waitconditions.qdoc

// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \example threads/waitconditions
    \examplecategory {Data Processing & I/O}
    \title Producer and Consumer using Wait Conditions
    \ingroup qtconcurrent-mtexamples

    \brief The Producer and Consumer using Wait Conditions example shows
    how to use QWaitCondition and QMutex to control access to a circular
    buffer shared by a producer thread and a consumer thread.

    The producer writes data to the buffer until it reaches the end
    of the buffer, at which point it restarts from the beginning,
    overwriting existing data. The consumer thread reads the data as
    it is produced and writes it to standard error.

    Wait conditions make it possible to have a higher level of
    concurrency than what is possible with mutexes alone. If accesses
    to the buffer were simply guarded by a QMutex, the consumer
    thread couldn't access the buffer at the same time as the
    producer thread. Yet, there is no harm in having both threads
    working on \e{different parts} of the buffer at the same time.

    The example comprises two classes: \c Producer and \c Consumer.
    Both inherit from QThread. The circular buffer used for
    communicating between these two classes and the synchronization
    tools that protect it are global variables.

    An alternative to using QWaitCondition and QMutex to solve the
    producer-consumer problem is to use QSemaphore. This is what the
    \l{Producer and Consumer using Semaphores} example does.

    \section1 Global Variables

    Let's start by reviewing the circular buffer and the associated
    synchronization tools:

    \snippet threads/waitconditions/waitconditions.cpp 0

    \c DataSize is the amount of data that the producer will generate.
    To keep the example as simple as possible, we make it a constant.
    \c BufferSize is the size of the circular buffer. It is less than
    \c DataSize, meaning that at some point the producer will reach
    the end of the buffer and restart from the beginning.

    To synchronize the producer and the consumer, we need two wait
    conditions and one mutex. The \c bufferNotEmpty condition is
    signalled when the producer has generated some data, telling the
    consumer that it can start reading it. The \c bufferNotFull
    condition is signalled when the consumer has read some data,
    telling the producer that it can generate more. The \c numUsedBytes
    is the number of bytes in the buffer that contain data.

    Together, the wait conditions, the mutex, and the \c numUsedBytes
    counter ensure that the producer is never more than \c BufferSize
    bytes ahead of the consumer, and that the consumer never reads
    data that the producer hasn't generated yet.

    \section1 Producer Class

    Let's review the code for the \c Producer class:

    \snippet threads/waitconditions/waitconditions.cpp 1
    \snippet threads/waitconditions/waitconditions.cpp 2

    The producer generates \c DataSize bytes of data. Before it
    writes a byte to the circular buffer, it must first check whether
    the buffer is full (i.e., \c numUsedBytes equals \c BufferSize).
    If the buffer is full, the thread waits on the \c bufferNotFull
    condition.

    At the end, the producer increments \c numUsedBytes and signalls
    that the condition \c bufferNotEmpty is true, since \c
    numUsedBytes is necessarily greater than 0.

    We guard all accesses to the \c numUsedBytes variable with a
    mutex. In addition, the QWaitCondition::wait() function accepts a
    mutex as its argument. This mutex is unlocked before the thread
    is put to sleep and locked when the thread wakes up. Furthermore,
    the transition from the locked state to the wait state is atomic,
    to prevent race conditions from occurring.

    \section1 Consumer Class

    Let's turn to the \c Consumer class:

    \snippet threads/waitconditions/waitconditions.cpp 3
    \snippet threads/waitconditions/waitconditions.cpp 4

    The code is very similar to the producer. Before we read the
    byte, we check whether the buffer is empty (\c numUsedBytes is 0)
    instead of whether it's full and wait on the \c bufferNotEmpty
    condition if it's empty. After we've read the byte, we decrement
    \c numUsedBytes (instead of incrementing it), and we signal the
    \c bufferNotFull condition (instead of the \c bufferNotEmpty
    condition).

    \section1 The main() Function

    In \c main(), we create the two threads and call QThread::wait()
    to ensure that both threads get time to finish before we exit:

    \snippet threads/waitconditions/waitconditions.cpp 5
    \snippet threads/waitconditions/waitconditions.cpp 6

    So what happens when we run the program? Initially, the producer
    thread is the only one that can do anything; the consumer is
    blocked waiting for the \c bufferNotEmpty condition to be
    signalled (\c numUsedBytes is 0). Once the producer has put one
    byte in the buffer, \c numUsedBytes is strictly greater than 0, and the
    \c bufferNotEmpty condition is signalled. At that point, two
    things can happen: Either the consumer thread takes over and
    reads that byte, or the producer gets to produce a second byte.

    The producer-consumer model presented in this example makes it
    possible to write highly concurrent multithreaded applications.
    On a multiprocessor machine, the program is potentially up to
    twice as fast as the equivalent mutex-based program, since the
    two threads can be active at the same time on different parts of
    the buffer.

    Be aware though that these benefits aren't always realized.
    Locking and unlocking a QMutex has a cost. In practice, it would
    probably be worthwhile to divide the buffer into chunks and to
    operate on chunks instead of individual bytes. The buffer size is
    also a parameter that must be selected carefully, based on
    experimentation.
*/