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

[David Hildenbrand <david@redhat.com>]

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.

> 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.

-- 
Cheers,

David / dhildenb