On Thu, Aug 1, 2024 at 10:27 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 01.08.24 18:22, Usama Arif wrote: > > > > > > On 01/08/2024 07:09, 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/ > > > > The objective of this series is to reduce memory usage, while trying to keep the performance benefits you get of using THP=always. Punishing any applications performance is the opposite of what I am trying to do here. > > For e.g. if there is only one main application running in production, and its using majority of the THPs, then reducing its performance doesn't make sense. > > > > I'm not sure if there would really be a performance degradation > regarding the THP, after all we zap PTEs either way. > > Shattering will take longer because real migration is involved IIUC. Correct, and that's by design. Also using it in the THP shrinker path isn't a problem. > > Also, just going through the commit, and found the line "The advantage of shattering is that it keeps the original THP intact" a bit confusing. I am guessing the THP is freed? i.e. if a 2M THP has 10 non-zero filled base pages and the rest are zero-filled, then after shattering we will have 10*4K memory and not 2M+10*4K? Is it the case the THP is reused at next fault? > > The idea is (as I understand it) to free the full THP abck to the buddy, > replacing the individual pieces that are kept to freshly allocated > order-0 pages from the buddy. Correct, and this is essential to our problem: we are under memory pressure with THP=always. Under this condition, we need to compare shatter with split + compaction, not with split alone. To summarize, the ideal use cases are: 1. split for THP=always with unlimited memory. 2. shatter for THP=always under memory pressure.
