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<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"><head><title>Single Thread Example</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="mt_allocator.html" title="Chapter 20. The mt_allocator"/><link rel="prev" href="bk01pt03ch20s03.html" title="Implementation"/><link rel="next" href="bk01pt03ch20s05.html" title="Multiple Thread Example"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Single Thread Example</th></tr><tr><td align="left"><a accesskey="p" href="bk01pt03ch20s03.html">Prev</a> </td><th width="60%" align="center">Chapter 20. The mt_allocator</th><td align="right"> <a accesskey="n" href="bk01pt03ch20s05.html">Next</a></td></tr></table><hr/></div><div class="section" title="Single Thread Example"><div class="titlepage"><div><div><h2 class="title"><a id="allocator.mt.example_single"/>Single Thread Example</h2></div></div></div><p> Let's start by describing how the data on a freelist is laid out in memory. This is the first two blocks in freelist for thread id 3 in bin 3 (8 bytes): </p><pre class="programlisting"> +----------------+ | next* ---------|--+ (_S_bin[ 3 ].first[ 3 ] points here) | | | | | | | | | +----------------+ | | thread_id = 3 | | | | | | | | | | | +----------------+ | | DATA | | (A pointer to here is what is returned to the | | | the application when needed) | | | | | | | | | | | | | | | | | | +----------------+ | +----------------+ | | next* |<-+ (If next == NULL it's the last one on the list) | | | | | | +----------------+ | thread_id = 3 | | | | | | | +----------------+ | DATA | | | | | | | | | | | | | | | +----------------+ </pre><p> With this in mind we simplify things a bit for a while and say that there is only one thread (a ST application). In this case all operations are made to what is referred to as the global pool - thread id 0 (No thread may be assigned this id since they span from 1 to _S_max_threads in a MT application). </p><p> When the application requests memory (calling allocate()) we first look at the requested size and if this is > _S_max_bytes we call new() directly and return. </p><p> If the requested size is within limits we start by finding out from which bin we should serve this request by looking in _S_binmap. </p><p> A quick look at _S_bin[ bin ].first[ 0 ] tells us if there are any blocks of this size on the freelist (0). If this is not NULL - fine, just remove the block that _S_bin[ bin ].first[ 0 ] points to from the list, update _S_bin[ bin ].first[ 0 ] and return a pointer to that blocks data. </p><p> If the freelist is empty (the pointer is NULL) we must get memory from the system and build us a freelist within this memory. All requests for new memory is made in chunks of _S_chunk_size. Knowing the size of a block_record and the bytes that this bin stores we then calculate how many blocks we can create within this chunk, build the list, remove the first block, update the pointer (_S_bin[ bin ].first[ 0 ]) and return a pointer to that blocks data. </p><p> Deallocation is equally simple; the pointer is casted back to a block_record pointer, lookup which bin to use based on the size, add the block to the front of the global freelist and update the pointer as needed (_S_bin[ bin ].first[ 0 ]). </p><p> The decision to add deallocated blocks to the front of the freelist was made after a set of performance measurements that showed that this is roughly 10% faster than maintaining a set of "last pointers" as well. </p></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="bk01pt03ch20s03.html">Prev</a> </td><td align="center"><a accesskey="u" href="mt_allocator.html">Up</a></td><td align="right"> <a accesskey="n" href="bk01pt03ch20s05.html">Next</a></td></tr><tr><td align="left" valign="top">Implementation </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Multiple Thread Example</td></tr></table></div></body></html>