Re: [PATCH v4 0/6] Swap-out mTHP without splitting

[Ryan Roberts]

On 12/03/2024 08:49, Ryan Roberts wrote:
> On 12/03/2024 08:01, Huang, Ying wrote:
>> Ryan Roberts <ryan.roberts@xxxxxxx> writes:
>>
>>> Hi All,
>>>
>>> This series adds support for swapping out multi-size THP (mTHP) without needing
>>> to first split the large folio via split_huge_page_to_list_to_order(). It
>>> closely follows the approach already used to swap-out PMD-sized THP.
>>>
>>> There are a couple of reasons for swapping out mTHP without splitting:
>>>
>>>   - Performance: It is expensive to split a large folio and under extreme memory
>>>     pressure some workloads regressed performance when using 64K mTHP vs 4K
>>>     small folios because of this extra cost in the swap-out path. This series
>>>     not only eliminates the regression but makes it faster to swap out 64K mTHP
>>>     vs 4K small folios.
>>>
>>>   - Memory fragmentation avoidance: If we can avoid splitting a large folio
>>>     memory is less likely to become fragmented, making it easier to re-allocate
>>>     a large folio in future.
>>>
>>>   - Performance: Enables a separate series [4] to swap-in whole mTHPs, which
>>>     means we won't lose the TLB-efficiency benefits of mTHP once the memory has
>>>     been through a swap cycle.
>>>
>>> I've done what I thought was the smallest change possible, and as a result, this
>>> approach is only employed when the swap is backed by a non-rotating block device
>>> (just as PMD-sized THP is supported today). Discussion against the RFC concluded
>>> that this is sufficient.
>>>
>>>
>>> Performance Testing
>>> ===================
>>>
>>> I've run some swap performance tests on Ampere Altra VM (arm64) with 8 CPUs. The
>>> VM is set up with a 35G block ram device as the swap device and the test is run
>>> from inside a memcg limited to 40G memory. I've then run `usemem` from
>>> vm-scalability with 70 processes, each allocating and writing 1G of memory. I've
>>> repeated everything 6 times and taken the mean performance improvement relative
>>> to 4K page baseline:
>>>
>>> | alloc size |            baseline |       + this series |
>>> |            |  v6.6-rc4+anonfolio |                     |
>>> |:-----------|--------------------:|--------------------:|
>>> | 4K Page    |                0.0% |                1.4% |
>>> | 64K THP    |              -14.6% |               44.2% |
>>> | 2M THP     |               87.4% |               97.7% |
>>>
>>> So with this change, the 64K swap performance goes from a 15% regression to a
>>> 44% improvement. 4K and 2M swap improves slightly too.
>>
>> I don't understand why the performance of 2M THP improves.  The swap
>> entry allocation becomes a little slower.  Can you provide some
>> perf-profile to root cause it?
> 
> I didn't post the stdev, which is quite large (~10%), so that may explain some
> of it:
> 
> | kernel   |   mean_rel |   std_rel |
> |:---------|-----------:|----------:|
> | base-4K  |       0.0% |      5.5% |
> | base-64K |     -14.6% |      3.8% |
> | base-2M  |      87.4% |     10.6% |
> | v4-4K    |       1.4% |      3.7% |
> | v4-64K   |      44.2% |     11.8% |
> | v4-2M    |      97.7% |     13.3% |
> 
> Regardless, I'll do some perf profiling and post results shortly.

I did a lot more runs (24 for each config) and meaned them to try to remove the
noise in the measurements. It's now only showing a 4% improvement for 2M. So I
don't think the 2M improvement is real:

| kernel   |   mean_rel |   std_rel |
|:---------|-----------:|----------:|
| base-4K  |       0.0% |      3.2% |
| base-64K |      -9.1% |     10.1% |
| base-2M  |      88.9% |      6.8% |
| v4-4K    |       0.5% |      3.1% |
| v4-64K   |      44.7% |      8.3% |
| v4-2M    |      93.3% |      7.8% |

Looking at the perf data, the only thing that sticks out is that a big chunk of
time is spent in during contpte_convert(), called as a result of
try_to_unmap_one(). This is present in both the before and after configs.

This is an arm64 function to "unfold" contpte mappings. Essentially, the PMD is
being split during shrink_folio_list()  with TTU_SPLIT_HUGE_PMD, meaning the
THPs are PTE-mapped in contpte blocks. Then we are unmapping each pte one-by-one
which means the contpte block needs to be unfolded. I think try_to_unmap_one()
could potentially be optimized to batch unmap a contiguously mapped folio and
avoid this unfold. But that would be an independent and separate piece of work.

