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<div class="section" id="block-allocator">
<h1>Block Allocator<a class="headerlink" href="#block-allocator" title="Permalink to this headline">¶</a></h1>
<div class="section" id="previous-design">
<h2>Previous Design<a class="headerlink" href="#previous-design" title="Permalink to this headline">¶</a></h2>
<p>Historically there have been two image allocators in Pillow:
<code class="docutils literal notranslate"><span class="pre">ImagingAllocateBlock</span></code> and <code class="docutils literal notranslate"><span class="pre">ImagingAllocateArray</span></code>. The first works
for images smaller than 16MB of data and allocates one large chunk of
memory of <code class="docutils literal notranslate"><span class="pre">im->linesize</span> <span class="pre">*</span> <span class="pre">im->ysize</span></code> bytes. The second works for
large images and make one allocation for each scan line of size
<code class="docutils literal notranslate"><span class="pre">im->linesize</span></code> 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.</p>
</div>
<div class="section" id="new-design">
<h2>New Design<a class="headerlink" href="#new-design" title="Permalink to this headline">¶</a></h2>
<p><code class="docutils literal notranslate"><span class="pre">ImagingAllocateArray</span></code> 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.</p>
<p><code class="docutils literal notranslate"><span class="pre">ImagingAllocateBlock</span></code> is now only used for those cases when we are
specifically requesting a single segment of memory for sharing with
other code.</p>
</div>
<div class="section" id="memory-pools">
<h2>Memory Pools<a class="headerlink" href="#memory-pools" title="Permalink to this headline">¶</a></h2>
<p>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:</p>
<blockquote>
<div><ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">PILLOW_ALIGNMENT</span></code>, in bytes. Specifies the alignment of memory
allocations. Valid values are powers of 2 between 1 and
128, inclusive. Defaults to 1.</li>
<li><code class="docutils literal notranslate"><span class="pre">PILLOW_BLOCK_SIZE</span></code>, in bytes, K, or M. Specifies the maximum
block size for <code class="docutils literal notranslate"><span class="pre">ImagingAllocateArray</span></code>. Valid values are
integers, with an optional <cite>k</cite> or <cite>m</cite> suffix. Defaults to 16M.</li>
<li><code class="docutils literal notranslate"><span class="pre">PILLOW_BLOCKS_MAX</span></code> 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.</li>
</ul>
</div></blockquote>
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