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48 lines
1.9 KiB
ReStructuredText
48 lines
1.9 KiB
ReStructuredText
Block Allocator
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===============
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Previous Design
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---------------
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Historically there have been two image allocators in Pillow:
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``ImagingAllocateBlock`` and ``ImagingAllocateArray``. The first works
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for images smaller than 16MB of data and allocates one large chunk of
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memory of ``im->linesize * im->ysize`` bytes. The second works for
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large images and makes one allocation for each scan line of size
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``im->linesize`` bytes. This makes for a very sharp transition
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between one allocation and potentially thousands of small allocations,
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leading to unpredictable performance penalties around the transition.
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New Design
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----------
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``ImagingAllocateArray`` now allocates space for images as a chain of
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blocks with a maximum size of 16MB. If there is a memory allocation
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error, it falls back to allocating a 4KB block, or at least one scan
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line. This is now the default for all internal allocations.
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``ImagingAllocateBlock`` is now only used for those cases when we are
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specifically requesting a single segment of memory for sharing with
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other code.
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Memory Pools
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------------
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There is now a memory pool to contain a supply of recently freed
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blocks, which can then be reused without going back to the OS for a
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fresh allocation. This caching of free blocks is currently disabled by
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default, but can be enabled and tweaked using three environment
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variables:
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* ``PILLOW_ALIGNMENT``, in bytes. Specifies the alignment of memory
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allocations. Valid values are powers of 2 between 1 and
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128, inclusive. Defaults to 1.
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* ``PILLOW_BLOCK_SIZE``, in bytes, K, or M. Specifies the maximum
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block size for ``ImagingAllocateArray``. Valid values are
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integers, with an optional `k` or `m` suffix. Defaults to 16M.
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* ``PILLOW_BLOCKS_MAX`` Specifies the number of freed blocks to
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retain to fill future memory requests. Any freed blocks over this
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threshold will be returned to the OS immediately. Defaults to 0.
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