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5.0 KiB
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78 lines
5.0 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Single Thread Example</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><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="mt_allocator_impl.html" title="Implementation" /><link rel="next" href="mt_allocator_ex_multi.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 width="20%" align="left"><a accesskey="p" href="mt_allocator_impl.html">Prev</a> </td><th width="60%" align="center">Chapter 20. The mt_allocator</th><td width="20%" align="right"> <a accesskey="n" href="mt_allocator_ex_multi.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="allocator.mt.example_single"></a>Single Thread Example</h2></div></div></div><p>
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Let's start by describing how the data on a freelist is laid out in memory.
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This is the first two blocks in freelist for thread id 3 in bin 3 (8 bytes):
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</p><pre class="programlisting">
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+----------------+
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| next* ---------|--+ (_S_bin[ 3 ].first[ 3 ] points here)
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+----------------+ |
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| thread_id = 3 | |
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+----------------+ |
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| DATA | | (A pointer to here is what is returned to the
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| | | the application when needed)
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+----------------+ |
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+----------------+ |
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| next* |<-+ (If next == NULL it's the last one on the list)
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+----------------+
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| thread_id = 3 |
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+----------------+
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| DATA |
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+----------------+
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</pre><p>
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With this in mind we simplify things a bit for a while and say that there is
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only one thread (a ST application). In this case all operations are made to
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what is referred to as the global pool - thread id 0 (No thread may be
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assigned this id since they span from 1 to _S_max_threads in a MT application).
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</p><p>
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When the application requests memory (calling allocate()) we first look at the
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requested size and if this is > _S_max_bytes we call new() directly and return.
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</p><p>
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If the requested size is within limits we start by finding out from which
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bin we should serve this request by looking in _S_binmap.
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</p><p>
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A quick look at _S_bin[ bin ].first[ 0 ] tells us if there are any blocks of
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this size on the freelist (0). If this is not NULL - fine, just remove the
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block that _S_bin[ bin ].first[ 0 ] points to from the list,
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update _S_bin[ bin ].first[ 0 ] and return a pointer to that blocks data.
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</p><p>
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If the freelist is empty (the pointer is NULL) we must get memory from the
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system and build us a freelist within this memory. All requests for new memory
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is made in chunks of _S_chunk_size. Knowing the size of a block_record and
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the bytes that this bin stores we then calculate how many blocks we can create
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within this chunk, build the list, remove the first block, update the pointer
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(_S_bin[ bin ].first[ 0 ]) and return a pointer to that blocks data.
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</p><p>
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Deallocation is equally simple; the pointer is casted back to a block_record
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pointer, lookup which bin to use based on the size, add the block to the front
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of the global freelist and update the pointer as needed
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(_S_bin[ bin ].first[ 0 ]).
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</p><p>
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The decision to add deallocated blocks to the front of the freelist was made
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after a set of performance measurements that showed that this is roughly 10%
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faster than maintaining a set of "last pointers" as well.
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</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="mt_allocator_impl.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="mt_allocator.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="mt_allocator_ex_multi.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Implementation </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Multiple Thread Example</td></tr></table></div></body></html> |