Why does multiprocessing.Queue have a small delay while (apparen

Why does multiprocessing.Queue have a small delay while (apparently) multiprocessing.Pipe does not?

Documentation for multiprocessing.Queue points out that there's a bit of a delay from when an item is enqueued until it's pickled representation is flushed to the underlying Pipe. Apparently though you can enqueue an item straight into a Pipe (it doesn't say otherwise and implies that's the case).

Why doesn't the Pipe need - or have - the same background thread to do the pickling? And is that the same reason that there's no similar delay when talking to a multiprocessor.SyncManager.Queue?

(Bonus question: What does the documentation mean when it says "After putting an object on an empty queue there may be an infinitesimal delay ..."? I've taken calculus; I know what infinitesimal means, and that meaning doesn't seem to fit here. So what is it talking about?)

Answer

The behavior you’re observing stems from the architectural and functional differences between multiprocessing.Queue, multiprocessing.Pipe, and multiprocessing.managers.SyncManager.Queue. Let’s address your questions one at a time:

1. Why doesn’t Pipe need or have the same background thread for pickling as Queue?

The key distinction lies in the purpose and design of Queue vs. Pipe:

• multiprocessing.Queue:

  • Designed as a high-level abstraction for safe inter-process communication.
  • It operates with a background thread to handle pickling and transferring data. When you call put() on a Queue, the object is serialized (pickled) and then transmitted through an underlying Pipe. This thread ensures the Queue is asynchronous, meaning your main process doesn’t block while waiting for the data to be serialized and sent.
  • This abstraction introduces a slight delay because the data is handed off to the background thread before being sent to the Pipe.

• multiprocessing.Pipe:

  • A lower-level primitive designed for direct communication between processes.
  • Pipe doesn’t use a background thread for pickling. When you send data through a Pipe, the pickling (if needed) and data transfer happen immediately in the context of your main thread. This is why there is no additional delay—there’s no intermediary managing the process.

In short, the Pipe doesn’t need a background thread because it doesn’t aim to provide the same level of abstraction or asynchronous behavior as the Queue.

2. Why is there no similar delay with multiprocessor.SyncManager.Queue?

A multiprocessor.SyncManager.Queue operates differently because it’s a proxy object backed by a server process:

• When you interact with a SyncManager.Queue, you're not directly putting or getting data from the queue. Instead, the put() and get() calls are proxied to a server process, which manages the actual queue. The manager process handles the pickling and communication with client processes.
• The absence of delay may arise from the synchronous nature of the calls. When you call put() on a SyncManager.Queue, the operation completes in the context of your process before returning. Any delays in pickling or transferring data are effectively "hidden" because they’re part of the put() execution time.

In contrast, multiprocessing.Queue uses a background thread to avoid blocking your process during put() operations, which creates the potential for a delay while the thread processes the data.

3. What does the documentation mean by "infinitesimal delay"?

The phrase "infinitesimal delay" is somewhat misleading in this context and appears to be a colloquial way of saying "very short delay" rather than using the mathematical definition of infinitesimal.

In this case, it refers to the extremely small, but non-zero, time it takes for the background thread of multiprocessing.Queue to notice that the Queue is no longer empty and begin processing the enqueued data. This delay is typically negligible, but it’s still worth mentioning because it technically exists and could theoretically impact high-frequency or latency-sensitive operations.