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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Multiple 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="bk01pt03ch20s04.html" title="Single Thread Example"/><link rel="next" href="bitmap_allocator.html" title="Chapter 21. The bitmap_allocator"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Multiple Thread Example</th></tr><tr><td align="left"><a accesskey="p" href="bk01pt03ch20s04.html">Prev</a> </td><th width="60%" align="center">Chapter 20. The mt_allocator</th><td align="right"> <a accesskey="n" href="bitmap_allocator.html">Next</a></td></tr></table><hr/></div><div class="section" title="Multiple Thread Example"><div class="titlepage"><div><div><h2 class="title"><a id="allocator.mt.example_multi"/>Multiple Thread Example</h2></div></div></div><p>
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In the ST example we never used the thread_id variable present in each block.
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Let's start by explaining the purpose of this in a MT application.
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</p><p>
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The concept of "ownership" was introduced since many MT applications
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allocate and deallocate memory to shared containers from different
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threads (such as a cache shared amongst all threads). This introduces
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a problem if the allocator only returns memory to the current threads
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freelist (I.e., there might be one thread doing all the allocation and
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thus obtaining ever more memory from the system and another thread
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that is getting a longer and longer freelist - this will in the end
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consume all available memory).
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</p><p>
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Each time a block is moved from the global list (where ownership is
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irrelevant), to a threads freelist (or when a new freelist is built
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from a chunk directly onto a threads freelist or when a deallocation
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occurs on a block which was not allocated by the same thread id as the
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one doing the deallocation) the thread id is set to the current one.
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</p><p>
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What's the use? Well, when a deallocation occurs we can now look at
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the thread id and find out if it was allocated by another thread id
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and decrease the used counter of that thread instead, thus keeping the
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free and used counters correct. And keeping the free and used counters
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corrects is very important since the relationship between these two
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variables decides if memory should be returned to the global pool or
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not when a deallocation occurs.
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</p><p>
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When the application requests memory (calling allocate()) we first
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look at the requested size and if this is >_S_max_bytes we call new()
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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 call to _S_get_thread_id() returns the thread id for the calling thread
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(and if no value has been set in _S_thread_key, a new id is assigned and
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returned).
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</p><p>
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A quick look at _S_bin[ bin ].first[ thread_id ] tells us if there are
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any blocks of this size on the current threads freelist. If this is
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not NULL - fine, just remove the block that _S_bin[ bin ].first[
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thread_id ] points to from the list, update _S_bin[ bin ].first[
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thread_id ], update the free and used counters and return a pointer to
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that blocks data.
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</p><p>
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If the freelist is empty (the pointer is NULL) we start by looking at
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the global freelist (0). If there are blocks available on the global
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freelist we lock this bins mutex and move up to block_count (the
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number of blocks of this bins size that will fit into a _S_chunk_size)
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or until end of list - whatever comes first - to the current threads
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freelist and at the same time change the thread_id ownership and
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update the counters and pointers. When the bins mutex has been
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unlocked, we remove the block that _S_bin[ bin ].first[ thread_id ]
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points to from the list, update _S_bin[ bin ].first[ thread_id ],
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update the free and used counters, and return a pointer to that blocks
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data.
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</p><p>
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The reason that the number of blocks moved to the current threads
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freelist is limited to block_count is to minimize the chance that a
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subsequent deallocate() call will return the excess blocks to the
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global freelist (based on the _S_freelist_headroom calculation, see
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below).
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</p><p>
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However if there isn't any memory on the global pool we need to get
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memory from the system - this is done in exactly the same way as in a
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single threaded application with one major difference; the list built
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in the newly allocated memory (of _S_chunk_size size) is added to the
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current threads freelist instead of to the global.
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</p><p>
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The basic process of a deallocation call is simple: always add the
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block to the front of the current threads freelist and update the
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counters and pointers (as described earlier with the specific check of
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ownership that causes the used counter of the thread that originally
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allocated the block to be decreased instead of the current threads
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counter).
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</p><p>
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And here comes the free and used counters to service. Each time a
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deallocation() call is made, the length of the current threads
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freelist is compared to the amount memory in use by this thread.
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</p><p>
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Let's go back to the example of an application that has one thread
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that does all the allocations and one that deallocates. Both these
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threads use say 516 32-byte blocks that was allocated during thread
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creation for example. Their used counters will both say 516 at this
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point. The allocation thread now grabs 1000 32-byte blocks and puts
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them in a shared container. The used counter for this thread is now
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1516.
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</p><p>
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The deallocation thread now deallocates 500 of these blocks. For each
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deallocation made the used counter of the allocating thread is
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decreased and the freelist of the deallocation thread gets longer and
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longer. But the calculation made in deallocate() will limit the length
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of the freelist in the deallocation thread to _S_freelist_headroom %
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of it's used counter. In this case, when the freelist (given that the
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_S_freelist_headroom is at it's default value of 10%) exceeds 52
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(516/10) blocks will be returned to the global pool where the
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allocating thread may pick them up and reuse them.
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</p><p>
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In order to reduce lock contention (since this requires this bins
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mutex to be locked) this operation is also made in chunks of blocks
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(just like when chunks of blocks are moved from the global freelist to
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a threads freelist mentioned above). The "formula" used can probably
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be improved to further reduce the risk of blocks being "bounced back
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and forth" between freelists.
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</p></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="bk01pt03ch20s04.html">Prev</a> </td><td align="center"><a accesskey="u" href="mt_allocator.html">Up</a></td><td align="right"> <a accesskey="n" href="bitmap_allocator.html">Next</a></td></tr><tr><td align="left" valign="top">Single Thread Example </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Chapter 21. The bitmap_allocator</td></tr></table></div></body></html>
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