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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.
A toy example that doesn’t really do anything useful:
::ProducerConsumerQueue<folly::fbstring> queue{size};
folly
std::thread reader([&queue] {
for (;;) {
::fbstring str;
follywhile (!queue.read(str)) {
//spin until we get a value
continue;
}
(str);
sink}
});
// producer thread:
for (;;) {
::fbstring str = source();
follywhile (!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 (;;) {
::fbstring* pval;
follydo {
= queue.frontPtr();
pval } while (!pval); // spin until we get a value;
(*pval);
sink.popFront();
queue}
});