> 
>>
>> --
>> Best Regards,
>> Huang, Ying
>>
>>> This test also acts as a good stress test for swap and, more generally mm. A
>>> couple of existing bugs were found as a result [5] [6].
>>>
>>>
>>> ---
>>> The series applies against mm-unstable (d7182786dd0a). Although I've
>>> additionally been running with a couple of extra fixes to avoid the issues at
>>> [6].
>>>
>>>
>>> Changes since v3 [3]
>>> ====================
>>>
>>>  - Renamed SWAP_NEXT_NULL -> SWAP_NEXT_INVALID (per Huang, Ying)
>>>  - Simplified max offset calculation (per Huang, Ying)
>>>  - Reinstated struct percpu_cluster to contain per-cluster, per-order `next`
>>>    offset (per Huang, Ying)
>>>  - Removed swap_alloc_large() and merged its functionality into
>>>    scan_swap_map_slots() (per Huang, Ying)
>>>  - Avoid extra cost of folio ref and lock due to removal of CLUSTER_FLAG_HUGE
>>>    by freeing swap entries in batches (see patch 2) (per DavidH)
>>>  - vmscan splits folio if its partially mapped (per Barry Song, DavidH)
>>>  - Avoid splitting in MADV_PAGEOUT path (per Barry Song)
>>>  - Dropped "mm: swap: Simplify ssd behavior when scanner steals entry" patch
>>>    since it's not actually a problem for THP as I first thought.
>>>
>>>
>>> Changes since v2 [2]
>>> ====================
>>>
>>>  - Reuse scan_swap_map_try_ssd_cluster() between order-0 and order > 0
>>>    allocation. This required some refactoring to make everything work nicely
>>>    (new patches 2 and 3).
>>>  - Fix bug where nr_swap_pages would say there are pages available but the
>>>    scanner would not be able to allocate them because they were reserved for the
>>>    per-cpu allocator. We now allow stealing of order-0 entries from the high
>>>    order per-cpu clusters (in addition to exisiting stealing from order-0
>>>    per-cpu clusters).
>>>
>>>
>>> Changes since v1 [1]
>>> ====================
>>>
>>>  - patch 1:
>>>     - Use cluster_set_count() instead of cluster_set_count_flag() in
>>>       swap_alloc_cluster() since we no longer have any flag to set. I was unable
>>>       to kill cluster_set_count_flag() as proposed against v1 as other call
>>>       sites depend explicitly setting flags to 0.
>>>  - patch 2:
>>>     - Moved large_next[] array into percpu_cluster to make it per-cpu
>>>       (recommended by Huang, Ying).
>>>     - large_next[] array is dynamically allocated because PMD_ORDER is not
>>>       compile-time constant for powerpc (fixes build error).
>>>
>>>
>>> [1] https://lore.kernel.org/linux-mm/20231010142111.3997780-1-ryan.roberts@xxxxxxx/
>>> [2] https://lore.kernel.org/linux-mm/20231017161302.2518826-1-ryan.roberts@xxxxxxx/
>>> [3] https://lore.kernel.org/linux-mm/20231025144546.577640-1-ryan.roberts@xxxxxxx/
>>> [4] https://lore.kernel.org/linux-mm/20240304081348.197341-1-21cnbao@xxxxxxxxx/
>>> [5] https://lore.kernel.org/linux-mm/20240311084426.447164-1-ying.huang@xxxxxxxxx/
>>> [6] https://lore.kernel.org/linux-mm/79dad067-1d26-4867-8eb1-941277b9a77b@xxxxxxx/
>>>
>>> Thanks,
>>> Ryan
>>>
>>>
>>> Ryan Roberts (6):
>>>   mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags
>>>   mm: swap: free_swap_and_cache_nr() as batched free_swap_and_cache()
>>>   mm: swap: Simplify struct percpu_cluster
>>>   mm: swap: Allow storage of all mTHP orders
>>>   mm: vmscan: Avoid split during shrink_folio_list()
>>>   mm: madvise: Avoid split during MADV_PAGEOUT and MADV_COLD
>>>
>>>  include/linux/pgtable.h |  28 ++++
>>>  include/linux/swap.h    |  33 +++--
>>>  mm/huge_memory.c        |   3 -
>>>  mm/internal.h           |  48 +++++++
>>>  mm/madvise.c            | 101 ++++++++------
>>>  mm/memory.c             |  13 +-
>>>  mm/swapfile.c           | 298 ++++++++++++++++++++++------------------
>>>  mm/vmscan.c             |   9 +-
>>>  8 files changed, 332 insertions(+), 201 deletions(-)
>>>
>>> --
>>> 2.25.1
>