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single-chunk scatter ABDs can be treated as linear
Scatter ABD's are allocated from a number of pages. In contrast to linear ABD's, these pages are disjoint in the kernel's virtual address space, so they can't be accessed as a contiguous buffer. Therefore routines that need a linear buffer (e.g. abd_borrow_buf() and friends) must allocate a separate linear buffer (with zio_buf_alloc()), and copy the contents of the pages to/from the linear buffer. This can have a measurable performance overhead on some workloads. 87c25d5 ("abd_alloc should use scatter for >1K allocations") increased the use of scatter ABD's, specifically switching 1.5K through 4K (inclusive) buffers from linear to scatter. For workloads that access blocks whose compressed sizes are in this range, that commit introduced an additional copy into the read code path. For example, the sequential_reads_arc_cached tests in the test suite were reduced by around 5% (this is doing reads of 8K-logical blocks, compressed to 3K, which are cached in the ARC). This commit treats single-chunk scattered buffers as linear buffers, because they are contiguous in the kernel's virtual address space. All single-page (4K) ABD's can be represented this way. Some multi-page ABD's can also be represented this way, if we were able to allocate a single "chunk" (higher-order "page" which represents a power-of-2 series of physically-contiguous pages). This is often the case for 2-page (8K) ABD's. Representing a single-entry scatter ABD as a linear ABD has the performance advantage of avoiding the copy (and allocation) in abd_borrow_buf_copy / abd_return_buf_copy. A performance increase of around 5% has been observed for ARC-cached reads (of small blocks which can take advantage of this), fixing the regression introduced by 87c25d5. Note that this optimization is only possible because all physical memory is always mapped into the kernel's address space. This is not the case for HIGHMEM pages, so the optimization can not be made on 32-bit systems. Reviewed-by: Chunwei Chen <email@example.com> Reviewed-by: Brian Behlendorf <firstname.lastname@example.org> Signed-off-by: Matthew Ahrens <email@example.com> Closes #8580
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