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

Huang, Ying
2024-03-12T08:03:12Z

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?

--
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