Warning: This is a long intro with long changelogs and this is not a trivial area to either analyse or fix. TLDR -- 95% reduction in fragmentation events, patches 1-3 should be relatively ok. Patch 4 and 5 need scrutiny but they are also independent or dropped. It has been noted before that fragmentation avoidance (aka anti-fragmentation) is far from perfect. Given a long enough time or an adverse enough workload, memory still gets fragmented and the long-term success of high-order allocations degrades. This series defines an adverse workload, a definition of external fragmentation events (including serious) ones and a series that reduces the level of those fragmentation events. This series is *not* directly related to the recent __GFP_THISNODE discussion and has no impact on the trivial test cases that were discussed there. This series was also evaluated without the candidate fixes from that discussion. The series does have consequences for high-order and THP allocations though that are important to consider so the same people are cc'd. It's also far from a complete solution but side-issues such as compaction, usability and other factors would require different series. It's also extremely important to note that this is analysed in the context of one adverse workload. While other patterns of fragmentation are possible (and workloads that are mostly slab allocations have a completely different solution space), they would need test cases to be properly considered. The details of the workload and the consequences are described in more detail in the changelogs. However, from patch 1, this is a high-level summary of the adverse workload. The exact details are found in the mmtests implementation. The broad details of the workload are as follows; 1. Create an XFS filesystem (not specified in the configuration but done as part of the testing for this patch) 2. Start 4 fio threads that write a number of 64K files inefficiently. Inefficiently means that files are created on first access and not created in advance (fio parameterr create_on_open=1) and fallocate is not used (fallocate=none). With multiple IO issuers this creates a mix of slab and page cache allocations over time. The total size of the files is 150% physical memory so that the slabs and page cache pages get mixed 3. Warm up a number of fio read-only threads accessing the same files created in step 2. This part runs for the same length of time it took to create the files. It'll fault back in old data and further interleave slab and page cache allocations. As it's now low on memory due to step 2, fragmentation occurs as pageblocks get stolen. 4. While step 3 is still running, start a process that tries to allocate 75% of memory as huge pages with a number of threads. The number of threads is based on a (NR_CPUS_SOCKET - NR_FIO_THREADS)/4 to avoid THP threads contending with fio, any other threads or forcing cross-NUMA scheduling. Note that the test has not been used on a machine with less than 8 cores. The benchmark records whether huge pages were allocated and what the fault latency was in microseconds 5. Measure the number of events potentially causing external fragmentation, the fault latency and the huge page allocation success rate. 6. Cleanup Overall the series reduces external fragmentation causing events by over 95% on 1 and 2 socket machines, which in turn impacts high-order allocation success rates over the long term. There are differences in latencies and high-order allocation success rates. Latencies are a mixed bag as they are vulnerable to exact system state and whether allocations succeeded so they are treated as a secondary metric. Patch 1 uses lower zones if they are populated and have free memory instead of fragmenting a higher zone. It's special cased to handle a Normal->DMA32 fallback with the reasons explained in the changelog. Patch 2+3 boosts watermarks temporarily when an external fragmentation event occurs. kswapd wakes to reclaim a small amount of old memory and then wakes kcompactd on completion to recover the system slightly. This introduces some overhead in the slowpath. The level of boosting can be tuned or disabled depending on the tolerance for fragmentation vs allocation latency. Patch 4 is more heavy handed. In the event of a movable allocation request that can stall, it'll wake kswapd as in patch 3. However, if the expected fragmentation event is serious then the request will stall briefly on pfmemalloc_wait until kswapd completes light reclaim work and retry the allocation without stalling. This can avoid the fragmentation event entirely in some cases. The definition of a serious fragmentation event can be tuned or disabled. Patch 5 is the hardest to prove it's a real benefit. In the event that fragmentation was unavoidable, it'll queue a pageblock for kcompactd to clean. It's a fixed-length queue that is neither guaranteed to have a slot available or successfully clean a pageblock. Patches 4 and 5 can be treated independently or dropped. The bulk of the improvement in fragmentation avoidance is from patches 1-3 (94-97% reduction in fragmentation events for an adverse workload on both a 1-socket and 2-socket machine). Documentation/sysctl/vm.txt | 42 +++++++ include/linux/compaction.h | 4 + include/linux/migrate.h | 7 +- include/linux/mm.h | 2 + include/linux/mmzone.h | 18 ++- include/trace/events/compaction.h | 62 +++++++++++ kernel/sysctl.c | 18 +++ mm/compaction.c | 148 +++++++++++++++++++++++-- mm/internal.h | 14 ++- mm/migrate.c | 6 +- mm/page_alloc.c | 228 ++++++++++++++++++++++++++++++++++---- mm/vmscan.c | 123 ++++++++++++++++++-- 12 files changed, 621 insertions(+), 51 deletions(-) -- 2.16.4
