1999-08-16 21:50:52 +00:00
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1999-08-16 21:50:52 +00:00
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2000-01-15 02:00:06 +00:00
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<link rel=File-List href="./mutex_files/filelist.xml">
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1999-08-16 21:50:52 +00:00
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<title>Mutex Documentation</title>
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2000-01-15 02:00:06 +00:00
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<o:DocumentProperties>
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<o:Author>Helena Shih</o:Author>
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<o:Template>Normal</o:Template>
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<o:LastAuthor>Helena Shih</o:LastAuthor>
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1999-08-16 21:50:52 +00:00
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</head>
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2000-01-15 02:00:06 +00:00
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<body bgcolor=white lang=EN-US style='tab-interval:.5in'>
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1999-08-16 21:50:52 +00:00
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2000-01-15 02:00:06 +00:00
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<div class=Section1>
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2000-02-21 21:38:59 +00:00
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<h1>International Components for Unicode Synchronization Issues</h1>
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1999-08-16 21:50:52 +00:00
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<h3>Introduction</h3>
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2000-01-15 02:00:06 +00:00
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<p>There are a number of functions in the IBM's Classes for Unicode that need
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to access or allocate global or static data. For example, there is a global
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cache of Collation rules, which ensures that we do not need to load collation
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data from a file each time that a new Collator object is created. The first
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time a given Collator is loaded it is stored in the cache, and subsequent
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accesses are extremely fast. </p>
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<p>In a single-threaded environment, this is all straightforward. However, in a
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multithreaded application there are synchronization issues to deal with. For
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example, the collation caching mechanism needs to be protected from
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simultaneous access by multiple threads; otherwise there could be problems with
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the data getting out of synch or with threads performing unnecessary work. </p>
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1999-08-16 21:50:52 +00:00
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<h3>Mutexes</h3>
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2000-01-15 02:00:06 +00:00
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<p>We prevent these problems by using a Mutex object. A Mutex is a
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"mutually exclusive" lock. Before accessing data which might be used
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by multiple threads, functions instantiate a Mutex object, which acquires the
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exclusive lock. An other thread that tries to access the data at the same time
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will also instantiate a Mutex, but the call will block until the first thread
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has released its lock. </p>
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1999-08-16 21:50:52 +00:00
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2000-01-15 02:00:06 +00:00
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<p>To save space, we use one underlying mutex implementation object for the
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entire application. An individual Mutex object simply acquires and releases the
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lock on this this global object. Since the implemention of a mutex is highly
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platform-dependent, developers who plan to use the International Classes for
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Unicode in a multithreaded environment are required to create their own mutex
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implementation object and register it with the system. </p>
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1999-08-16 21:50:52 +00:00
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<h3>Re-Entrancy</h3>
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2000-01-15 02:00:06 +00:00
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<p>Using a single, global lock object can, of course, cause reentrancy
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problems. Deadlock could occur where the Mutex aquire is attempted twice within
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the same thread before it is released. For example, Win32 critical sections are
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reentrant, but our testing shows that some POSIX mutex implementations are not.
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POSIX would require additional code, at a performance loss. </p>
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1999-08-16 21:50:52 +00:00
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2000-01-15 02:00:06 +00:00
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<p>To avoid these problems, the Mutex is only aquired during a pointer
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assignment, where possible. In the few cases where this is not true, care is
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taken to not call any other functions inside the mutex that could possibly
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aquire the mutex. </p>
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1999-08-16 21:50:52 +00:00
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2000-01-15 02:00:06 +00:00
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<p>The result of this design principle is that the mutex may be aquired more
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times than necessary, however time spent inside the mutex is then minimized. </p>
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1999-08-16 21:50:52 +00:00
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<p>Developers implementing the Mutex are not required to provide reentrant-safe
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implementations. </p>
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1999-10-28 18:06:45 +00:00
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<h3>Using collators in multi-threaded environment</h3>
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2000-01-15 02:00:06 +00:00
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<p>Instances of Collator class are meant to be used on per thread basis.
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Although it is possible to have multiple threads access one Collator there is
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no guarante that such a construct will work, especially if number of threads
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grows over 10. There are no limitations on number of threads if each thread
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creates its own separate instance of Collator class.</p>
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1999-10-28 18:06:45 +00:00
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2000-01-15 02:00:06 +00:00
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<p>Test results have shown that case with 50 threads accessing 1 collator fails
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with a crash after 20 threads are reached. However, a test with 50 threads
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creating separate instances works well.</p>
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1999-10-28 18:06:45 +00:00
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1999-08-16 21:50:52 +00:00
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<h3>Implementations</h3>
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2000-01-15 02:00:06 +00:00
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<p>On Win32 platforms, a reentrant mutex is most naturally implemented on top
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of a Critical Section.<br>
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1999-08-16 21:50:52 +00:00
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On POSIX platforms, pthread_mutex provides an implementation. </p>
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2000-01-15 02:00:06 +00:00
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<p>The International Classes for Unicode are provided with reference
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implementations for Win32 and POSIX. </p>
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1999-08-16 21:50:52 +00:00
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2000-01-15 02:00:06 +00:00
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<p><b>See also:</b> </p>
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<ul type=disc>
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<li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
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mso-list:l0 level1 lfo1;tab-stops:list .5in'><b>mutex.h </b>- Mutex API</li>
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<li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
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mso-list:l0 level1 lfo1;tab-stops:list .5in'><b>muteximp.h</b> - The API's
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and instructions for providing your own mutexes.</li>
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<li class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;
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mso-list:l0 level1 lfo1;tab-stops:list .5in'><b>mutex.cpp</b> - includes
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reference implementations for Win32 and POSIX.</li>
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1999-08-16 21:50:52 +00:00
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</ul>
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</div>
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1999-08-16 21:50:52 +00:00
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</body>
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2000-01-15 02:00:06 +00:00
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1999-08-16 21:50:52 +00:00
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</html>
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