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    SubjectRe: [PATCH] [15/58] i386: Rewrite sched_clock (cmpxchg8b)
    * Nick Piggin ( wrote:
    > Mathieu Desnoyers wrote:
    > >I tried it with and without the LOCK prefix on my Pentium 4.
    > >
    > >Locked cmpxchg8b : 90 cycles
    > >Non locked cmpxchg8b: 30 cycles
    > >sti: 166 cycles
    > >cli: 159 cycles
    > >
    > >So, hrm, even if we use the locked version, it is still much faster than
    > >the sti/cli. I am thoughtful about the comment in asm-i386/system.h:
    > Curious: what does it look like if the memory is not in cache? I
    > found that cmpxchg is relatively slower than other rmw instructions
    > in that case.

    Actually, I have just seen that cmpxchg64 and cmpxchg64_local are
    doing exactly this and they are already implemented in asm-i386/system.h.

    A quick test: I am doing clflush in a loop (substracting its time from the
    following loops) to have a memory hit when I do cmpxchg. This is the
    result of just the cmpxchg8b:

    non locked cmpxchg8b: 583.37 cycles
    locked cmpxchg8b: 650.48 cycles
    rmw in 3 operations: 581.43 cycles

    So the locked cmpxchg is 67 cycles slower than the non locked cmpxchg,
    which fits with my 30 vs 90 cycles. rmw is a tiny bit faster than
    cmpxchg8b (2 cycles), but nothing to call home about.


    Mathieu Desnoyers
    Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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