Introduction to Composites

Composite objects in the MATLAB^® client session let you directly access data values on the workers. Most often you assigned these variables within spmd statements. In their display and usage, Composites resemble cell arrays. There are two ways to create Composites:

Create Composites in spmd Statements

When you define or assign values to variables inside an spmd statement, the data values are stored on the workers.

After the spmd statement, those data values are accessible on the client as Composites. Composite objects resemble cell arrays, and behave similarly. On the client, a Composite has one element per worker. For example, suppose you create a parallel pool of three local workers and run an spmd statement on that pool:

parpool("Processes",3)

spmd  % Uses all 3 workers
    MM = magic(spmdIndex+2); % MM is a variable on each worker
end
MM{1} % In the client, MM is a Composite with one element per worker

ans =     
     8     1     6
     3     5     7
     4     9     2

MM{2}

ans =
    16     2     3    13
     5    11    10     8
     9     7     6    12
     4    14    15     1

A variable might not be defined on every worker. For the workers on which a variable is not defined, the corresponding Composite element has no value. Trying to read that element throws an error.

spmd
    if spmdIndex > 1
         HH = rand(4);
    end
end
HH

HH =
     Worker 1: No data
     Worker 2: class = double, size = [4  4]
     Worker 3: class = double, size = [4  4] 

You can also set values of Composite elements from the client. This causes a transfer of data, storing the value on the appropriate worker even though it is not executed within an spmd statement:

MM{3} = eye(4);

In this case, MM must already exist as a Composite, otherwise MATLAB interprets it as a cell array.

Now when you do enter an spmd statement, the value of the variable MM on worker 3 is as set:

spmd
    if spmdIndex == 3, MM, end
end

Worker 3: 
    MM =
         1     0     0     0
         0     1     0     0
         0     0     1     0
         0     0     0     1

Data transfers from worker to client when you explicitly assign a variable in the client workspace using a Composite element:

M = MM{1} % Transfer data from worker 1 to variable M on the client

M =
     8     1     6
     3     5     7
     4     9     2

Assigning an entire Composite to another Composite does not cause a data transfer. Instead, the client merely duplicates the Composite as a reference to the appropriate data stored on the workers:

NN = MM % Set entire Composite equal to another, without transfer

However, accessing a Composite's elements to assign values to other Composites does result in a transfer of data from the workers to the client, even if the assignment then goes to the same worker. In this case, NN must already exist as a Composite:

NN{1} = MM{1} % Transfer data to the client and then to worker

When finished, you can delete the pool:

delete(gcp)

Variable Persistence and Sequences of spmd

The values stored on the workers are retained between spmd statements. This allows you to use multiple spmd statements in sequence, and continue to use the same variables defined in previous spmd blocks.

The values are retained on the workers until the corresponding Composites are cleared on the client, or until the parallel pool is deleted. The following example illustrates data value lifespan with spmd blocks, using a pool of four workers:

parpool('Processes',4)

spmd
    AA = spmdIndex; % Initial setting
end
AA(:)  % Composite

ans =

  4×1 cell array

    {[1]}
    {[2]}
    {[3]}
    {[4]}

spmd
    AA = AA * 2; % Multiply existing value
end
AA(:)  % Composite

ans =

  4×1 cell array

    {[2]}
    {[4]}
    {[6]}
    {[8]}

clear AA % Clearing in client also clears on workers

spmd
    AA = AA * 2; 
end   % Generates error

delete(gcp)

Analyzing and transferring files to the workers ...done.
Error detected on workers 2 3 4.

Caused by:
    An UndefinedFunction error was thrown on the workers for 'AA'.  
    This may be because the file containing 'AA' is not accessible on the workers. 
    Specify the required files for this parallel pool 
    using the command: addAttachedFiles(pool, ...).  
    See the documentation for parpool for more details.
        Unrecognized function or variable 'AA'.

Create Composites Outside spmd Statements

The Composite function creates Composite objects without using an spmd statement. This might be useful to prepopulate values of variables on workers before an spmd statement begins executing on those workers. Assume a parallel pool is already running:

PP = Composite()

By default, this creates a Composite with an element for each worker in the parallel pool. You can also create Composites on only a subset of the workers in the pool. See the Composite reference page for more details. The elements of the Composite can now be set as usual on the client, or as variables inside an spmd statement. When you set an element of a Composite, the data is immediately transferred to the appropriate worker:

for ii = 1:numel(PP)
    PP{ii} = ii; 
end