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Maps is a collection of maps primarily sorted by the starting address of the map. Prior to this change the maps were held in an rbtree requiring 4 pointers per node. Prior to reference count checking, the rbnode was embedded in the map so 3 pointers per node were necessary. This change switches the rbtree to an array lazily sorted by address, much as the array sorting nodes by name. 1 pointer is needed per node, but to avoid excessive resizing the backing array may be twice the number of used elements. Meaning the memory overhead is roughly half that of the rbtree. For a perf record with "--no-bpf-event -g -a" of true, the memory overhead of perf inject is reduce fom 3.3MB to 3MB, so 10% or 300KB is saved. Map inserts always happen at the end of the array. The code tracks whether the insertion violates the sorting property. O(log n) rb-tree complexity is switched to O(1). Remove slides the array, so O(log n) rb-tree complexity is degraded to O(n). A find may need to sort the array using qsort which is O(n*log n), but in general the maps should be sorted and so average performance should be O(log n) as with the rbtree. An rbtree node consumes a cache line, but with the array 4 nodes fit on a cache line. Iteration is simplified to scanning an array rather than pointer chasing. Overall it is expected the performance after the change should be comparable to before, but with half of the memory consumed. To avoid a list and repeated logic around splitting maps, maps__merge_in is rewritten in terms of maps__fixup_overlap_and_insert. maps_merge_in splits the given mapping inserting remaining gaps. maps__fixup_overlap_and_insert splits the existing mappings, then adds the incoming mapping. By adding the new mapping first, then re-inserting the existing mappings the splitting behavior matches. Signed-off-by: Ian Rogers <irogers@google.com> Acked-by: Namhyung Kim <namhyung@kernel.org> Cc: K Prateek Nayak <kprateek.nayak@amd.com> Cc: James Clark <james.clark@arm.com> Cc: Vincent Whitchurch <vincent.whitchurch@axis.com> Cc: Alexey Dobriyan <adobriyan@gmail.com> Cc: Colin Ian King <colin.i.king@gmail.com> Cc: Changbin Du <changbin.du@huawei.com> Cc: Masami Hiramatsu <mhiramat@kernel.org> Cc: Song Liu <song@kernel.org> Cc: Leo Yan <leo.yan@linux.dev> Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com> Cc: Liam Howlett <liam.howlett@oracle.com> Cc: Artem Savkov <asavkov@redhat.com> Cc: bpf@vger.kernel.org Signed-off-by: Namhyung Kim <namhyung@kernel.org> Link: https://lore.kernel.org/r/20240210031746.4057262-2-irogers@google.com |
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| README | ||
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 Restructured Text 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.