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87c25d567f
abd_alloc() normally does scatter allocations, thus solving the problem that ABD originally set out to: the bulk of ZFS's allocations are single pages, which are faster to allocate and free, and don't suffer from internal fragmentation (and the inability to reclaim memory because some buffers in the slab are still allocated). However, the current code does linear allocations for 4KB and smaller allocations, defeating the purpose of ABD. Scatter ABD's use at least one page each, so sub-page allocations waste some space when allocated as scatter (e.g. 2KB scatter allocation wastes half of each page). Using linear ABD's for small allocations means that they will be put on slabs which contain many allocations. This can improve memory efficiency, but it also makes it much harder for ARC evictions to actually free pages, because all the buffers on one slab need to be freed in order for the slab (and underlying pages) to be freed. Typically, 512B and 1KB kmem caches have 16 buffers per slab, so it's possible for them to actually waste more memory than scatter (one page per buf = wasting 3/4 or 7/8th; one buf per slab = wasting 15/16th). Spill blocks are typically 512B and are heavily used on systems running selinux with the default dnode size and the `xattr=sa` property set. By default we will use linear allocations for 512B and 1KB, and scatter allocations for larger (1.5KB and up). Reviewed-by: George Melikov <mail@gmelikov.ru> Reviewed-by: DHE <git@dehacked.net> Reviewed-by: Chunwei Chen <tuxoko@gmail.com> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Don Brady <don.brady@delphix.com> Signed-off-by: Matthew Ahrens <mahrens@delphix.com> Closes #8455 |
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