Scope

In shared memory programming multiple CPUs can access the same variables. This makes the program more efficient and saves copying. In some cases, each thread needs its own copy of the variables – such as the loop variables in parallel for() loops.

Clauses specified in OpenMP directives (see the descriptions Table 1) define the properties of these variables. You can append clauses to the OpenMP #pragma, for example:

#pragma omp parallel
for shared(x, y) private(z)

Errors in shared()/private() variable declarations are some of the most common causes of errors in parallelized programming.

Reduction

Now you now know how to create threads and distribute the workload over multiple threads. However, how can you get all the threads to work on a collated result – for example, to total the values in an array? reduction() (Listing 2) handles this.

The compiler creates a local copy of each variable in reduction() and initializes it independently of the operator (e.g., 0 for "+", 1 for "*"). If, say, three threads are each handling one third of the loop, the master thread adds up the subtotals at the end.

Who is Faster?

Debugging parallelized programs is an art form in its own right. It is particularly difficult to find errors that do not occur in serial programs and do not occur regularly in parallel processing. This category includes what are known as race conditions: different results on repeated runs of a program with multiple blocks that are executed parallel to one another, depending on which thread is fastest each time. The code in Listing 3 starts by filling an array in parallel and then goes on to calculate the sum of these values in parallel.

Without the OpenMP #pragma omp critical (sum_total) statement in line 13, the following race condition could occur:

• Thread 1 loads the current value of sum into a CPU register.
• Thread 2 loads the current value of sum into a CPU register.
• Thread 2 adds a[i+1] to the value in the register.
• Thread 2 writes the value in the register back to the sum variable.
• Thread 1 adds a[i] to the value in the register.
• Thread 1 writes the value in the register to the sum variable.

Because thread 2 overtakes thread 1 here, thus winning the "race," a[i+1] would not be added correctly. Although thread 2 calculates the sum and stores it in the sum variable, thread 1 overwrites it with an incorrect value.

The #pragma omp critical statement makes sure that this does not happen. All threads execute the critical code, but only one at any time. The example in Listing 3 thus performs the addition correctly without parallel threads messing up the results. For elementary operations (e.g., i++) #pragma omp atomic will atomically execute a command. Write access to shared() variables also should be protected by a #pragma omp critical statement.