2018-03-22 03:22:44 +08:00
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.. _z3fold:
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======
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2016-05-21 07:58:30 +08:00
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z3fold
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2018-03-22 03:22:44 +08:00
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======
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2016-05-21 07:58:30 +08:00
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z3fold is a special purpose allocator for storing compressed pages.
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It is designed to store up to three compressed pages per physical page.
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It is a zbud derivative which allows for higher compression
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ratio keeping the simplicity and determinism of its predecessor.
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The main differences between z3fold and zbud are:
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2018-03-22 03:22:44 +08:00
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2016-05-21 07:58:30 +08:00
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* unlike zbud, z3fold allows for up to PAGE_SIZE allocations
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* z3fold can hold up to 3 compressed pages in its page
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* z3fold doesn't export any API itself and is thus intended to be used
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via the zpool API.
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To keep the determinism and simplicity, z3fold, just like zbud, always
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stores an integral number of compressed pages per page, but it can store
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up to 3 pages unlike zbud which can store at most 2. Therefore the
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compression ratio goes to around 2.7x while zbud's one is around 1.7x.
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Unlike zbud (but like zsmalloc for that matter) z3fold_alloc() does not
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return a dereferenceable pointer. Instead, it returns an unsigned long
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handle which encodes actual location of the allocated object.
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Keeping effective compression ratio close to zsmalloc's, z3fold doesn't
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depend on MMU enabled and provides more predictable reclaim behavior
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which makes it a better fit for small and response-critical systems.
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