On Tue, Aug 6, 2024 at 11:38 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > On Thu, Aug 01, 2024 at 12:09:16AM -0600, Yu Zhao wrote: > > 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 > > I agree with Rik, this seems like a possible optimization, not a > pre-requisite. > > > 1) whoever underutilized their THPs get punished for the overhead; > > Is that true? Yes :) > The downgrade is done in a shrinker. Ideally, should we charge for the CPU usage of the shrinker and who should we charge it to? > With or without > shattering, the compaction effort will be on the allocation side. If compaction is needed at all. > > 2) underutilized THPs are kept intact and can be reused by others. > > If migration of the subpages is possible, then compaction can clear > the block as quickly as shattering can. Compaction needs to scan and find the block first, assuming that block is still movable when it gets there. > The only difference is that > compaction would do the work on-demand And can often fail to produce 2MB blocks *under memory pressure* > whereas shattering would do it unconditionally And always produces a 2MB block > whether a THP has been requested or not... With the former condition being the priority because of *THP=always*.
