ExecutionPolicy

The concept of an ExecutionPolicy is the fundamental abstraction to represent “how” the execution of a Kokkos parallel pattern takes place. This page talks practically about how to use the common features of execution policies in Kokkos; for a more formal and theoretical treatment, see this document.

Disclaimer: There is nothing new about the term “concept” in C++; anyone who has ever used templates in C++ has used concepts whether they knew it or not. Please do not be confused by the word “concept” itself, which is now more often associated with a shiny new C++20 language feature. Here, “concept” just means “what you’re allowed to do with a type that is a template parameter in certain places”.

What is an ExecutionPolicy?

The dominant parallel dispatch mechanism in Kokkos, described elsewhere in the programming guide, involves a parallel_pattern (e.g., something like Kokkos::parallel_for or Kokkos::parallel_reduce), an ExecutionPolicy, and a Functor. In a hand-wavy sense:

parallel_pattern(
    ExecutionPolicy(),
    Functor()
);

The most basic (“beginner”) case is actually a shortcut:

Kokkos::parallel_for(
    42,
    KOKKOS_LAMBDA (int n) { /* ... */ }
);

is a “shortcut” for

Kokkos::parallel_for(
    Kokkos::RangePolicy<Kokkos::DefaultExecutionSpace>(
        Kokkos::DefaultExecutionSpace(), 0, 42
    ),
    KOKKOS_LAMBDA(int n) { /* ... */ }
);

In this example, Kokkos::RangePolicy<Kokkos::DefaultExecutionSpace> is the ExecutionPolicy type.

Functionality

All ExecutionPolicy types provide a nested type named index_type.