CSC/ECE 506 Spring 2010/ch 2 aj/Data Parallel Programming

Data-Parallel Programming Model

Introduction

Relationship with SIMD

Examples

This section shows a simple example illustrating the data-parallel programming model. All codes below are written in pseudo-code style.

Suppose we want to perform the following task on an array a: updating each element of a by the product of itself and its subscript, and adding together the elements of a into the variable sum. Corresponding code is shown below.

// a simple code
sum = 0;
for (i = 0; i < a.length; i++)
{
   a[i] = a[i] * i;
   sum = sum + a[i];
}

When we implement the task using data-parallel programming model, the program can be divided into two parts. The first part performs the same operations on separate elements of the array for each processing element, and the second part reorganizes data among all processing elements (In our example data reorganization is summing up values). Since data-parallel programming model only defines the overall effects of parallel steps, the second part can be accomplished either through shared memory or message passing. The three code fragments below are examples for the first part of the program, shared memory version of the second part, and message passing for the second part, respectively.

// data parallel programming: let each PE perform the same task on different pieces of distributed data
pe_id = getid();
my_sum = 0;
for (i = pe_id; i < a.length; i += number_of_pe)
{
   a[i] = a[i] * i;
   my_sum = my_sum + a[i];
}

// data reorganization via message passing
if (pe_id != 0) send_msg (0, my_sum);
else
   for (i = 1; i < number_of_pe; i++)
   {
      recv_msg (i, temp);
      my_sum = my_sum + temp;
   }
sum = my_sum;

// data reorganization via shared memory
shared sum;
lock();
sum = sum + my_sum;
unlock();
barrier;

As illustrated above, each processing element performs actions on array's separate elements which are identified using the processing element's id. In our example, elements assigned to each processing element are staggered instead of continuous. For example, if the system contains only 2 process elements, then one of them would operate on all the odd entries and the other the even entries of the array.

References

1) Old textbook

2) Internet article