On Wed, Oct 12, 2011 at 02:57:53PM -0500, Christoph Lameter wrote: > On Wed, 12 Oct 2011, Andrew Morton wrote: > > > > 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. > > The global vm_stat array is keeping the state for the zone. It would be > even more expensive to calculate this at every point where we need such > data. > > > 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. > > We already have a contention avoiding scheme for counter updates in > vmstat.c. The problem here is that vm_stat is frequently read. Updates > from other cpus that fold counter updates in a deferred way into the > global statistics cause cacheline eviction. The updates occur too frequent > in this load. The test I did slowed down the reads by __vm_enough_memory by caching the values and updating them every two seconds (in the OVERCOMMIT_GUESS area). > > > otoh, I think we've been round this loop before and I don't recall why > > nothing happened. > > The update behavior can be tuned using /proc/sys/vm/stat_interval. > Increase the interval to reduce the folding into the global counter (set > maybe to 10?). This will reduce contention. The other approach is to Increasing this interval to 10 (or even 100) had no effect on the vm_stat contention on a 640 cpu test system, so vmstat_update() is not the culprit. > increase the allowed delta per zone if frequent updates occur via the > overflow checks in vmstat.c. See calculate_*_threshold there. I tried changing the threshold in both directions, with slower throughput in both cases. > > Note that the deltas are current reduced for memory pressure situations > (after recent patches by Mel). This will cause a significant increase in > vm_stat cacheline contention compared to earlier kernels. > -- 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>