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 forImagingAllocateArray. Valid values are integers, with an optionalkormsuffix. Defaults to 16M.
PILLOW_BLOCKS_MAXSpecifies 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.