90 lines
4.0 KiB
HTML
90 lines
4.0 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="GENERATOR" content="Microsoft FrontPage 3.0">
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<title>Mutex Documentation</title>
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</head>
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<body bgcolor="#FFFFFF">
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<h1>IBM's Classes for Unicode Synchronization Issues</h1>
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<h3>Introduction</h3>
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<p>There are a number of functions in the IBM's Classes for Unicode that need to access or
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allocate global or static data. For example, there is a global cache of Collation rules,
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which ensures that we do not need to load collation data from a file each time that a new
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Collator object is created. The first time a given Collator is loaded it is stored in the
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cache, and subsequent 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 example, the
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collation caching mechanism needs to be protected from simultaneous access by multiple
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threads; otherwise there could be problems with the data getting out of synch or with
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threads performing unnecessary work. </p>
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<h3>Mutexes</h3>
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<p>We prevent these problems by using a Mutex object. A Mutex is a "mutually
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exclusive" lock. Before accessing data which might be used by multiple threads,
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functions instantiate a Mutex object, which acquires the exclusive lock. An other thread
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that tries to access the data at the same time will also instantiate a Mutex, but the call
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will block until the first thread has released its lock. </p>
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<p>To save space, we use one underlying mutex implementation object for the entire
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application. An individual Mutex object simply acquires and releases the lock on this this
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global object. Since the implemention of a mutex is highly platform-dependent, developers
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who plan to use the International Classes for Unicode in a multithreaded environment are
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required to create their own mutex implementation object and register it with the system. </p>
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<h3>Re-Entrancy</h3>
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<p>Using a single, global lock object can, of course, cause reentrancy problems. Deadlock
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could occur where the Mutex aquire is attempted twice within the same thread before it is
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released. For example, Win32 critical sections are reentrant, but our testing shows that
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some POSIX mutex implementations are not. POSIX would require additional code, at a
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performance loss. </p>
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<p>To avoid these problems, the Mutex is only aquired during a pointer assignment, where
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possible. In the few cases where this is not true, care is taken to not call any other
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functions inside the mutex that could possibly aquire the mutex. </p>
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<p>The result of this design principle is that the mutex may be aquired more times than
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necessary, however time spent inside the mutex is then minimized. </p>
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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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<h3>Using collators in multi-threaded environment</h3>
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<p>Instances of Collator class are meant to be used on per thread basis. Although it is
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possible to have multiple threads access one Collator there is no guarante that such a
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construct will work, especially if number of threads grows over 10. There are no
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limitations on number of threads if each thread creates its own separate instance of
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Collator class.</p>
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<p>Test results have shown that case with 50 threads accessing 1 collator fails with a
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crash after 20 threads are reached. However, a test with 50 threads creating separate
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instances works well.</p>
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<h3>Implementations</h3>
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<p>On Win32 platforms, a reentrant mutex is most naturally implemented on top of a
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Critical Section.<br>
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On POSIX platforms, pthread_mutex provides an implementation. </p>
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<p>The International Classes for Unicode are provided with reference implementations for
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Win32 and POSIX. </p>
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<p><b>See also:</b>
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<ul>
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<li><b>mutex.h </b>- Mutex API</li>
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<li><b>muteximp.h</b> - The API's and instructions for providing your own mutexes.</li>
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<li><b>mutex.cpp</b> - includes reference implementations for Win32 and POSIX.</li>
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</ul>
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</body>
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</html>
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