Messages in this thread | | | Date | Fri, 30 Mar 2007 03:59:02 +0200 | From | Nick Piggin <> | Subject | Re: [rfc][patch] queued spinlocks (i386) |
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On Thu, Mar 29, 2007 at 05:27:24PM -0700, Davide Libenzi wrote: > 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: > > http://www.xmailserver.org/qspins.c
Slightly more than slightly ;)
You want to have a delay _outside_ the critical section as well, for multi-thread tests, otherwise the releasing CPU often just retakes the lock (in the unqueued lock case). As I said, most kernel code should _not_ be dropping and retaking locks.
> 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.
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