On 11/23/18 12:45 PM, Mel Gorman wrote: > An external fragmentation event was previously described as > > When the page allocator fragments memory, it records the event using > the mm_page_alloc_extfrag event. If the fallback_order is smaller > than a pageblock order (order-9 on 64-bit x86) then it's considered > an event that will cause external fragmentation issues in the future. > > The kernel reduces the probability of such events by increasing the > watermark sizes by calling set_recommended_min_free_kbytes early in the > lifetime of the system. This works reasonably well in general but if there > are enough sparsely populated pageblocks then the problem can still occur > as enough memory is free overall and kswapd stays asleep. > > This patch introduces a watermark_boost_factor sysctl that allows a zone > watermark to be temporarily boosted when an external fragmentation causing > events occurs. The boosting will stall allocations that would decrease > free memory below the boosted low watermark and kswapd is woken if the > calling context allows to reclaim an amount of memory relative to the > size of the high watermark and the watermark_boost_factor until the boost > is cleared. When kswapd finishes, it wakes kcompactd at the pageblock > order to clean some of the pageblocks that may have been affected by > the fragmentation event. kswapd avoids any writeback, slab shrinkage and > swap from reclaim context during this operation to avoid excessive system > disruption in the name of fragmentation avoidance. Care is taken so that > kswapd will do normal reclaim work if the system is really low on memory. > > This was evaluated using the same workloads as "mm, page_alloc: Spread > allocations across zones before introducing fragmentation". > > 1-socket Skylake machine > config-global-dhp__workload_thpfioscale XFS (no special madvise) > 4 fio threads, 1 THP allocating thread > -------------------------------------- > > 4.20-rc3 extfrag events < order 9: 804694 > 4.20-rc3+patch: 408912 (49% reduction) > 4.20-rc3+patch1-4: 18421 (98% reduction) > > 4.20.0-rc3 4.20.0-rc3 > lowzone-v5r8 boost-v5r8 > Amean fault-base-1 653.58 ( 0.00%) 652.71 ( 0.13%) > Amean fault-huge-1 0.00 ( 0.00%) 178.93 * -99.00%* > > 4.20.0-rc3 4.20.0-rc3 > lowzone-v5r8 boost-v5r8 > Percentage huge-1 0.00 ( 0.00%) 5.12 ( 100.00%) > > Note that external fragmentation causing events are massively reduced > by this path whether in comparison to the previous kernel or the vanilla > kernel. The fault latency for huge pages appears to be increased but that > is only because THP allocations were successful with the patch applied. > > 1-socket Skylake machine > global-dhp__workload_thpfioscale-madvhugepage-xfs (MADV_HUGEPAGE) > ----------------------------------------------------------------- > > 4.20-rc3 extfrag events < order 9: 291392 > 4.20-rc3+patch: 191187 (34% reduction) > 4.20-rc3+patch1-4: 13464 (95% reduction) > > thpfioscale Fault Latencies > 4.20.0-rc3 4.20.0-rc3 > lowzone-v5r8 boost-v5r8 > Min fault-base-1 912.00 ( 0.00%) 905.00 ( 0.77%) > Min fault-huge-1 127.00 ( 0.00%) 135.00 ( -6.30%) > Amean fault-base-1 1467.55 ( 0.00%) 1481.67 ( -0.96%) > Amean fault-huge-1 1127.11 ( 0.00%) 1063.88 * 5.61%* > > 4.20.0-rc3 4.20.0-rc3 > lowzone-v5r8 boost-v5r8 > Percentage huge-1 77.64 ( 0.00%) 83.46 ( 7.49%) > > As before, massive reduction in external fragmentation events, some jitter > on latencies and an increase in THP allocation success rates. > > 2-socket Haswell machine > config-global-dhp__workload_thpfioscale XFS (no special madvise) > 4 fio threads, 5 THP allocating threads > ---------------------------------------------------------------- > > 4.20-rc3 extfrag events < order 9: 215698 > 4.20-rc3+patch: 200210 (7% reduction) > 4.20-rc3+patch1-4: 14263 (93% reduction) > > 4.20.0-rc3 4.20.0-rc3 > lowzone-v5r8 boost-v5r8 > Amean fault-base-5 1346.45 ( 0.00%) 1306.87 ( 2.94%) > Amean fault-huge-5 3418.60 ( 0.00%) 1348.94 ( 60.54%) > > 4.20.0-rc3 4.20.0-rc3 > lowzone-v5r8 boost-v5r8 > Percentage huge-5 0.78 ( 0.00%) 7.91 ( 910.64%) > > There is a 93% reduction in fragmentation causing events, there > is a big reduction in the huge page fault latency and allocation > success rate is higher. > > 2-socket Haswell machine > global-dhp__workload_thpfioscale-madvhugepage-xfs (MADV_HUGEPAGE) > ----------------------------------------------------------------- > > 4.20-rc3 extfrag events < order 9: 166352 > 4.20-rc3+patch: 147463 (11% reduction) > 4.20-rc3+patch1-4: 11095 (93% reduction) > > thpfioscale Fault Latencies > 4.20.0-rc3 4.20.0-rc3 > lowzone-v5r8 boost-v5r8 > Amean fault-base-5 6217.43 ( 0.00%) 7419.67 * -19.34%* > Amean fault-huge-5 3163.33 ( 0.00%) 3263.80 ( -3.18%) > > 4.20.0-rc3 4.20.0-rc3 > lowzone-v5r8 boost-v5r8 > Percentage huge-5 95.14 ( 0.00%) 87.98 ( -7.53%) > > There is a large reduction in fragmentation events with some jitter around > the latencies and success rates. As before, the high THP allocation > success rate does mean the system is under a lot of pressure. However, > as the fragmentation events are reduced, it would be expected that the > long-term allocation success rate would be higher. > > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
