Reduction

Reduction, which is the process of combining (or reducing) the results of several sub-calculations into a single combined (reduced) result, is very common, and is the second half of the very powerful map-reduce form of parallel programming. In the exercise in the last section you used reduction to form the global sum of the total number of points inside and outside the circle, calculated as the sum of the number of points inside and outside the circle calculated by each thread’s iterations of the loop. While it may appear easy to write your own reduction code, it is actually very hard to write efficient reduction code. This is because reduction requires the use of a critical section, where only one thread is allowed to update the global sum at a time. Reduction can actually be implemented much more efficiently, e.g. perhaps by dividing threads into pairs, and getting each pair to sum their results, and then dividing into pairs of pairs, and summing the pairs of pairs results, etc. (this method is known as binary tree reduction - see here for a more in-depth discussion of reduction algorithms).

So reduction is actually quite complicated to implement if you want it to be efficient and work well. Fortunately, you don’t have to implement it, as OpenMP provides a reduction directive which has implemented it for you! The reduction directive is added to the end of the OpenMP parallel directive and has the following form;

reduction( operator : variable list )

where operator can be any binary operator (e.g. +, -, *), and variable list is a single variable or list of variables that will be used for the reduction, e.g.

reduction( + : sum )

will tell the compiler that sum will hold the global result of a reduction which will be formed by adding together the results of thread-private calculations, while,

reduction( - : balance, diff )

will tell the compiler that both balance and diff will hold the global results of reductions which are formed by subtracting the results of thread-private calculations.

To make this clear, the following links provide the code for the fixed loop counting examples from the last section which use reduction rather than thread-private variables with an OpenMP critical section;


Exercise

Edit your program to estimate pi so that it uses reduction to form the sum of the number of points inside and outside the circle.

Here’s the answers for checking (no peeking before you’ve finished!)


Compare with MPI


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