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| Extensions |
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| </th><td width="20%" align="right"> <a accesskey="n" href="bitmap_allocator_impl.html">Next</a></td></tr></table><hr /></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a id="manual.ext.allocator.bitmap"></a>Chapter 21. The bitmap_allocator</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><dt><span class="section"><a href="bitmap_allocator.html#allocator.bitmap.design">Design</a></span></dt><dt><span class="section"><a href="bitmap_allocator_impl.html">Implementation</a></span></dt><dd><dl><dt><span class="section"><a href="bitmap_allocator_impl.html#bitmap.impl.free_list_store">Free List Store</a></span></dt><dt><span class="section"><a href="bitmap_allocator_impl.html#bitmap.impl.super_block">Super Block</a></span></dt><dt><span class="section"><a href="bitmap_allocator_impl.html#bitmap.impl.super_block_data">Super Block Data Layout</a></span></dt><dt><span class="section"><a href="bitmap_allocator_impl.html#bitmap.impl.max_wasted">Maximum Wasted Percentage</a></span></dt><dt><span class="section"><a href="bitmap_allocator_impl.html#bitmap.impl.allocate"><code class="function">allocate</code></a></span></dt><dt><span class="section"><a href="bitmap_allocator_impl.html#bitmap.impl.deallocate"><code class="function">deallocate</code></a></span></dt><dt><span class="section"><a href="bitmap_allocator_impl.html#bitmap.impl.questions">Questions</a></span></dt><dd><dl><dt><span class="section"><a href="bitmap_allocator_impl.html#bitmap.impl.question.1">1</a></span></dt><dt><span class="section"><a href="bitmap_allocator_impl.html#bitmap.impl.question.2">2</a></span></dt><dt><span class="section"><a href="bitmap_allocator_impl.html#bitmap.impl.question.3">3</a></span></dt></dl></dd><dt><span class="section"><a href="bitmap_allocator_impl.html#bitmap.impl.locality">Locality</a></span></dt><dt><span class="section"><a href="bitmap_allocator_impl.html#bitmap.impl.grow_policy">Overhead and Grow Policy</a></span></dt></dl></dd></dl></div><p> |
| </p><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="allocator.bitmap.design"></a>Design</h2></div></div></div><p> |
| As this name suggests, this allocator uses a bit-map to keep track |
| of the used and unused memory locations for its book-keeping |
| purposes. |
| </p><p> |
| This allocator will make use of 1 single bit to keep track of |
| whether it has been allocated or not. A bit 1 indicates free, |
| while 0 indicates allocated. This has been done so that you can |
| easily check a collection of bits for a free block. This kind of |
| Bitmapped strategy works best for single object allocations, and |
| with the STL type parameterized allocators, we do not need to |
| choose any size for the block which will be represented by a |
| single bit. This will be the size of the parameter around which |
| the allocator has been parameterized. Thus, close to optimal |
| performance will result. Hence, this should be used for node based |
| containers which call the allocate function with an argument of 1. |
| </p><p> |
| The bitmapped allocator's internal pool is exponentially growing. |
| Meaning that internally, the blocks acquired from the Free List |
| Store will double every time the bitmapped allocator runs out of |
| memory. |
| </p><p> |
| The macro <code class="literal">__GTHREADS</code> decides whether to use |
| Mutex Protection around every allocation/deallocation. The state |
| of the macro is picked up automatically from the gthr abstraction |
| layer. |
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