Parallel Programming Guide for HP-UX Systems

Troubleshooting

False cache line sharing

Chapter 9 179

REAL*4 S(8,8)

C$DIR LOOP_PARALLEL

DO I = 1, N

S(1,MY_THREAD()+1) = ... ! EACH THREAD ASSIGNS

ONE ELEMENT OF S

ENDDO

C$DIR NO_PARALLEL

DO J = 1, NUM_THREADS()

= ...S(1,J) ! THREAD 0 POST-PROCESSES

ENDDO

Scalars sharing a cache line

Sometimes parallel tasks assign unique scalar variables that are in the

same cache line, as in the following code:

COMMON /RESULTS/ SUM, PRODUCT

C$DIR BEGIN_TASKS

DO I = 1, N

SUM = SUM + ...

ENDDO

C$DIR NEXT_TASK

DO J = 1, M

PRODUCT = PRODUCT * ...