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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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