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    SubjectRe: [PATCH 1/5] Add a global synchronization point for pvclock
    On 04/19/2010 01:56 PM, Peter Zijlstra wrote:
    >
    >
    >>> Right, do bear in mind that the x86 implementation of atomic64_read() is
    >>> terrifyingly expensive, it is better to not do that read and simply use
    >>> the result of the cmpxchg.
    >>>
    >>>
    >>>
    >> atomic64_read() _is_ cmpxchg64b. Are you thinking of some clever
    >> implementation for smp i386?
    >>
    >
    > No, what I was suggesting was to rewrite that loop no to need the
    > initial read but use the cmpxchg result of the previous iteration.
    >
    > Something like:
    >
    > u64 last = 0;
    >
    > /* more stuff */
    >
    > do {
    > if (ret< last)
    > return last;
    > last = cmpxchg64(&last_value, last, ret);
    > } while (last != ret);
    >
    > That only has a single cmpxchg8 in there per loop instead of two
    > (avoiding the atomic64_read() one).
    >

    Still have two cmpxchgs in the common case. The first iteration will
    fail, fetching last_value, the second will work.

    It will be better when we have contention, though, so it's worthwhile.

    --
    error compiling committee.c: too many arguments to function



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