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    SubjectRe: [rfc][patch] queued spinlocks (i386)
    On Thu, 29 Mar 2007, Nick Piggin wrote:

    > On Thu, Mar 29, 2007 at 03:36:52AM +0200, Nick Piggin wrote:
    > > In most cases, no. For the uncontended case they should be about the
    > > same. They have the same spinning behaviour. However there is a little
    > > window where they might be a bit slower I think... actually perhaps I'm
    > > wrong!
    > >
    > > Currently if you have 4 CPUs spinning and the lock is released, all 4
    > > CPU cachelines will be invalidated, then they will be loaded again, and
    > > found to be 0, so they all try to atomic_dec_return the counter, each
    > > one invalidating others' cachelines. 1 gets through.
    > >
    > > With my queued locks, all 4 cachelines are invalidated and loaded, but
    > > only one will be allowed to proceed, and there are 0 atomic operations
    > > or stores of any kind.
    > >
    > > So I take that back: our current spinlocks have a worse thundering herd
    > > behaviour under contention than my queued ones. So I'll definitely
    > > push the patch through.
    > OK, it isn't a big difference, but a user-space test is showing slightly
    > (~2%) improvement in the contended case on a 16 core Opteron.
    > There is a case where the present spinlocks are almost twice as fast on
    > this machine (in terms of aggregate throughput), and that is when a lock
    > is taken right after it is released. This is because the same CPU will
    > often be able to retake the lock without transitioning the cache. This is
    > going to be a rare case for us, and would suggest suboptimal code anyway
    > (ie. the lock should just be kept rather than dropped and retaken).
    > Actually, one situation where it comes up is when we drop and retake a
    > lock that needs_lockbreak. Of course, the queued lock behaviour is
    > desired in that case anyway.
    > However single-thread performance is presently a bit down. OTOH, the
    > assembly generated by gcc looks like it could be improved upon (even by
    > me :P).
    > This is what I've got so far. Should work for i386 and x86_64. Any
    > enhancements or results from other CPUs would be interesting.

    I slightly modified it to use cycles:

    Here (Dual Opteron 252) queued locks (ticklocks) are about 10% slower in
    both cases. This is really a microbench, and assembly matter a lot. I did
    not have time to look at the generated one yet, but optimizing branches
    can help in those cases.

    - Davide

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