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? -- 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
