Re: [LSF/MM/BPF TOPIC]mTHP reliable allocation and reclamation

[Barry Song <21cnbao@gmail.com>]

On Sat, May 11, 2024 at 9:18 AM Yang Shi <shy828301@gmail.com> wrote:
>
> On Thu, May 9, 2024 at 7:22 PM Barry Song <21cnbao@gmail.com> wrote:
> >
> > Hi,
> >
> > I'd like to propose a session about the allocation and reclamation of
> > mTHP. This is related to Yu Zhao's
> > TAO[1] but not the same.
> >
> > OPPO has implemented mTHP-like large folios across thousands of
> > genuine Android devices, utilizing
> > ARM64 CONT-PTE. However, we've encountered challenges:
> >
> > - The allocation of mTHP isn't consistently reliable; even after
> > prolonged use, obtaining large folios
> >   remains uncertain.
> >   As an instance, following a few hours of operation, the likelihood
> > of successfully allocating large
> >   folios on a phone may decrease to just 2%.
> >
> > - Mixing large and small folios in the same LRU list can lead to
> > mutual blocking and unpredictable
> >   latency during reclamation/allocation.
>
> I'm also curious how much large folios can improve reclamation
> efficiency. Having large folios is supposed to reduce the scan time
> since there should be fewer folios on LRU. But IIRC I haven't seen too
> much data or benchmark (particularly real life workloads) regarding
> this.

Hi Yang,

We lack direct data on this matter, but information from Ryan's THP_SWPOUT
series [1] provides insights as follows:

| alloc size |                baseline |           + this series |
|            | mm-unstable (~v6.9-rc1) |                         |
|:-----------|------------------------:|------------------------:|
| 4K Page    |                    0.0% |                    1.3% |
| 64K THP    |                  -13.6% |                   46.3% |
| 2M THP     |                   91.4% |                   89.6% |


I suspect the -13.6% performance decrease is due to the split
operation. Once the split
is eliminated, the patchset observed a 46.3% increase. It is presumed
that the overhead
required to reclaim 64K is reduced compared to reclaiming 16 * 4K.

However, at present, in actual android devices, we are observing
nearly 100% occurrence
of anon_thp_swpout_fallback after the device has been in operation for
several hours[2].

Hence, it is likely that we will experience regression instead of
improvement due to the
absence of measures to mitigate swap fragmentation.

[1] https://lore.kernel.org/all/20240408183946.2991168-1-ryan.roberts@arm.com/
[2] https://lore.kernel.org/lkml/CAGsJ_4zAcJkuW016Cfi6wicRr8N9X+GJJhgMQdSMp+Ah+NSgNQ@mail.gmail.com/

>
> >
> >   For instance, if you require large folios, the LRU list's tail could
> > be filled with small folios.
> >   LRU(LF- large folio, SF- small folio):
> >
> >    LF - LF - LF -  SF - SF - SF - SF - SF - SF -SF - SF - SF - SF - SF - SF - SF
> >
> >  You might end up reclaiming many small folios yet still struggle to
> > allocate large folios. Conversely,
> >  the inverse scenario can occur when the LRU list's tail is populated
> > with large folios.
> >
> >    SF - SF - SF -  LF - LF - LF - LF - LF - LF -LF - LF - LF - LF - LF - LF - LF
> >
> > In OPPO's products, we allocate dedicated pageblocks solely for large
> > folios allocation, and we've
> > fine-tuned the LRU mechanism to support dual LRU—one for small folios
> > and another for large ones.
> > Dedicated page blocks offer a fundamental guarantee of allocating
> > large folios. Additionally, segregating
> > small and large folios into two LRUs ensures that both can be
> > efficiently reclaimed for their respective
> > users' requests.  However, while the implementation may lack aesthetic
> > appeal and is primarily tailored
> > for product purposes, it isn't fully upstreamable.
> >
> > You can obtain the architectural diagram of OPPO's approach from link[2].
> >
> > Therefore, my plan is to present:
> >
> > - Introduce the architecture of OPPO's mTHP-like approach, which
> > encompasses additional optimizations
> >   we've made to address swap fragmentation issues and improve swap
> > performance, such as dual-zRAM
> >   and compression/decompression of large folios [3].
> >
> > - Present OPPO's method of utilizing dedicated page blocks and a
> > dual-LRU system for mTHP.
> >
> > - Share our observations from employing Yu Zhao's TAO on Pixel 6 phones.
> >
> > - Discuss our future direction—are we leaning towards TAO or dedicated
> > page blocks? If we opt for page
> >   blocks, how do we plan to resolve the LRU issue?
> >
> > [1] https://lore.kernel.org/linux-mm/20240229183436.4110845-1-yuzhao@google.com/
> > [2] https://github.com/21cnbao/mTHP/blob/main/largefoliosarch.png
> > [3] https://lore.kernel.org/linux-mm/20240327214816.31191-1-21cnbao@gmail.com/
> >
Thanks,
Barry