优雅地退出无限循环线程

Question

I keep running into this problem of trying to run a thread with the following properties:

1. runs in an infinite loop, checking some external resource, eg data from the network or a device,
2. gets updates from its resource promptly,
3. exits promptly when asked to,
4. uses the CPU efficiently.

First approach

One solution I have seen for this is something like the following:

void class::run()
{
    while(!exit_flag)
    {
        if (resource_ready)
            use_resource();
    }
}

This satisfies points 1, 2 and 3, but being a busy waiting loop, uses 100% CPU.

Second approach

A potential fix for this is to put a sleep statement in:

void class::run()
{
    while(!exit_flag)
    {
        if (resource_ready)
            use_resource();
        else
            sleep(a_short_while);
    }
}

We now don't hammer the CPU, so we address 1 and 4, but we could wait up to a_short_while unnecessarily when the resource is ready or we are asked to quit.

Third approach

A third option is to do a blocking read on the resource:

void class::run()
{
    while(!exit_flag)
    {
        obtain_resource();
        use_resource();
    }
}

This will satisfy 1, 2, and 4 elegantly, but now we can't ask the thread to quit if the resource does not become available.

Question

The best approach seems to be the second one, with a short sleep, so long as the tradeoff between CPU usage and responsiveness can be achieved. However, this still seems suboptimal, and inelegant to me. This seems like it would be a common problem to solve. Is there a more elegant way to solve it? Is there an approach which can address all four of those requirements?

Answer 1

This depends on the specifics of the resources the thread is accessing, but basically to do it efficiently with minimal latency, the resources need to provide an API for either doing an interruptible blocking wait.

On POSIX systems, you can use the select(2) or poll(2) system calls to do that, if the resources you're using are files or file descriptors (including sockets). To allow the wait to be preempted, you also create a dummy pipe which you can write to.

For example, here's how you might wait for a file descriptor or socket to become ready or for the code to be interrupted:

// Dummy pipe used for sending interrupt message
int interrupt_pipe[2];
int should_exit = 0;

void class::run()
{
    // Set up the interrupt pipe
    if (pipe(interrupt_pipe) != 0)
        ;  // Handle error

    int fd = ...;  // File descriptor or socket etc.
    while (!should_exit)
    {
        // Set up a file descriptor set with fd and the read end of the dummy
        // pipe in it
        fd_set fds;
        FD_CLR(&fds);
        FD_SET(fd, &fds);
        FD_SET(interrupt_pipe[1], &fds);
        int maxfd = max(fd, interrupt_pipe[1]);

        // Wait until one of the file descriptors is ready to be read
        int num_ready = select(maxfd + 1, &fds, NULL, NULL, NULL);
        if (num_ready == -1)
            ; // Handle error

        if (FD_ISSET(fd, &fds))
        {
            // fd can now be read/recv'ed from without blocking
            read(fd, ...);
        }
    }
}

void class::interrupt()
{
    should_exit = 1;

    // Send a dummy message to the pipe to wake up the select() call
    char msg = 0;
    write(interrupt_pipe[0], &msg, 1);
}

class::~class()
{
    // Clean up pipe etc.
    close(interrupt_pipe[0]);
    close(interrupt_pipe[1]);
}

If you're on Windows, the select() function still works for sockets, but only for sockets, so you should install use WaitForMultipleObjects to wait on a resource handle and an event handle. For example:

// Event used for sending interrupt message
HANDLE interrupt_event;
int should_exit = 0;

void class::run()
{
    // Set up the interrupt event as an auto-reset event
    interrupt_event = CreateEvent(NULL, FALSE, FALSE, NULL);
    if (interrupt_event == NULL)
        ;  // Handle error

    HANDLE resource = ...;  // File or resource handle etc.
    while (!should_exit)
    {
        // Wait until one of the handles becomes signaled
        HANDLE handles[2] = {resource, interrupt_event};
        int which_ready = WaitForMultipleObjects(2, handles, FALSE, INFINITE);    
        if (which_ready == WAIT_FAILED)
            ; // Handle error
        else if (which_ready == WAIT_OBJECT_0))
        {
            // resource can now be read from without blocking
            ReadFile(resource, ...);
        }
    }
}

void class::interrupt()
{
    // Signal the event to wake up the waiting thread
    should_exit = 1;
    SetEvent(interrupt_event);
}

class::~class()
{
    // Clean up event etc.
    CloseHandle(interrupt_event);
}

Answer 2

You get a efficient solution if your obtain_ressource() function supports a timeout value:

while(!exit_flag)
{
    obtain_resource_with_timeout(a_short_while);
    if (resource_ready)
        use_resource();
}

This effectively combines the sleep() with the obtain_ressurce() call.

