On Mon, 2 Jul 2007, Suleiman Souhlal wrote: >> I don't think here you need an atomic instruction, a memory barrier throug >> sfence is good enough in order to make thread migration consistent. > > SFENCE is not needed. Stores are already strongly ordered wrt other stores > on x86 (unless you use write-combining memory or non-temporal stores). The > main advantage of using an atomic operation when unlocking is that it should > make the store visible to other CPUs faster (so they don't spin as long), > although I think you'll have a hard time noticing a difference in a > macrobenchmark. FYI, in my experience the difference between using an atomic operation and a non-atomic operation to force out the release of a lock *is* measurable in a macrobenchmark. I measured a several percentage slowdown in buildworld time when I made the mutex release use a non-atomic operation, even though the cycle count for the release operation went up, which I put down to additional waiting time across several CPUs. Our kernel was quite different then -- much less fine-grained locking, such as Giant over VFS still -- and what is now several-year old SMP hardware. It would be very interesting to re-run micro- and macrobenchmarks to continue to reevaluate some of the decisions we made a few years ago on that generation of MP hardware and with the incomplete locking work now that MP hardware has changed a lot and our locking is much more mature. Robert N M Watson Computer Laboratory University of CambridgeReceived on Tue Jul 03 2007 - 07:33:20 UTC
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