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

Yu Zhao
2024-08-06T18:06:59Z

On Tue, Aug 6, 2024 at 11:38 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Thu, Aug 01, 2024 at 12:09:16AM -0600, 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
>
> I agree with Rik, this seems like a possible optimization, not a
> pre-requisite.
>
> > 1) whoever underutilized their THPs get punished for the overhead;
>
> Is that true?

Yes :)

> The downgrade is done in a shrinker.

Ideally, should we charge for the CPU usage of the shrinker and who
should we charge it to?

> With or without
> shattering, the compaction effort will be on the allocation side.

If compaction is needed at all.

> > 2) underutilized THPs are kept intact and can be reused by others.
>
> If migration of the subpages is possible, then compaction can clear
> the block as quickly as shattering can.

Compaction needs to scan and find the block first, assuming that block
is still movable when it gets there.

> The only difference is that
> compaction would do the work on-demand

And can often fail to produce 2MB blocks *under memory pressure*

> whereas shattering would do it unconditionally

And always produces a 2MB block

> whether a THP has been requested or not...

With the former condition being the priority because of *THP=always*.