Syntax

Description

C = parallel.pool.Constant(X) copies the value X to each worker and returns a parallel.pool.Constant object, C, which allows each worker to access the value X within a parallel language construct (parfor, spmd, parfeval) using the property C.Value. This can improve performance when you have multiple parfor-loops accessing the same constant set of data, because X is transferred only once to the workers.

C = parallel.pool.Constant(FH) evaluates function handle FH on each worker and stores the result in C.Value. This is also useful for creating and using any handle-type resources on the workers, such as file handles and database connections.

C = parallel.pool.Constant(FH,CLEANUP) evaluates function handle FH on each worker and stores the result in C.Value. When C is cleared, the function handle CLEANUP is evaluated with a single argument C.Value on each worker.

C = parallel.pool.Constant(COMP) uses the values stored in the Composite COMP, and stores them in C.Value on each worker. This is especially useful when the data that you need to use inside a parfor-loop can be constructed only on the workers, such as when the data is too large to conveniently fit in the client, or when it is being loaded from a file that only the workers can access. If COMP does not have a defined value on every worker, an error results.

Tips

parallel.pool.Constant must be called in the MATLAB client session.

parallel.pool.Constant automatically starts a parallel pool, if possible. If no pool is running, and one cannot be started automatically, parallel.pool.Constant generates an error.

Examples

Make parallel.pool.Constant from array in client

This example shows how to create a numeric parallel.pool.Constant, and use it in multiple parfor-loops on the same pool.

First, create some large data on the client, then build a parallel.pool.Constant, transferring the data to the pool only once.

data = rand(1000);
c = parallel.pool.Constant(data);
for ii = 1:10
    % Run multiple PARFOR loops accessing the data.
    parfor jj = 1:10
        x(ii,jj) = c.Value(ii,jj);
    end
end

Make parallel.pool.Constant from function handle

This examples shows how to create a parallel.pool.Constant with a function handle and a cleanup function.

Create a temporary file on each worker. By passing @fclose as the second argument, the file is automatically closed when c goes out of scope.

c = parallel.pool.Constant(@() fopen(tempname(pwd),'wt'),@fclose);
spmd
   disp(fopen(c.Value)); % Displays the temporary filenames.
end

parfor idx = 1:1000
   fprintf(c.Value,'Iteration: %d\n',idx);
end
clear c;   % Closes the temporary files.

Make parallel.pool.Constant from Composite

This example shows how to build large data sets as a Composite on pool workers inside an spmd block, and then use that data as a parallel.pool.Constant inside a parfor-loop.

spmd
  if labindex == 1
    x = labBroadcast(1,rand(5000));
  else
    x = labBroadcast(1);
  end
end
xc = parallel.pool.Constant(x);
parfor idx = 1:10
  s(idx) = sum(xc.Value(:,idx));
end
s

s =
   1.0e+03 *

    2.5108    2.5031    2.5123    2.4909    2.4957    2.5462    2.4859    2.5320    2.5076    2.5432

See Also

parcluster | parfeval | parfor | parpool | spmd