On Tue, Jul 30, 2024 at 6:54 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote: > > The current upstream default policy for THP is always. However, Meta > uses madvise in production as the current THP=always policy vastly > overprovisions THPs in sparsely accessed memory areas, resulting in > excessive memory pressure and premature OOM killing. > Using madvise + relying on khugepaged has certain drawbacks over > THP=always. Using madvise hints mean THPs aren't "transparent" and > require userspace changes. Waiting for khugepaged to scan memory and > collapse pages into THP can be slow and unpredictable in terms of performance > (i.e. you dont know when the collapse will happen), while production > environments require predictable performance. If there is enough memory > available, its better for both performance and predictability to have > a THP from fault time, i.e. THP=always rather than wait for khugepaged > to collapse it, and deal with sparsely populated THPs when the system is > running out of memory. > > This patch-series is an attempt to mitigate the issue of running out of > memory when THP is always enabled. During runtime whenever a THP is being > faulted in or collapsed by khugepaged, the THP is added to a list. > Whenever memory reclaim happens, the kernel runs the deferred_split > shrinker which goes through the list and checks if the THP was underutilized, > i.e. how many of the base 4K pages of the entire THP were zero-filled. > If this number goes above a certain threshold, the shrinker will attempt > to split that THP. Then at remap time, the pages that were zero-filled are > not remapped, hence saving memory. This method avoids the downside of > wasting memory in areas where THP is sparsely filled when THP is always > enabled, while still providing the upside THPs like reduced TLB misses without > having to use madvise. > > Meta production workloads that were CPU bound (>99% CPU utilzation) were > tested with THP shrinker. The results after 2 hours are as follows: > > | THP=madvise | THP=always | THP=always > | | | + shrinker series > | | | + max_ptes_none=409 > ----------------------------------------------------------------------------- > Performance improvement | - | +1.8% | +1.7% > (over THP=madvise) | | | > ----------------------------------------------------------------------------- > Memory usage | 54.6G | 58.8G (+7.7%) | 55.9G (+2.4%) > ----------------------------------------------------------------------------- > max_ptes_none=409 means that any THP that has more than 409 out of 512 > (80%) zero filled filled pages will be split. > > To test out the patches, the below commands without the shrinker will > invoke OOM killer immediately and kill stress, but will not fail with > the shrinker: > > echo 450 > /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none > mkdir /sys/fs/cgroup/test > echo $$ > /sys/fs/cgroup/test/cgroup.procs > echo 20M > /sys/fs/cgroup/test/memory.max > echo 0 > /sys/fs/cgroup/test/memory.swap.max > # allocate twice memory.max for each stress worker and touch 40/512 of > # each THP, i.e. vm-stride 50K. > # With the shrinker, max_ptes_none of 470 and below won't invoke OOM > # killer. > # Without the shrinker, OOM killer is invoked immediately irrespective > # of max_ptes_none value and kill stress. > stress --vm 1 --vm-bytes 40M --vm-stride 50K > > Patches 1-2 add back helper functions that were previously removed > to operate on page lists (needed by patch 3). > Patch 3 is an optimization to free zapped tail pages rather than > waiting for page reclaim or migration. > Patch 4 is a prerequisite for THP shrinker to not remap zero-filled > subpages when splitting THP. > Patches 6 adds support for THP shrinker. > > (This patch-series restarts the work on having a THP shrinker in kernel > originally done in > https://lore.kernel.org/all/cover.1667454613.git.alexlzhu@xxxxxx/. > The THP shrinker in this series is significantly different than the > original one, hence its labelled v1 (although the prerequisite to not > remap clean subpages is the same).) > > Alexander Zhu (1): > mm: add selftests to split_huge_page() to verify unmap/zap of zero > pages > > Usama Arif (3): > Revert "memcg: remove mem_cgroup_uncharge_list()" > Revert "mm: remove free_unref_page_list()" > mm: split underutilized THPs > > Yu Zhao (2): > mm: free zapped tail pages when splitting isolated thp > mm: don't remap unused subpages when splitting isolated thp I would recommend shatter [1] instead of splitting so that 1) whoever underutilized their THPs get punished for the overhead; 2) underutilized THPs are kept intact and can be reused by others. [1] https://lore.kernel.org/20240229183436.4110845-3-yuzhao@xxxxxxxxxx/
