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We prefer a free cluster over a nonfull cluster whenever a CPU local cluster is drained to respect the SSD discard behavior [1]. It's not a best practice for non-discarding devices. And this is causing a higher fragmentation rate. So for a non-discarding device, prefer nonfull over free clusters. This reduces the fragmentation issue by a lot. Testing with make -j96, defconfig, using 64k mTHP, 8G ZRAM: Before: sys time: 6176.34s 64kB/swpout: 1659757 64kB/swpout_fallback: 139503 After: sys time: 6194.11s 64kB/swpout: 1689470 64kB/swpout_fallback: 56147 Testing with make -j96, defconfig, using 64k mTHP, 10G ZRAM: After: sys time: 5531.49s 64kB/swpout: 1791142 64kB/swpout_fallback: 17676 After: sys time: 5587.53s 64kB/swpout: 1811598 64kB/swpout_fallback: 0 Performance is basically unchanged, and the large allocation failure rate is lower. Enabling all mTHP sizes showed a more significant result. Using the same test setup with 10G ZRAM and enabling all mTHP sizes: 128kB swap failure rate: Before: swpout:451599 swpout_fallback:54525 After: swpout:502710 swpout_fallback:870 256kB swap failure rate: Before: swpout:63652 swpout_fallback:2708 After: swpout:65913 swpout_fallback:20 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 The success rate of large allocations is much higher. Link: https://lore.kernel.org/linux-mm/87v8242vng.fsf@yhuang6-desk2.ccr.corp.intel.com/ [1] Link: https://lkml.kernel.org/r/20250806161748.76651-4-ryncsn@gmail.com Signed-off-by: Kairui Song <kasong@tencent.com> Acked-by: Chris Li <chrisl@kernel.org> Reviewed-by: Nhat Pham <nphamcs@gmail.com> Cc: Baoquan He <bhe@redhat.com> Cc: Barry Song <baohua@kernel.org> Cc: "Huang, Ying" <ying.huang@linux.alibaba.com> Cc: Kemeng Shi <shikemeng@huaweicloud.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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.