Re: [PATCH 0/6] mm: split underutilized THPs

Yu Zhao
2024-08-04T21:55:35Z

On Thu, Aug 1, 2024 at 9:47 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 01.08.24 08:09, Yu Zhao wrote:
> > On Tue, Jul 30, 2024 at 6:54 AM Usama Arif <usamaarif642@gmail.com> 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@fb.com/.
> >> 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@google.com/
> >
>
> Do you have any plans to upstream the shattering also during "ordinary"
> deferred splitting?

Yes, once we finish verifying it in our production.