On Thu, 27 Jul 2017, Vlastimil Babka wrote: > As we discussed at last LSF/MM [1], the goal here is to shift more compaction > work to kcompactd, which currently just makes a single high-order page > available and then goes to sleep. The last patch, evolved from the initial RFC > [2] does this by recording for each order > 0 how many allocations would have > potentially be able to skip direct compaction, if the memory wasn't fragmented. > Kcompactd then tries to compact as long as it takes to make that many > allocations satisfiable. This approach avoids any hooks in allocator fast > paths. There are more details to this, see the last patch. > I think I would have liked to have seen "less proactive" :) Kcompactd currently has the problem that it is MIGRATE_SYNC_LIGHT so it continues until it can defragment memory. On a host with 128GB of memory and 100GB of it sitting in a hugetlb pool, we constantly get kcompactd wakeups for order-2 memory allocation. The stats are pretty bad: compact_migrate_scanned 2931254031294 compact_free_scanned 102707804816705 compact_isolated 1309145254 0.0012% of memory scanned is ever actually isolated. We constantly see very high cpu for compaction_alloc() because kcompactd is almost always running in the background and iterating most memory completely needlessly (define needless as 0.0012% of memory scanned being isolated). vm.extfrag_threshold isn't a solution to the problem because it sees memory as being free in the 28GB of memory remaining and isolates/migrates even if order-2 memory will not become available, so it would need to be set at >850 for it to prevent compaction. If memory is freed from the hugetlb pool we would need to adjust the threshold at runtime. (Why is kcompactd setting ignore_skip_hint, again?) I think we need to look at making kcompactd do less work on each wakeup, perhaps by not forcing full scans of memory with MIGRATE_SYNC_LIGHT and defer compaction for longer if most scanning is completely pointless. -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
