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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Chapter 21. The bitmap_allocator</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content=" ISO C++ , allocator "/><meta name="keywords" content=" ISO C++ , library "/><meta name="keywords" content=" ISO C++ , runtime , library "/><link rel="home" href="../index.html" title="The GNU C++ Library"/><link rel="up" href="extensions.html" title="Part III. Extensions"/><link rel="prev" href="bk01pt03ch20s05.html" title="Multiple Thread Example"/><link rel="next" href="bk01pt03ch21s02.html" title="Implementation"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 21. The bitmap_allocator</th></tr><tr><td align="left"><a accesskey="p" href="bk01pt03ch20s05.html">Prev</a> </td><th width="60%" align="center">Part III.
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Extensions
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</th><td align="right"> <a accesskey="n" href="bk01pt03ch21s02.html">Next</a></td></tr></table><hr/></div><div class="chapter" title="Chapter 21. The bitmap_allocator"><div class="titlepage"><div><div><h2 class="title"><a id="manual.ext.allocator.bitmap"/>Chapter 21. The bitmap_allocator</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl><dt><span class="section"><a href="bitmap_allocator.html#allocator.bitmap.design">Design</a></span></dt><dt><span class="section"><a href="bk01pt03ch21s02.html">Implementation</a></span></dt><dd><dl><dt><span class="section"><a href="bk01pt03ch21s02.html#bitmap.impl.free_list_store">Free List Store</a></span></dt><dt><span class="section"><a href="bk01pt03ch21s02.html#bitmap.impl.super_block">Super Block</a></span></dt><dt><span class="section"><a href="bk01pt03ch21s02.html#bitmap.impl.super_block_data">Super Block Data Layout</a></span></dt><dt><span class="section"><a href="bk01pt03ch21s02.html#bitmap.impl.max_wasted">Maximum Wasted Percentage</a></span></dt><dt><span class="section"><a href="bk01pt03ch21s02.html#bitmap.impl.allocate"><code class="function">allocate</code></a></span></dt><dt><span class="section"><a href="bk01pt03ch21s02.html#bitmap.impl.deallocate"><code class="function">deallocate</code></a></span></dt><dt><span class="section"><a href="bk01pt03ch21s02.html#bitmap.impl.questions">Questions</a></span></dt><dd><dl><dt><span class="section"><a href="bk01pt03ch21s02.html#bitmap.impl.question.1">1</a></span></dt><dt><span class="section"><a href="bk01pt03ch21s02.html#bitmap.impl.question.2">2</a></span></dt><dt><span class="section"><a href="bk01pt03ch21s02.html#bitmap.impl.question.3">3</a></span></dt></dl></dd><dt><span class="section"><a href="bk01pt03ch21s02.html#bitmap.impl.locality">Locality</a></span></dt><dt><span class="section"><a href="bk01pt03ch21s02.html#bitmap.impl.grow_policy">Overhead and Grow Policy</a></span></dt></dl></dd></dl></div><p>
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</p><div class="section" title="Design"><div class="titlepage"><div><div><h2 class="title"><a id="allocator.bitmap.design"/>Design</h2></div></div></div><p>
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As this name suggests, this allocator uses a bit-map to keep track
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of the used and unused memory locations for its book-keeping
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purposes.
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</p><p>
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This allocator will make use of 1 single bit to keep track of
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whether it has been allocated or not. A bit 1 indicates free,
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while 0 indicates allocated. This has been done so that you can
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easily check a collection of bits for a free block. This kind of
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Bitmapped strategy works best for single object allocations, and
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with the STL type parameterized allocators, we do not need to
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choose any size for the block which will be represented by a
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single bit. This will be the size of the parameter around which
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the allocator has been parameterized. Thus, close to optimal
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performance will result. Hence, this should be used for node based
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containers which call the allocate function with an argument of 1.
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</p><p>
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The bitmapped allocator's internal pool is exponentially growing.
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Meaning that internally, the blocks acquired from the Free List
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Store will double every time the bitmapped allocator runs out of
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memory.
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</p><p>
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The macro <code class="literal">__GTHREADS</code> decides whether to use
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Mutex Protection around every allocation/deallocation. The state
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of the macro is picked up automatically from the gthr abstraction
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layer.
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</p></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="bk01pt03ch20s05.html">Prev</a> </td><td align="center"><a accesskey="u" href="extensions.html">Up</a></td><td align="right"> <a accesskey="n" href="bk01pt03ch21s02.html">Next</a></td></tr><tr><td align="left" valign="top">Multiple Thread Example </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Implementation</td></tr></table></div></body></html>
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