Template:ElementTypeCommunication

When a variable is assigned to another, depending on where each variable is allocated to, there may be communication required to achieve this assignment. Table \ref{tab:eltypecomm} details the communication rules in the assignment \emph{assignmed variable := assigning variable}. If the communication is issued from MPMD programming style then this will be one sided. The default communication listed here is guaranteed to be safe, which may result in a small performance hit.

Assigned Variable	Assigning Variable	Semantics
multiple[]	multiple[]	local assignment
single[on[i]]	multiple[]	individual processes write values to process i
multiple[]	single[on[i]]	individual processes read values from process i
single[on[i]]	single[on[i]]	local assignment where i==i
single[on[i]]	single[on[j]]	communication from j to i where i!=j

Communication Example

var a:Int;
var b:Int :: allocated[single[on[2]]];
var p;
par p from 0 to 3 {
   if (p==2) b:=p;
   a:=b;
   sync;
};

This code will result in each process reading the value of b from process 2 and then writing this into a. As already noted, in absence of allocation information the default of allocating to all processes is used. In this example the variable a can be assumed to additionally have the type allocated[multiple]. Note that communication groups are the same as multiple in this context and share the same semantics. All variables marked multiple are private to their containing process.