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href="../_sources/reference/block_allocator.rst.txt" rel="nofollow"> View page source</a> </li> </ul> <hr/> </div> <div role="main" class="document" itemscope="itemscope" itemtype="http://schema.org/Article"> <div itemprop="articleBody"> <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> </div> </div> </div> </div> <footer> <div class="rst-footer-buttons" role="navigation" aria-label="footer navigation"> <a href="../porting.html" class="btn btn-neutral float-right" title="Porting" accesskey="n" rel="next">Next <span class="fa fa-arrow-circle-right"></span></a> <a href="limits.html" class="btn btn-neutral float-left" title="Limits" accesskey="p" rel="prev"><span class="fa fa-arrow-circle-left"></span> Previous</a> </div> <hr/> <div role="contentinfo"> <p> © Copyright 1995-2011 Fredrik Lundh, 2010-2018 Alex Clark and Contributors </p> </div> Built with <a href="http://sphinx-doc.org/">Sphinx</a> using a <a href="https://github.com/rtfd/sphinx_rtd_theme">theme</a> provided by <a href="https://readthedocs.org">Read the Docs</a>. </footer> </div> </div> </section> </div> <script type="text/javascript"> jQuery(function () { SphinxRtdTheme.Navigation.enable(true); }); </script> </body> </html>