Answer 3

Check out the manpage for nanosleep :

If the nanosleep() function returns because it has been interrupted by a signal, the function returns a value of -1 and sets errno to indicate the interruption.

In other words, you can interrupt sleeping threads by sending a signal (the sleep manpage says something similar). This means you can use your 2nd approach, and use an interrupt to immediately wake the thread if it's sleeping.

Answer 4

Use the Gang of Four Observer Pattern:

http://home.comcast.net/~codewrangler/tech_info/patterns_code.html#Observer

Callback, don't block.

Answer 5

Self-Pipe trick can be used here. http://cr.yp.to/docs/selfpipe.html Assuming that you are reading the data from file descriptor.

Create a pipe and select() for readability on the pipe input as well as on the resource you are interested. Then when data comes on resource, the thread wakes up and does the processing. Else it sleeps. To terminate the thread send it a signal and in signal handler, write something on the pipe (I would say something which will never come from the resource you are interested in, something like NULL for illustrating the point). The select call returns and thread on reading the input knows that it got the poison pill and it is time to exit and calls pthread_exit().

EDIT: Better way will be just to see that the data came on the pipe and hence just exit rather than checking the value which came on that pipe.

Answer 6

The Win32 API uses more or less this approach:

someThreadLoop( ... )
{
  MSG msg;
  int retVal;

  while( (retVal = ::GetMessage( &msg, TaskContext::winHandle_, 0, 0 )) > 0 )
  {
    ::TranslateMessage( &msg );
    ::DispatchMessage( &msg );
  }
}

GetMessage itself blocks until any type of message is received therefore not using any processing ( refer ). If a WM_QUIT is received, it returns false, exiting the thread function gracefully. This is a variant of the producer/consumer mentioned elsewhere.

You can use any variant of a producer/consumer, and the pattern is often similar. One could argue that one would want to split the responsibility concerning quitting and obtaining of a resource, but OTOH quitting could depend on obtaining a resource too (or could be regarded as one of the resources - but a special one). I would at least abstract the producer consumer pattern and have various implementations thereof.

Therefore:

AbstractConsumer:

void AbstractConsumer::threadHandler()
{
  do
  {
    try
    {        
      process( dequeNextCommand() ); 
    }
    catch( const base_except& ex )
    {
      log( ex );
      if( ex.isCritical() ){ throw; }
      //else we don't want loop to exit...
    }
    catch( const std::exception& ex )
    {
      log( ex );
      throw; 
    }
  }
  while( !terminated() );
}

virtual void /*AbstractConsumer::*/process( std::unique_ptr<Command>&& command ) = 0;
//Note: 
// Either may or may not block until resource arrives, but typically blocks on
// a queue that is signalled as soon as a resource is available.
virtual std::unique_ptr<Command> /*AbstractConsumer::*/dequeNextCommand() = 0;
virtual bool /*AbstractConsumer::*/terminated() const = 0;

I usually encapsulate command to execute a function in the context of the consumer, but the pattern in the consumer is always the same.

Answer 7

Any (welln at least, most) approaches mentioned above will do the following: thread is created, then it's blocked wwiting for resource, then it's deleted.

If you're worried about efficiency, this is not a best approach when waiting for IO. On Windows at least, you'll allocate around 1mb of memory in user mode, some in kernel for just one additional thread. What if you have many such resources? Having many waiting threads will also increase context switches and slow down your program. What if resource takes longer to be available and many requests are made? You may end up with tons of waiting threads.

Now, the solution to it (again, on Windows, but I'm sure there should be something similar on other OSes) is using threadpool (the one provided by Windows). On Windows this will not only create limited amount of threads, it'll be able to detect when thread is waiting for IO and will stwal thread from there and reuse it for other operations while waitting.

See http://msdn.microsoft.com/en-us/library/windows/desktop/ms686766(v=vs.85).aspx

Also, for more fine-grained control bit still having ability give up thread when waiting for IO, see IO completion ports (I think they'll anyway use threadpool inside): http://msdn.microsoft.com/en-us/library/windows/desktop/aa365198(v=vs.85).aspx