On Thu, Aug 1, 2024 at 9:47 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 01.08.24 08:09, 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 > > 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/ > > > > Do you have any plans to upstream the shattering also during "ordinary" > deferred splitting? Yes, once we finish verifying it in our production.
