On Sat, Aug 20, 2016 at 7:28 AM, Gabi Melman <[email protected]> wrote: > Hi Dmitry, > > I am the the author of spdlog (https://github.com/gabime/spdlog) lib which > use your mpmc queue (a siglhly modified version: > https://github.com/gabime/spdlog/blob/master/include/spdlog/details/mpmc_bounded_q.h) > > I have quetion My about what would be the best/accurate way to get the size > of the queue. > I came up with : > > > size_t approx_size() > { > size_t first_pos = dequeue_pos_.load(std::memory_order_relaxed); > size_t last_pos = enqueue_pos_.load(std::memory_order_relaxed); > return last_pos > first_pos ? last_pos - first_pos : 0; > } > > but I am not sure it is accurate or whether it could be replaced with > something better.
+lock-free group Hi Gabi, Your code is a reasonable approximation on the queue size. You can also cap it by maximum capacity, because you can read dequeue_pos_, then lots of items are pushed/popped, then you read enqueue_pos_ and the result can be arbitrary large. If you want more precise size, you can get an atomic snapshot using the following technique. It will yield some actual size of the queue while size operation runs. Here is an example from Go runtime: https://github.com/golang/go/blob/master/src/runtime/proc.go#L4005 And here is a similar one from TensorFlow runtime: https://bitbucket.org/eigen/eigen/src/cd5824eb9fb275e5b0edc8a7af95e8af85fe9231/unsupported/Eigen/CXX11/src/ThreadPool/RunQueue.h?at=default&fileviewer=file-view-default#RunQueue.h-155 In both these cases that size does not spuriously return 0 if the queue never become empty while the operation runs. In either case I would suggest to write some stress tests. For example, you can write a test where the queue never becomes empty and ensure that size never returns 0. Of you can maintain size between 5 and 10 and ensure that size returns values in that range. -- --- You received this message because you are subscribed to the Google Groups "Scalable Synchronization Algorithms" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/lock-free/CAEeQi3sOeTMw_Tr4SBneo8tJJVGji9VMfyM-arYV-idoNg7Xnw%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
