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    SubjectRe: [PATCH RFC 0/7] x86: convert ticketlocks to C and remove duplicate code

    * Jeremy Fitzhardinge <> wrote:

    > 2. With NR_CPUS < 256 the ticket size is 8 bits. The compiler doesn't
    > use the same trick as the hand-coded asm to directly compare the high
    > and low bytes in the word, but does a bit of extra shuffling around.
    > However, the Intel optimisation guide and several x86 experts have
    > opined that its best to avoid the high-byte operations anyway, since
    > they will cause a partial word stall, and the gcc-generated code should
    > be better.
    > Overall the compiler-generated code is very similar to the hand-coded
    > versions, with the partial byte operations being the only significant
    > difference. (Curiously, gcc does generate a high-byte compare for me
    > in trylock, so it can if it wants to.)
    > I've been running with this code in place for several months on 4 core
    > systems without any problems.

    Please do measurements both in terms of disassembly based instruction
    count(s) in the fastpath(s) (via looking at the before/after
    disassembly) and actual cycle, instruction and branch counts (via
    perf measurements).

    > I couldn't measure a consistent performance difference between the two
    > implemenations; there seemed to be +/- ~1% +/-, which is the level of
    > variation I see from simply recompiling the kernel with slightly
    > different code alignment.

    Then you've done the micro-cost measurements the wrong way - we can
    and do detect much finer effects than 1%, see the methods used in
    this commit for example:

    c8b281161dfa: sched: Increase SCHED_LOAD_SCALE resolution

    Please also ensure that the cold-cache behavior is fairly measured
    via hot-cache benchmarks (that is not always guaranteed).



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