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The page allocator processes free pages in groups of pageblocks, where the size of a pageblock is typically quite large (1024 pages without hugetlbpage support). Pageblocks are processed atomically with the zone lock held, which can cause severe scheduling delays on both the CPU going through the pageblock and any other CPUs waiting to acquire the zone lock. A frequent offender is move_freepages_block(), which is used by rmqueue() for page allocation. As it turns out, there's no requirement for pageblocks to be so large, so the pageblock order can simply be reduced to ease the scheduling delays and zone lock contention. PAGE_ALLOC_COSTLY_ORDER is used as a reasonable setting to ensure non-costly page allocation requests can still be serviced without always needing to free up more than one pageblock's worth of pages at a time. This has a noticeable effect on overall system latency when memory pressure is elevated. The various mm functions which operate on pageblocks no longer appear in the preemptoff tracer, where previously they would spend up to 100 ms on a mobile arm64 CPU processing a pageblock with preemption disabled and the zone lock held. Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.