Advanced Topic: Other Kinds of Iteration Spaces

Advanced Topic: Other Kinds of Iteration Spaces#

The examples so far have used the class blocked_range<T> to specify ranges. This class is useful in many situations, but it does not fit every situation. You can use oneAPI Threading Building Blocks (oneTBB) to define your own iteration space objects. The object must specify how it can be split into subspaces by providing a basic splitting constructor, an optional proportional splitting constructor, and two predicate methods. If your class is called R, the methods and constructors should be as follows:

class R {
    // True if range is empty
    bool empty() const;
    // True if range can be split into non-empty subranges
    bool is_divisible() const;
    // Splits r into subranges r and *this
    R( R& r, split );
    // (optional) Splits r into subranges r and *this in proportion p
    R( R& r, proportional_split p );

The method empty should return true if the range is empty. The method is_divisible should return true if the range can be split into two non-empty subspaces, and such a split is worth the overhead. The basic splitting constructor should take two arguments:

  • The first of type R

  • The second of type oneapi::tbb::split

The second argument is not used; it serves only to distinguish the constructor from an ordinary copy constructor. The basic splitting constructor should attempt to split r roughly into two halves, and update r to be the first half, and set the constructed object as the second half.

Unlike the basic splitting constructor, the proportional splitting constructor is optional and takes the second argument of type oneapi::tbb::proportional_split. The type has methods left and right that return the values of the proportion. These values should be used to split r accordingly, so that the updated r corresponds to the left part of the proportion, and the constructed object corresponds to the right part.

Both splitting constructors should guarantee that the updated r part and the constructed object are not empty. The parallel algorithm templates call the splitting constructors on r only if r.is_divisible is true.

The iteration space does not have to be linear. Look at oneapi/tbb/blocked_range2d.h for an example of a range that is two-dimensional. Its splitting constructor attempts to split the range along its longest axis. When used with parallel_for, it causes the loop to be “recursively blocked” in a way that improves cache usage. This nice cache behavior means that using parallel_for over a blocked_range2d<T> can make a loop run faster than the sequential equivalent, even on a single processor.