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

[Ryan Roberts <ryan.roberts@arm.com>]

On 12/03/2024 08:01, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts@arm.com> 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.

> 
> --
> 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@arm.com/
>> [2] https://lore.kernel.org/linux-mm/20231017161302.2518826-1-ryan.roberts@arm.com/
>> [3] https://lore.kernel.org/linux-mm/20231025144546.577640-1-ryan.roberts@arm.com/
>> [4] https://lore.kernel.org/linux-mm/20240304081348.197341-1-21cnbao@gmail.com/
>> [5] https://lore.kernel.org/linux-mm/20240311084426.447164-1-ying.huang@intel.com/
>> [6] https://lore.kernel.org/linux-mm/79dad067-1d26-4867-8eb1-941277b9a77b@arm.com/
>>
>> 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