Re: [RFC PATCH 0/4] Enable >0 order folio memory compaction

[Huang, Ying]

Something wrong with my mail box.  Sorry, if you received duplicated
mail.

Zi Yan <ziy@xxxxxxxxxx> writes:

> On 9 Oct 2023, at 3:12, Huang, Ying wrote:
>
>> Hi, Zi,
>>
>> Thanks for your patch!
>>
>> Zi Yan <zi.yan@xxxxxxxx> writes:
>>
>>> From: Zi Yan <ziy@xxxxxxxxxx>
>>>
>>> Hi all,
>>>
>>> This patchset enables >0 order folio memory compaction, which is one of
>>> the prerequisitions for large folio support[1]. It is on top of
>>> mm-everything-2023-09-11-22-56.
>>>
>>> Overview
>>> ===
>>>
>>> To support >0 order folio compaction, the patchset changes how free pages used
>>> for migration are kept during compaction.
>>
>> migrate_pages() can split the large folio for allocation failure.  So
>> the minimal implementation could be
>>
>> - allow to migrate large folios in compaction
>> - return -ENOMEM for order > 0 in compaction_alloc()
>>
>> The performance may be not desirable.  But that may be a baseline for
>> further optimization.
>
> I would imagine it might cause a regression since compaction might gradually
> split high order folios in the system.

I may not call it a pure regression, since large folio can be migrated
during compaction with that, but it's possible that this hurts
performance.

Anyway, this can be a not-so-good minimal baseline.

> But I can move Patch 4 first to make this the baseline and see how
> system performance changes.

Thanks!

>>
>> And, if we can measure the performance for each step of optimization,
>> that will be even better.
>
> Do you have any benchmark in mind for the performance tests? vm-scalability?

I remember Mel Gorman has done some tests for defragmentation before.
But that's for order-0 pages.

>>> Free pages used to be split into
>>> order-0 pages that are post allocation processed (i.e., PageBuddy flag cleared,
>>> page order stored in page->private is zeroed, and page reference is set to 1).
>>> Now all free pages are kept in a MAX_ORDER+1 array of page lists based
>>> on their order without post allocation process. When migrate_pages() asks for
>>> a new page, one of the free pages, based on the requested page order, is
>>> then processed and given out.
>>>
>>>
>>> Optimizations
>>> ===
>>>
>>> 1. Free page split is added to increase migration success rate in case
>>> a source page does not have a matched free page in the free page lists.
>>> Free page merge is possible but not implemented, since existing
>>> PFN-based buddy page merge algorithm requires the identification of
>>> buddy pages, but free pages kept for memory compaction cannot have
>>> PageBuddy set to avoid confusing other PFN scanners.
>>>
>>> 2. Sort source pages in ascending order before migration is added to
>>
>> Trivial.
>>
>> s/ascending/descending/
>>
>>> reduce free page split. Otherwise, high order free pages might be
>>> prematurely split, causing undesired high order folio migration failures.
>>>
>>>
>>> TODOs
>>> ===
>>>
>>> 1. Refactor free page post allocation and free page preparation code so
>>> that compaction_alloc() and compaction_free() can call functions instead
>>> of hard coding.
>>>
>>> 2. One possible optimization is to allow migrate_pages() to continue
>>> even if get_new_folio() returns a NULL. In general, that means there is
>>> not enough memory. But in >0 order folio compaction case, that means
>>> there is no suitable free page at source page order. It might be better
>>> to skip that page and finish the rest of migration to achieve a better
>>> compaction result.
>>
>> We can split the source folio if get_new_folio() returns NULL.  So, do
>> we really need this?
>
> It depends. The situation it can benefit is that when the system is going
> to allocate a high order free page and trigger a compaction, it is possible to
> get the high order free page by migrating a bunch of base pages instead of
> splitting a existing high order folio.
>
>>
>> In general, we may reconsider all further optimizations given splitting
>> is available already.
>
> In my mind, split should be avoided as much as possible.

If so, should we use "nosplit" logic in migrate_pages_batch() in some
situation?

> But it really depends
> on the actual situation, e.g., how much effort and cost the compaction wants
> to pay to get memory defragmented. If the system really wants to get a high
> order free page at any cost, split can be used without any issue. But applications
> might lose performance because existing large folios are split just to a
> new one.

Is it possible that splitting is desirable in some situation?  For
example, allocate some large DMA buffers at the cost of large anonymous
folios?

> Like I said in the email, there are tons of optimizations and policies for us
> to explore. We can start with the bare minimum support (if no performance
> regression is observed, we can even start with split all high folios like you
> suggested) and add optimizations one by one.

Sound good to me!  Thanks!

>>
>>> 3. Another possible optimization is to enable free page merge. It is
>>> possible that a to-be-migrated page causes free page split then fails to
>>> migrate eventually. We would lose a high order free page without free
>>> page merge function. But a way of identifying free pages for memory
>>> compaction is needed to reuse existing PFN-based buddy page merge.
>>>
>>> 4. The implemented >0 order folio compaction algorithm is quite naive
>>> and does not consider all possible situations. A better algorithm can
>>> improve compaction success rate.
>>>
>>>
>>> Feel free to give comments and ask questions.
>>>
>>> Thanks.
>>>
>>>
>>> [1] https://lore.kernel.org/linux-mm/f8d47176-03a8-99bf-a813-b5942830fd73@xxxxxxx/
>>>
>>> Zi Yan (4):
>>>   mm/compaction: add support for >0 order folio memory compaction.
>>>   mm/compaction: optimize >0 order folio compaction with free page
>>>     split.
>>>   mm/compaction: optimize >0 order folio compaction by sorting source
>>>     pages.
>>>   mm/compaction: enable compacting >0 order folios.
>>>
>>>  mm/compaction.c | 205 +++++++++++++++++++++++++++++++++++++++---------
>>>  mm/internal.h   |   7 +-
>>>  2 files changed, 176 insertions(+), 36 deletions(-)

--
Best Regards,
Huang, Ying