Hello Andrew, On Wed, Aug 10, 2011 at 11:20:33AM -0700, Andrew Bresticker wrote: > While back-porting Johannes Weiner's patch "mm: memcg-aware global reclaim" > for an internal effort, we noticed a significant performance regression > during page-reclaim heavy workloads due to high contention of the ss->id_lock. > This lock protects idr map, and serializes calls to idr_get_next() in > css_get_next() (which is used during the memcg hierarchy walk). Since > idr_get_next() is just doing a look up, we need only serialize it with > respect to idr_remove()/idr_get_new(). By making the ss->id_lock a > rwlock, contention is greatly reduced and performance improves. > > Tested: cat a 256m file from a ramdisk in a 128m container 50 times > on each core (one file + container per core) in parallel on a NUMA > machine. Result is the time for the test to complete in 1 of the > containers. Both kernels included Johannes' memcg-aware global > reclaim patches. > Before rwlock patch: 1710.778s > After rwlock patch: 152.227s The reason why there is much more hierarchy walking going on is because there was actually a design bug in the hierarchy reclaim. The old code would pick one memcg and scan it at decreasing priority levels until SCAN_CLUSTER_MAX pages were reclaimed. For each memcg scanned with priority level 12, there were SWAP_CLUSTER_MAX pages reclaimed. My last revision would bail the whole hierarchy walk once it reclaimed SWAP_CLUSTER_MAX. Also, at the time, small memcgs were not force-scanned yet. So 128m containers would force the priority level to 10 before scanning anything at all (128M / pagesize >> priority), and then bail after one or two scanned memcgs. This means that for each SWAP_CLUSTER_MAX reclaimed pages there was a nr_of_containers * 2 overhead of just walking the hierarchy to no avail. I changed this and removed the bail condition based on the number of reclaimed pages. Instead, the cycle ends when all reclaimers together made a full round-trip through the hierarchy. The more cgroups, the more likely that there are several tasks going into reclaim concurrently, it should be a reasonable share of work for each one. The number of reclaim invocations, thus the number of hierarchy walks, is back to sane levels again and the id_lock contention should be less of an issue. Your patch still makes sense, but it's probably less urgent. -- 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>