Prefix sums

Overview

Prefix sums is a very simple, basic parallel algorithm commonly used as the building block of many applications. Also known as a scan, each process will sumate their value with every preceding processes' value. For instance, p=0 returns its value, p=1 returns p=1 + p=0 values, p=2 returns p=2 + p=1 + p=0 values. The MPI reduce command often implements the communication via the logarithmic structure shown below.

Source Code

function void main[var arga,var argb]
{
 var m:=10;
 var a:Int :: allocated[multiple[]];
 var p;
 par p from 0 to m - 1
 {
   var mine:Int;
   mine:= randomnumber[0,toInt[argb#1]];
   var i;
   for i from 0 to m - 1
   {
     var myvalue:=mine;
     if (i < p) myvalue:=0;
     (a :: reduce[i, "sum"]):=myvalue;
   };
   print[p," = ",a,"\n"];
 };
};

Notes

The function main has been included here so that the user can provide, via command line options, the range of the random number to find. The complexity of the prefix sums is taken away by using the reduce primitive communication type.

Download

Download the entire prefix sums source code here