Parallel Programming in Fortran 95 using OpenMP - People

52 3. PRIVATE, SHARED & Co.
Pieces of work
1 2
3 4 5 6
7 8
9 10 11
12
✻
256 iter.
512 iter.
246 iter.
1024 iter.
2048 iter.
Figure 3.8: Graphical representation of the effect of the SCHEDULE(GUIDED,chunk) clause on a
do-loop with 10230 iterations and with chunk = 256.
To see in figure 3.8 is that the last piece of work is smaller than the value specified
in chunk. This is necessary in order to fulfill the total number of iterations of the
do-loop, namely 10230.
Once the pieces of work have been created, the way in which they are distributed
among the available threads is the same as in the previous scheduling method
DYNAMIC.
RUNTIME : Any of the previous three scheduling methods needs to be fixed at the time of
compiling the source code. But it may be of interest to be able to modify during
runtime the way in which the work is distributed among the threads. This can be
achieved by specifying the RUNTIME scheduling method:
!$OMP DO SCHEDULE(RUNTIME)
If this is done, then the content of the environment variable OMP SCHEDULE specifies
the scheduling method to be used. More information about the OMP SCHEDULE
environment variable can be found on page 70.
3.2.5 ORDERED
When a do-loop includes statements, which need to be executed sequentially, like in the
following example:
do i = 1, 1000
A(i) = 2 * A(i-1)
enddo