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    <p><strong>A standard BSD-style memory allocator basically works like this:</strong></p> <ul> <li>It keeps a linked list of pre-allocated memory blocks for sizes 2^k for k&lt;=12 (for example).</li> <li>In reality, each list for a given k is composed of memory-blocks from different <em>areas</em>, see below.</li> <li>A malloc request for n bytes is serviced by calculating n', the closest 2^k >= n, then looking up the first <em>area</em> in the list for k, and then returning the first free block in the free-list for the given <em>area</em>.</li> <li>When there is no pre-allocated memory block for size 2^k, an <em>area</em> is allocated, an area being some larger piece of continuous memory, say a 4kB piece of memory. This piece of memory is then chopped up into pieces that are 2^k bytes. At the beginning of the continuous memory area there is book-keeping information such as where to find the linked list of free blocks within the area. A bitmap can also be used, but a linked list typically has better cache behavior (you want the next allocated block to return memory that is already in the cache).</li> <li>The reason for using <em>areas</em> is that free(ptr) can be implemented efficiently. ptr &amp; 0xfffff000 in this example points to the beginning of the <em>area</em> which contains the book-keeping structures and makes it possible to link the memory block back into the area.</li> <li>The BSD allocator will waste space by always returning a memory block 2^k in size, but it can reuse the memory of the block to keep the free-list, which is a nice property. Also allocation is blazingly fast.</li> </ul> <p><strong>Modifications to the above general idea include:</strong></p> <ul> <li>Using anonymous mmap for large allocations. This shifts the work over to the kernel for handling large mallocs and avoids wasting a lot of memory in these cases.</li> <li>The GNU version of malloc have special cases for non-power-of-two buckets. There is nothing inherent in the BSD allocator that requires returning 2^k memory blocks, only that there are pre-defined bucket sizes. The GNU allocator has more buckets and thus waste less space.</li> <li>Sharing memory between threads is a tricky subject. Lock-contention during allocation is an important consideration, so in the GNU allocator for example will eagerly create extra <em>areas</em> for different threads for a given bucket size if it ever encounters lock-contention during allocation. </li> </ul>
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