On Fri, Jun 19, 2015 at 11:50:02AM -0400, Waiman Long wrote: > The current cmpxchg() loop in setting the _QW_WAITING flag for writers > in queue_write_lock_slowpath() will contend with incoming readers > causing possibly extra cmpxchg() operations that are wasteful. This > patch changes the code to do a byte cmpxchg() to eliminate contention > with new readers. > > A multithreaded microbenchmark running 5M read_lock/write_lock loop > on a 8-socket 80-core Westmere-EX machine running 4.0 based kernel > with the qspinlock patch have the following execution times (in ms) > with and without the patch: > > With R:W ratio = 5:1 > > Threads w/o patch with patch % change > ------- --------- ---------- -------- > 2 990 895 -9.6% > 3 2136 1912 -10.5% > 4 3166 2830 -10.6% > 5 3953 3629 -8.2% > 6 4628 4405 -4.8% > 7 5344 5197 -2.8% > 8 6065 6004 -1.0% > 9 6826 6811 -0.2% > 10 7599 7599 0.0% > 15 9757 9766 +0.1% > 20 13767 13817 +0.4% > > With small number of contending threads, this patch can improve > locking performance by up to 10%. With more contending threads, > however, the gain diminishes. > > With the extended qrwlock structure defined in asm-generic/qrwlock, > the queue_write_unlock() function is also simplified to a > smp_store_release() call. > > Signed-off-by: Waiman Long <Waiman.Long@xxxxxx> This one does not in fact apply, seeing how I applied a previous version. Please send an incremental patch if you still want to change things to this form. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
