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

[Ryan Roberts]

On 11/03/2024 15:00, Ryan Roberts wrote:
> 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 |                     |

Oops, just noticed I failed to update these column headers. The baseline is
actually mm-unstable (d7182786dd0a) which is based on v6.8-rc5 and already
contains "anonfolio" - now called mTHP.


> |:-----------|--------------------:|--------------------:|
> | 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.
> 
> 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
>