parallel.pool.PollableDataQueue

Class that enables sending and polling for data between client and workers

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Description

parallel.pool.PollableDataQueue enables sending and polling for data or messages between workers and client in a parallel pool while a computation is being carried out. You can get intermediate values and progress of the computation.

To send data from a parallel pool worker back to the client, first construct a PollableDataQueue in the client. Pass this PollableDataQueue into a parfor-loop or other parallel language construct, such as parfeval. From the workers, call send to send data back to the client. At the client, use poll to retrieve the result of a message or data sent from a worker.

  • You can call send from the process that calls the constructor, if required.

  • You can construct the queue on the workers and send it back to the client to enable communication in the reverse direction. However, you cannot send a queue from one worker to another. Use spmd, labSend, or labReceive instead.

  • Unlike all other handle objects, PollableDataQueue instances do remain connected when they are sent to workers.

Construction

p = parallel.pool.PollableDataQueue

The constructor for a PollableDataQueue takes no arguments and returns an object that can be used to send and poll for messages (or data) from different workers. You call the constructor only in the process where you want to receive the data. In the usual workflow, the workers should not be calling the constructor, but should be handed an existing PollableDataQueue instance instead.

Properties

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Read-only property that indicates how many items of data are waiting to be removed from the queue.

Example: 

pollableQ = parallel.pool.PollableDataQueue;
% No messages in queue because nothing has been sent yet.
pollableQ.QueueLength

ans =

     0
pollableQ.send('A message')
% Now QueueLength = 1 because one message is sent and not yet polled.
pollableQ.QueueLength

ans =

     1
msg = pollableQ.poll();
disp(msg)

A message

pollableQ.QueueLength
% Now QueueLength = 0 because there are no more pending messages.

ans =

     0

Methods

A parallel.pool.PollableDataQueue object has the following methods.

poll Retrieve data sent from a worker
sendSend data from worker to client using a data queue

Copy Semantics

Handle. To learn how handle classes affect copy operations, see Copying Objects (MATLAB).

Examples

Send a Message in a parfor-loop, and Poll for the Result

Construct a PollableDataQueue. 

p = parallel.pool.PollableDataQueue;

Start a parfor-loop, and send a message, such as data with the value 1. 

parfor i = 1
    send(p, i); 
end
Poll for the result.

poll(p)
1

For more details on polling for data using a PollableDataQueue, see poll.

See Also

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Topics

Introduced in R2017a

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