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Patch series "mm, swap: improve cluster scan strategy", v2. This series improves the large allocation performance and reduces the failure rate. Some design of the cluster alloactor was later found to be improvable after thorough testing. The allocator spent too much effort scanning the fragment list, which is not helpful in most setups, but causes serious contention of the list lock (si->lock). Besides, the allocator prefers free clusters when searching for a new cluster due to historical reasons, which causes fragmentation issues. So make the allocator only scan one cluster for high order allocation, and prefer nonfull cluster. This both improves the performance and reduces fragmentation. For example, build kernel test with make -j96 and 10G ZRAM with 64kB mTHP enabled shows better performance and a lower failure rate: Before: sys time: 11609.69s 64kB/swpout: 1787051 64kB/swpout_fallback: 20917 After: sys time: 5587.53s 64kB/swpout: 1811598 64kB/swpout_fallback: 0 System time is cut in half, and the failure rate drops to zero. Larger allocations in a hybrid workload also showed a major improvement: 512kB swap failure rate: Before: swpout:11663 swpout_fallback:1767 After: swpout:14480 swpout_fallback:6 2M swap failure rate: Before: swpout:24 swpout_fallback:1442 After: swpout:1329 swpout_fallback:7 This patch (of 3): Fragment clusters were mostly failing high order allocation already. The reason we scan it through now is that a swap slot may get freed without releasing the swap cache, so a swap map entry will end up in HAS_CACHE only status, and the cluster won't be moved back to non-full or free cluster list. This may cause a higher allocation failure rate. Usually only !SWP_SYNCHRONOUS_IO devices may have a large number of slots stuck in HAS_CACHE only status. Because when a !SWP_SYNCHRONOUS_IO device's usage is low (!vm_swap_full()), it will try to lazy free the swap cache. But this fragment list scan out is a bit overkill. Fragmentation is only an issue for the allocator when the device is getting full, and by that time, swap will be releasing the swap cache aggressively already. Only scanning one fragment cluster at a time is good enough to reclaim already pinned slots, and move the cluster back to nonfull. And besides, only high order allocation requires iterating over the list, order 0 allocation will succeed on the first attempt. And high order allocation failure isn't a serious problem. So the iteration of fragment clusters is trivial, but it will slow down large allocation by a lot when the fragment cluster list is long. So it's better to drop this fragment cluster iteration design. Test on a 48c96t system, build linux kernel using 10G ZRAM, make -j48, defconfig with 768M cgroup memory limit, on top of tmpfs, 4K folio only: Before: sys time: 4432.56s After: sys time: 4430.18s Change to make -j96, 2G memory limit, 64kB mTHP enabled, and 10G ZRAM: Before: sys time: 11609.69s 64kB/swpout: 1787051 64kB/swpout_fallback: 20917 After: sys time: 5572.85s 64kB/swpout: 1797612 64kB/swpout_fallback: 19254 Change to 8G ZRAM: Before: sys time: 21524.35s 64kB/swpout: 1687142 64kB/swpout_fallback: 128496 After: sys time: 6278.45s 64kB/swpout: 1679127 64kB/swpout_fallback: 130942 Change to use 10G brd device with SWP_SYNCHRONOUS_IO flag removed: Before: sys time: 7393.50s 64kB/swpout:1788246 swpout_fallback: 0 After: sys time: 7399.88s 64kB/swpout:1784257 swpout_fallback: 0 Change to use 8G brd device with SWP_SYNCHRONOUS_IO flag removed: Before: sys time: 26292.26s 64kB/swpout:1645236 swpout_fallback: 138945 After: sys time: 9463.16s 64kB/swpout:1581376 swpout_fallback: 259979 The performance is a lot better for large folios, and the large order allocation failure rate is only very slightly higher or unchanged even for !SWP_SYNCHRONOUS_IO devices high pressure. Link: https://lkml.kernel.org/r/20250806161748.76651-1-ryncsn@gmail.com Link: https://lkml.kernel.org/r/20250806161748.76651-2-ryncsn@gmail.com Signed-off-by: Kairui Song <kasong@tencent.com> Acked-by: Nhat Pham <nphamcs@gmail.com> Acked-by: Chris Li <chrisl@kernel.org> Cc: Baoquan He <bhe@redhat.com> Cc: Barry Song <baohua@kernel.org> Cc: "Huang, Ying" <ying.huang@linux.alibaba.com> Cc: Kairui Song <kasong@tencent.com> Cc: Kemeng Shi <shikemeng@huaweicloud.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.