mirror of
https://github.com/python-pillow/Pillow.git
synced 2024-09-21 19:38:57 +03:00
48 lines
1.9 KiB
ReStructuredText
48 lines
1.9 KiB
ReStructuredText
Block Allocator
|
|
===============
|
|
|
|
Previous Design
|
|
---------------
|
|
|
|
Historically there have been two image allocators in Pillow:
|
|
``ImagingAllocateBlock`` and ``ImagingAllocateArray``. The first works
|
|
for images smaller than 16MB of data and allocates one large chunk of
|
|
memory of ``im->linesize * im->ysize`` bytes. The second works for
|
|
large images and make one allocation for each scan line of size
|
|
``im->linesize`` bytes. This makes for a very sharp transition
|
|
between one allocation and potentially thousands of small allocations,
|
|
leading to unpredictable performance penalties around the transition.
|
|
|
|
New Design
|
|
----------
|
|
|
|
``ImagingAllocateArray`` now allocates space for images as a chain of
|
|
blocks with a maximum size of 16MB. If there is a memory allocation
|
|
error, it falls back to allocating a 4KB block, or at least one scan
|
|
line. This is now the default for all internal allocations.
|
|
|
|
``ImagingAllocateBlock`` is now only used for those cases when we are
|
|
specifically requesting a single segment of memory for sharing with
|
|
other code.
|
|
|
|
Memory Pools
|
|
------------
|
|
|
|
There is now a memory pool to contain a supply of recently freed
|
|
blocks, which can then be reused without going back to the OS for a
|
|
fresh allocation. This caching of free blocks is currently disabled by
|
|
default, but can be enabled and tweaked using three environment
|
|
variables:
|
|
|
|
* ``PILLOW_ALIGNMENT``, in bytes. Specifies the alignment of memory
|
|
allocations. Valid values are powers of 2 between 1 and
|
|
128, inclusive. Defaults to 1.
|
|
|
|
* ``PILLOW_BLOCK_SIZE``, in bytes, K, or M. Specifies the maximum
|
|
block size for ``ImagingAllocateArray``. Valid values are
|
|
integers, with an optional `k` or `m` suffix. Defaults to 16M.
|
|
|
|
* ``PILLOW_BLOCKS_MAX`` Specifies the number of freed blocks to
|
|
retain to fill future memory requests. Any freed blocks over this
|
|
threshold will be returned to the OS immediately. Defaults to 0.
|