10. task/tasks construct

A task construct generates a task, which is executed by the specified nodes. A tasks construct asserts that surrounding task constructs can be executed in parallel.

This page shows some examples of the task construct other than those in Tutorial (Global-view).

10.1. task construct

The on clause of the task construct specifies the node set that executes the task.

  • XMP/C program
#include <stdio.h>
#pragma xmp nodes p[4]

int main(){
  int num = xmpc_node_num();
#pragma xmp task on p[1:3]
{
  printf("%d: Hello\n", num);
}

  return 0;
}
  • XMP/Fortran program
program main
!$xmp nodes p(4)
  integer :: num

  num = xmp_node_num()
!$xmp task on p(2:4)
  write(*,*) num, ": Hello"
!$xmp end task

end program main

In the above example, nodes p[1], p[2], and p[3] execute the printf() function and p[1] outputs “1: Hello” in XMP/C, and p(2), p(3), and p(4) execute the write statement, and p(2) outputs “2: Hello” in XMP/Fortran.

Note that a new node set is generated by each task construct. Consider inserting a bcast construct into a task.

  • Excerpt from an XMP/C program
#pragma xmp task on p[1:3]
{
#pragma xmp bcast (num)
}
  • Excerpt from and XMP/Fortran program
!$xmp task on p(2:4)
!$xmp bcast (num)
!$xmp end task

This bcast construct is executed by the node set specified by the task construct.

the node p[1] broadcasts the value to p[2] and p[3] in XMP/C, and p(2) to p(3) and p(4) in XMP/Fortran.

_images/task.png

The bcast construct in the above code is equivalent to that in the following code, where it is executed by a new node set that is explicitly declared.

  • Excerpt from an XMP/C program
#pragma xmp nodes q[3] = p[1:3]
#pragma xmp bcast (num) on q
  • Excerpt from an XMP/Fortran program
!$xmp nodes q(3) = p(2:4)
!$xmp bcast (num) on q

Note that the task is executed by the node set specified by the on clause, and, therefore, xmpc_node_num() and xmp_node_num() returns the id in the node set.

For example, consider inserting xmpc_node_num() or xmp_node_num() into the task in the first program.

  • XMP/C program
#include <stdio.h>
#pragma xmp nodes p[4]

int main(){
#pragma xmp task on p[1:3]
{
  printf("%d: Hello\n", xmpc_node_num());
}

  return 0;
}
  • XMP/Fortran program
program main
!$xmp nodes p(4)

!$xmp task on p(2:4)
  write(*,*) xmp_node_num(), ": Hello"
!$xmp end task

end program main

The node p[1] outputs “0: Hello” in XMP/C and p(2) “1: Hello” in XMP/Fortran.

Note

A new node set should be collectively generated by all of the executing nodes at the point of task construct unless it is surrounded by a tasks construct. In the above example, p[0] in XMP/C and p(1) in XMP/Fortran must execute the task construct.

10.2. tasks construct

Consider that each of two tasks invokes a function.

  • XMP/C program
#pragma xmp nodes p[4]

#pragma xmp task on p[0:2]
{
  func_a();
}
#pragma xmp task on p[2:2]
{
  func_b();
}
  • XMP/Fortran program
!$xmp nodes p(4)

!$xmp task on p(1:2)
  call func_a()
!$xmp end task
!$xmp task on p(3:4)
  call func_b()
!$xmp end task

In the above example, the two tasks cannot be executed in parallel because those on clauses must be evaluated by all of the executing nodes.

_images/task_noparallel.png

Use the tasks construct to execute multiple task in parallel.

  • XMP/C program
#pragma xmp nodes p[4]

#pragma xmp tasks
{
#pragma xmp task on p[0:2]
{
  func_a();
}
#pragma xmp task on p[2:2]
{
  func_b();
}
}
  • XMP/Fortran program
!$xmp nodes p(4)

!$xmp tasks
!$xmp task on p(1:2)
  call func_a()
!$xmp end task
!$xmp task on p(3:4)
  call func_b()
!$xmp end task
!$xmp end tasks

the node sets specified by the on clauses of task constructs surrounded by a task construct can be executed in parallel.

_images/tasks.png