On Wed, 12 Oct 2011 11:02:02 -0500 Dimitri Sivanich <sivanich@xxxxxxx> wrote: > Tmpfs I/O throughput testing on UV systems has shown writeback contention > between multiple writer threads (even when each thread writes to a separate > tmpfs mount point). > > A large part of this is caused by cacheline contention reading the vm_stat > array in the __vm_enough_memory check. > > The attached test patch illustrates a possible avenue for improvement in this > area. By locally caching the values read from vm_stat (and refreshing the > values after 2 seconds), I was able to improve tmpfs writeback performance from > ~300 MB/sec to ~700 MB/sec with 120 threads writing data simultaneously to > files on separate tmpfs mount points (tested on 3.1.0-rc9). > > Note that this patch is simply to illustrate the gains that can be made here. > What I'm looking for is some guidance on an acceptable way to accomplish the > task of reducing contention in this area, either by caching these values in a > way similar to the attached patch, or by some other mechanism if this is > unacceptable. Yes, the global vm_stat[] array is a problem - I'm surprised it's hung around for this long. Altering the sysctl_overcommit_memory mode will hide the problem, but that's no good. I think we've discussed switching vm_stat[] to a contention-avoiding counter scheme. Simply using <percpu_counter.h> would be the simplest approach. They'll introduce inaccuracies but hopefully any problems from that will be minor for the global page counters. otoh, I think we've been round this loop before and I don't recall why nothing happened. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>