Commits on Jun 10, 2019

1. 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.
   
   openzfs@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.
   
   Signed-off-by: Matthew Ahrens <mahrens@delphix.com>

   

   ahrens committed Jun 10, 2019
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