Executing a loop in parallel incurs overhead that varies with
different systems. If a loop has little computational work,
the overhead required to set up parallel execution may make
the loop execute more slowly than it executes serially. The
minconcurrent qualifier sets the level of work in
a loop above which KAP should execute the loop in parallel.
Setting the /minconcurrent qualifier causes KAP to
automatically set the /concurrentize qualifier for you.
The range of values for /minconcurrent is
all integers greater than or equal to 0. The higher the
minconcurrent value, the more iterations and/or
statements the loop body must have to run concurrently.
At compilation time, KAP estimates the amount of work inside
a loop on the basis of loop computations and loop iterations.
KAP multiplies the loop iteration count by the sum of the
noindex operands/results and the nonassignment operators. KAP
compares its estimation with the minconcurrent
value. If the estimated amount of work is greater than the
minconcurrent value, KAP generates parallel code for
the loop. Otherwise, the loop execution is serial. This is called a
two-version loop.
If the DO loop bounds are known at compilation time, KAP computes the exact iteration count. However, if the DO loop bounds are unknown, KAP generates a block IF around the parallel code. The block IF allows a run-time decision whether or not to execute the loop in parallel.
To disable the generation of two-version loops throughout
the program, use the command-line qualifier
/minconcurrent=0 . To disable this action in specific DO
loops, use the C*$* minconcurrent(0) directive.
The following loop illustrates this qualifier using the
minconcurrent default of 1000:
DO 10 I = 1,N
A(I) = B(I) + C(I)
10 CONTINUE
Becomes:
IF (N .GE. 425) THEN
CALL mppfrk (P$PLP10,0)
ELSE
DO 2 I=1,N-3,4
A(I) = B(I) + C(I)
A(I+1) = B(I+1) + C(I+1)
A(I+2) = B(I+2) + C(I+2)
A(I+3) = B(I+3) + C(I+3)
2 CONTINUE
ENDIF
At run time, if the iteration count N is greater than or equal to 425 (1000/4), the concurrent loop executes in parallel; otherwise, it executes serially.