Multiprocessing python queue9/17/2023 ![]() Or newly created producer processes can fail to deserialise the Process object of the consumer process if the synchronisation resources of the queue that comes with it as an attribute are garbage collected before, raising a FileNotFoundError: import multiprocessing.queues Pass # reached before the producer process adds the item to the queue P = multiprocessing.Process(target=f, args=(q,)) ![]() Multiprocessing.Process(target=f, args=(q, e)).start() Q.put('X' * 1000000) # block the feeder thread (size > pipe capacity) In this situation, a reliable way to empty the queue is to make each producer process add a sentinel item when it is done and make the consumer process remove items (regular and sentinel items) until it has removed as many sentinel items as there are producer processes: import multiprocessing In some applications, a consumer process may not know how many items have been added to a queue by producer processes. Remember also that non-daemonic processes will be joined automatically.Īn example which will deadlock is the following: from multiprocessing import Process, QueueĪ fix here would be to swap the last two lines (or simply remove the p.join() line). ![]() Otherwise you cannot be sure that processes which have put items on the queue will terminate. This means that whenever you use a queue you need to make sure that all items which have been put on the queue will eventually be removed before the process is joined. (The child process can call the Queue.cancel_join_thread method of the queue to avoid this behaviour.) That is why the multiprocessing Python library documentation recommends to make a consumer process empty each Queue object with Queue.get calls before its feeder threads are joined in producer processes (implicitly with garbage collection or explicitly with Queue.join_thread calls):īear in mind that a process that has put items in a queue will wait before terminating until all the buffered items are fed by the “feeder” thread to the underlying pipe. Since Linux 2.6.35, the default pipe capacity is 16 pages, but the capacity can be queried and set using the fcntl(2) F_GETPIPE_SZ and F_SETPIPE_SZ operations. Since Linux 2.6.11, the pipe capacity is 16 pages (i.e., 65,536 bytes in a system with a page size of 4096 bytes). In Linux versions before 2.6.11, the capacity of a pipe was the same as the system page size (e.g., 4096 bytes on i386). Applications should not rely on a particular capacity: an application should be designed so that a reading process consumes data as soon as it is available, so that a writing process does not remain blocked. Different implementations have different limits for the pipe capacity. If the pipe is full, then a write(2) will block or fail, depending on whether the O_NONBLOCK flag is set (see below). Nonblocking I/O is possible by using the fcntl(2) F_SETFL operation to enable the O_NONBLOCK open file status flag.Ī pipe has a limited capacity. If a process attempts to write to a full pipe (see below), then write(2) blocks until sufficient data has been read from the pipe to allow the write to complete. If a process attempts to read from an empty pipe, then read(2) will block until data is available. Pipe(7) Linux manual page specifies that a pipe has a limited capacity (65,536 bytes by default) and that writing to a full pipe blocks until enough data has been read from the pipe to allow the write to complete:
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