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labReceive

Receive data from another worker in an spmd block

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    Syntax

    B = labReceive
    B = labReceive(source)
    B = labReceive('any')
    B = labReceive('any',tag)
    B = labReceive(source,tag)
    [B,source,tag] = labReceive(___)

    Description

    example

    B = labReceive receives data sent from any worker in the current spmd block or communicating job.

    Tip

    When you offload computations using parfor and parfeval, each computation is run by only one worker at a time. These workers are independent and do not communicate with each other. If you use labReceive on these workers, the function has no effect.

    When a worker runs labReceive, execution of other commands is blocked until the worker receives the data.

    To use labReceive, numlabs must be greater than 1.

    B = labReceive(source) receives data sent from the worker with labindex equal to source.

    example

    B = labReceive('any') receives data from any worker.

    example

    B = labReceive('any',tag) receives data sent with the tag tag from any worker.

    B = labReceive(source,tag) receives data sent with the tag tag from the worker with labindex equal to source.

    [B,source,tag] = labReceive(___) receives data sent from another worker, returns the index source of the source worker, and returns the tag tagwith the data.

    Examples

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    This example shows how to send data between workers in an spmd block or communicating job.

    Create a parallel pool with 4 workers. By default, spmd is supported on all process-backed pools.

    parpool(4);

    When you execute an spmd block after creating a parallel pool, by default all available workers in the pool will run the code inside the spmd block.

    Create an spmd block. On the worker with labindex equal to 1, create an array. Use labSend to send the array to the worker with labindex equal to 2.

    Use labReceive to collect the data.

    spmd
    switch labindex
        case 1
            A = magic(3)
            labSend(A,2);
        case 2
            B = labReceive
    end
end
    Worker 1: 
  
  A =
  
       8     1     6
       3     5     7
       4     9     2
  
Worker 2: 
  
  B =
  
       8     1     6
       3     5     7
       4     9     2

    This example shows how to tag and send data between workers in an spmd block or communicating job.

    Create a parallel pool with 4 workers. By default, spmd is supported on all process-backed pools.

    parpool(4);

    When you execute an spmd block after creating a parallel pool, by default all available workers in the pool will run the code inside the spmd block.

    Create an spmd block. On the worker with labindex equal to 1, create two arrays A1 and A2. Before and between creating arrays, use pause again to simulate some work. Then, use labSend to send the matrices to the worker with labindex equal to 2.

    Tag each matrix with an integer. Tags have many uses, including the following.

    • Use tags to save memory by only loading arrays on workers when you need the data.

    • Use tags to create code that doesn't depend on the index of the sending worker.

    On the worker with labindex equal to 2, use labReceive to collect the data. 

    tic
spmd
    switch labindex
        case 1
            pause(5);
            A1 = magic(1)
            pause(5);
            A2 = magic(2)
            labSend(A1,2,1);
            labSend(A2,2,2);
        case 2
            B1 = labReceive('any',1)
            B2 = labReceive('any',2)
  
    end
end
toc
    Worker 1: 
  
  A1 =
  
       1
  
  
  A2 =
  
       1     3
       4     2
  
Worker 2: 
  
  B1 =
  
       1

  B2 =
  
       1     3
       4     2
  
Elapsed time is 10.061523 seconds.

    In some cases, you can improve the performance of your code by moving some work from one worker to another. Move some work from the worker with labindex equal to 1 to the worker with labindex equal to 3. When you use tags, you can easily move calculations from one worker to another without updating code on the receiving worker.

    tic
spmd
    switch labindex
        case 1
            pause(5);
            A1 = magic(1)
            labSend(A1,2,1);
        case 2
            B2 = labReceive('any',2)
            B1 = labReceive('any',1)
        case 3
            pause(5);
            A2 = magic(2)
            labSend(A2,2,2);
    end
end
toc
    Worker 1: 
  
  A1 =
  
       1
  
Worker 2: 
  
  B2 =
  
       1     3
       4     2
  
  
  B1 =
  
       1
  
Worker 3: 
  
  A2 =
  
       1     3
       4     2
  
Elapsed time is 5.117787 seconds.

    Input Arguments

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    Index of the worker sending data, specified as a positive integer scalar. The value must be less than or equal to the value given by numlabs, the number of workers running the current spmd block or communicating job. When specified, labReceive returns data sent from the worker with labindex equal to source. When not specified, labReceive receives data sent from any worker.

    Example: 1

    Tag attached to data, specified as 0 or a positive integer scalar. When specified, labReceive returns data with sent to the current worker using labSend with the tag argument equal to tag.

    Example: 314159

    Output Arguments

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    Index of the worker sending data, specified as a positive integer scalar or the character vector 'any'. The value is equal to labindex on the worker that sent the received data.

    Tag attached to data received by the current worker, specified as a positive integer scalar.

    Extended Capabilities

    Version History

    Introduced before R2006a

    See Also

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