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

Yu Zhao
2024-08-04T23:32:50Z

On Thu, Aug 1, 2024 at 10:27 AM David Hildenbrand <david@redhat.com> 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@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/
> >
> > 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.