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

[David Hildenbrand <david@redhat.com>]

On 30.07.24 14:45, Usama Arif 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).)

As shared previously, there is one issue with uffd (even when currently 
not active for a VMA!), where we must not zap present page table entries.

Something that is always possible (assuming no GUP pins of course, 
which) is replacing the zero-filled subpages by shared zeropages.

Is that being done in this patch set already, or are we creating 
pte_none() entries?

-- 
Cheers,

David / dhildenb