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 thpI 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?
-- Cheers, David / dhildenb
