No major change from v3 really, mostly resending to see if there is any review reaction. It's rebased but a partial test indicated that the behaviour is similar to the previous baseline Changelog since v3 o Rebase to 4.20-rc3 o Remove a stupid warning from the last patch Changelog since v2 o Drop patch 5 as it was borderline o Decrease timeout when stalling on fragmentation events Changelog since v1 o Rebase to v4.20-rc1 for the THP __GFP_THISNODE patch in particular o Add tracepoint to record fragmentation stall durations o Add vmstat event to record that a fragmentation stall occurred o Stalls now alter watermark boosting o Stalls occur only when the allocation is about to fail It has been noted before that fragmentation avoidance (aka anti-fragmentation) is not perfect. Given sufficient time or an adverse workload, memory 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. 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. 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). The primary benefit of patch 4 is the increase in THP success rates and the fact it reduces fragmentation events to almost negligible levels with the option of eliminating them. Documentation/sysctl/vm.txt | 42 ++++++++ include/linux/mm.h | 2 + include/linux/mmzone.h | 14 ++- include/linux/vm_event_item.h | 1 + include/trace/events/kmem.h | 21 ++++ kernel/sysctl.c | 18 ++++ mm/compaction.c | 2 +- mm/internal.h | 14 ++- mm/page_alloc.c | 238 ++++++++++++++++++++++++++++++++++++++---- mm/vmscan.c | 123 ++++++++++++++++++++-- mm/vmstat.c | 1 + 11 files changed, 436 insertions(+), 40 deletions(-) -- 2.16.4
