148 lines
6.3 KiB
Plaintext
148 lines
6.3 KiB
Plaintext
/****************************************************************************
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**
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** Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
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** This file is part of the documentation of the Qt Toolkit.
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**
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****************************************************************************/
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/*!
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\example semaphores
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\title Semaphores Example
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\brief Demonstrates multi-thread programming using Qt
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\ingroup qtconcurrent-mtexamples
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\brief The Semaphores example shows how to use QSemaphore to control
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access to a circular buffer shared by a producer thread and a
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consumer thread.
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The producer writes data to the buffer until it reaches the end
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of the buffer, at which point it restarts from the beginning,
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overwriting existing data. The consumer thread reads the data as
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it is produced and writes it to standard error.
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Semaphores make it possible to have a higher level of concurrency
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than mutexes. If accesses to the buffer were guarded by a QMutex,
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the consumer thread couldn't access the buffer at the same time
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as the producer thread. Yet, there is no harm in having both
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threads working on \e{different parts} of the buffer at the same
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time.
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The example comprises two classes: \c Producer and \c Consumer.
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Both inherit from QThread. The circular buffer used for
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communicating between these two classes and the semaphores that
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protect it are global variables.
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An alternative to using QSemaphore to solve the producer-consumer
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problem is to use QWaitCondition and QMutex. This is what the
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\l{threads/waitconditions}{Wait Conditions} example does.
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\section1 Global Variables
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Let's start by reviewing the circular buffer and the associated
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semaphores:
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\snippet semaphores/semaphores.cpp 0
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\c DataSize is the amout of data that the producer will generate.
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To keep the example as simple as possible, we make it a constant.
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\c BufferSize is the size of the circular buffer. It is less than
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\c DataSize, meaning that at some point the producer will reach
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the end of the buffer and restart from the beginning.
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To synchronize the producer and the consumer, we need two
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semaphores. The \c freeBytes semaphore controls the "free" area
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of the buffer (the area that the producer hasn't filled with data
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yet or that the consumer has already read). The \c usedBytes
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semaphore controls the "used" area of the buffer (the area that
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the producer has filled but that the consumer hasn't read yet).
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Together, the semaphores ensure that the producer is never more
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than \c BufferSize bytes ahead of the consumer, and that the
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consumer never reads data that the producer hasn't generated yet.
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The \c freeBytes semaphore is initialized with \c BufferSize,
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because initially the entire buffer is empty. The \c usedBytes
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semaphore is initialized to 0 (the default value if none is
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specified).
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\section1 Producer Class
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Let's review the code for the \c Producer class:
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\snippet semaphores/semaphores.cpp 1
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\snippet semaphores/semaphores.cpp 2
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The producer generates \c DataSize bytes of data. Before it
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writes a byte to the circular buffer, it must acquire a "free"
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byte using the \c freeBytes semaphore. The QSemaphore::acquire()
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call might block if the consumer hasn't kept up the pace with the
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producer.
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At the end, the producer releases a byte using the \c usedBytes
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semaphore. The "free" byte has successfully been transformed into
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a "used" byte, ready to be read by the consumer.
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\section1 Consumer Class
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Let's now turn to the \c Consumer class:
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\snippet semaphores/semaphores.cpp 3
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\snippet semaphores/semaphores.cpp 4
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The code is very similar to the producer, except that this time
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we acquire a "used" byte and release a "free" byte, instead of
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the opposite.
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\section1 The main() Function
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In \c main(), we create the two threads and call QThread::wait()
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to ensure that both threads get time to finish before we exit:
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\snippet semaphores/semaphores.cpp 5
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\snippet semaphores/semaphores.cpp 6
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So what happens when we run the program? Initially, the producer
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thread is the only one that can do anything; the consumer is
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blocked waiting for the \c usedBytes semaphore to be released (its
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initial \l{QSemaphore::available()}{available()} count is 0).
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Once the producer has put one byte in the buffer,
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\c{freeBytes.available()} is \c BufferSize - 1 and
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\c{usedBytes.available()} is 1. At that point, two things can
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happen: Either the consumer thread takes over and reads that
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byte, or the consumer gets to produce a second byte.
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The producer-consumer model presented in this example makes it
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possible to write highly concurrent multithreaded applications.
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On a multiprocessor machine, the program is potentially up to
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twice as fast as the equivalent mutex-based program, since the
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two threads can be active at the same time on different parts of
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the buffer.
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Be aware though that these benefits aren't always realized.
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Acquiring and releasing a QSemaphore has a cost. In practice, it
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would probably be worthwhile to divide the buffer into chunks and
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to operate on chunks instead of individual bytes. The buffer size
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is also a parameter that must be selected carefully, based on
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experimentation.
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*/
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