
.. _block_allocator:

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 makes 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.
