Messages in this thread | | | Date | Wed, 11 Dec 2013 22:47:19 +0000 | From | Mel Gorman <> | Subject | Re: [patch] mm: page_alloc: exclude unreclaimable allocations from zone fairness policy |
| |
On Wed, Dec 11, 2013 at 01:09:16PM -0500, Johannes Weiner wrote: > Dave Hansen noted a regression in a microbenchmark that loops around > open() and close() on an 8-node NUMA machine and bisected it down to > 81c0a2bb515f ("mm: page_alloc: fair zone allocator policy"). That > change forces the slab allocations of the file descriptor to spread > out to all 8 nodes, causing remote references in the page allocator > and slab. >
The original patch was primarily concerned with the fair aging of LRU pages of zones within a node. This patch uses GFP_MOVABLE_MASK which includes __GFP_RECLAIMABLE meaning any slab created with SLAB_RECLAIM_ACCOUNT is still getting the round-robin treatment. Those pages have a different lifecycle to LRU pages and the shrinkers are only node aware, not zone aware. While I get this patch probably helps this specific benchmark, was the use of GFP_MOVABLE_MASK intentional or did you mean to use __GFP_MOVABLE?
Looking at the original patch again I think I made a major mistake when reviewing it. Considering the effect of the following for NUMA machines
for_each_zone_zonelist_nodemask(zone, z, zonelist, high_zoneidx, nodemask) { .... if (alloc_flags & ALLOC_WMARK_LOW) { if (zone_page_state(zone, NR_ALLOC_BATCH) <= 0) continue; if (zone_reclaim_mode && !zone_local(preferred_zone, zone)) continue; }
Enabling zone_reclaim_mode sucks badly for workloads that are not paritioned to fit within NUMA nodes. Consequently, I expect the common case it that it's disabled by default due to small NUMA distances or manually disabled.
However, the effect of that block is that we allocate NR_ALLOC_BATCH from local zones then fallback to batch allocating remote nodes! I bet the numa_hit stats in /proc/vmstat have sucked recently. The original problem was because the page allocator would try allocating from the highest zone while kswapd reclaimed from it causing LRU-aging problems. The problem is not the same between nodes. How do you feel about dropping the zone_reclaim_mode check above and only round-robin in batches between zones on the local node?
-- Mel Gorman SUSE Labs
| |