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June 21, 2012
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folly/ProducerConsumerQueue.h

The folly::ProducerConsumerQueue class is a one-producer one-consumer queue with very low synchronization overhead.

The queue must be created with a fixed maximum size (and allocates that many cells of sizeof(T)), and it provides just a few simple operations:

  • read: Attempt to read the value at the front to the queue into a variable, returns false iff queue was empty.
  • write: Emplace a value at the end of the queue, returns false iff the queue was full.
  • frontPtr: Retrieve a pointer to the item at the front of the queue, or nullptr if it is empty.
  • popFront: Remove the item from the front of the queue (queue must not be empty).
  • isEmpty: Check if the queue is empty.
  • isFull: Check if the queue is full.
  • sizeGuess: Returns the number of entries in the queue. Because of the way we coordinate threads, this guess could be slightly wrong when called by the producer/consumer thread, and it could be wildly inaccurate if called from any other threads. Hence, only call from producer/consumer threads!

All of these operations are wait-free. The read operations (including frontPtr and popFront) and write operations must only be called by the reader and writer thread, respectively. isFull, isEmpty, and sizeGuess may be called by either thread, but the return values from read, write, or frontPtr are sufficient for most cases.

write may fail if the queue is full, and read may fail if the queue is empty, so in many situations it is important to choose the queue size such that the queue filling or staying empty for long is unlikely.

Example


A toy example that doesn't really do anything useful:

    folly::ProducerConsumerQueue<folly::fbstring> queue;

    std::thread reader([&queue] {
      for (;;) {
        folly::fbstring str;
        while (!queue.read(str)) {
          //spin until we get a value
          continue;
        }

        sink(str);
      }
    });

    // producer thread:
    for (;;) {
      folly::fbstring str = source();
      while (!queue.write(str)) {
        //spin until the queue has room
        continue;
      }
    }

Alternatively, the consumer may be written as follows to use the 'front' value in place, thus avoiding moves or copies:

    std::thread reader([&queue] {
      for (;;) {
        folly::fbstring* pval;
        do {
          pval = queue.frontPtr();
        } while (!pval); // spin until we get a value;

        sink(*pval);
        queue.popFront();
      }
    });
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