Parfor: Converting broadcast variables into temporary variables or sliced variables

Is there a difference in efficiency if I just assign a broadcast varible to a temporary variable inside a parfor loop vs re-writing it as a sliced variable that is indexed the same as the main index of the parfor loop? Matlab does not categorize it as a broadcast variable either way, but I'm not sure if the temporary variable method still avoids sending more data than is necesary to workers.

Answer 1

[Matt J]

Temporary variables are not sent to the workers. They are created on the workers (and are destroyed there).

[Brandon]

As a highly simplified example, suppose that I have a broadcast variable x=1, with the following parfor loop:

parfor ih=1:10
    if mod(ih,2)==0
        a=x; 
    else
        a=0;
    end
 store{ih} = f(a);  % some operations on a
end

I get that a is deleted after the parfor loop, but I'm trying to understand if x would get sent all all the workers for each iteration of ih, or only the iterations for when ih is an even number.

[Matt J]

PARFOR broadcasts variables to the workers before any iterations begin. So, in your example, a copy of x will be sent to each worker once and only once. As the worker loops through the sub-batch of iterations assigned to it, it will draw upon x when needed, just like in an ordinary for-loop.

[Brandon]

Thanks. Follow up question: Suppose that the example above is modifed slightly so that the temporary variable is always assigned a constant and the broadcast variable is never used:

parfor ih=1:10
    if ih>0
        a=0; 
    else
        a=x;
    end
 store{ih} = f(a);  % some operations on a
end

Does the broadcast variable, x, still get sent to every worker?

[Walter Roberson]

Yes, the broadcast variable will (probably) be sent. MATLAB does some static flow analysis, but you cannot count on it being advanced enough to be able to reason that ih>0 will always be true.