 4850 262.67
    string_hash17 0.966050 1.03 4805 263.68
    string_hash31 1.008544 1.03 4807 259.37
    fnv32 0.774806 1.03 4817 259.17

    what do you think the difference between 'string10', 'string10a' and
    'string10b' are?

    None. None what-so-ever. The source code is identical, and gcc generates
    identical assembly language. Yet those timings are extremely stable for
    me, and 'string10b' is 25% faster than the identical string10 and
    string10a functions.

    The only difference? 'string10a' starts aligned to just 16 bytes, but that
    in turn happens to mean that the tight inner loop ends up aligned on a
    128-byte boundary. And being cacheline aligned just there seems to matters
    for some subtle micro-architectural reason.

    The reason I noticed this is that I wondered what small modifications to
    'string10' would do for performance, and noticed that even _without_ the
    small modifications, performance fluctuated.

    Lesson? Microbenchmarks like this can be dangerous and misleading. That's
    _especially_ true if the loop ends up being just tight enough that it can
    fit in some trace cache or similar. In real life, the name hash is
    performance-critical, but at the same time almost certainly won't be run
    in a tight enough loop that you'd ever notice things like that.


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