Parallelism in Ada: Program Design and Meaning

The art of designing parallel programs is underdeveloped because we do not understand parallelism clearly. This paper suggests a programming methodology and it gives a precise definition of the ADA form of parallelism. The methodology is based on ideas of Milner and it can be used when designing parallel programs in languages other than ADA, Computer Science Department~ Aarhus University~ Ny Munkegade~ 8000 Aarhus C~ Denmark. • . P a r t 1: D E S I G N T h e a r t o f d e s i g n i n g p a r a l l e l p r o g r a m s is u n d e r d e v e l o p e d b e c a u s e w e d o no t u n d e r s t a n d p a r a l l e l i s m c l e a r l y . T h i s p a p e r s u g g e s t s a p r o g r a m m i n g m e t h o d o l o g y and i t g i v e s a p r e c i s e d e f i n i t i o n o f the A D A f o r m o f p a r a l l e l i s m . T h e m e t h o d o l o g y is b a s e d on the i d e a s o f M i l n e r and i t c a n be u s e d w h e n d e s i g n i n g p a r a l l e l p r o g r a m s in l a n g u a g e s o t h e r t h a n A D A . F o r us a p a r a l l e l p r o g r a m c o n s i s t s o f o n e o r m o r e t a s k s and s e v e r a l a r r o w s f r o m one t a s k to a n o t h e r . We s h a l l u s e the e x a m p l e o f p r o d u c e r s and c o n s u m e r s ~ c o m m u n i c a t i n g t h r o u g h a p o s t b o x ~ to i l l u s t r a t e o u r d e s i g n m e t h o d , 257 In a l l o u r p i c t u r e s d i f f e r e n t a r r o w s may h a v e the s a m e head bu t t h e y a l w a y s h a v e d i f f e r e n t t a i l s , F r o m o u r p i c t u r e f o r p r o d u c e r s and c o n s u m e r s w e s e e t ha t the p o s t b o x d e c i d e s w h i c h c o n s u m e r to c o m m u n i c a t e w i t h . I f w e r e v e r s e d the a r r o w s to the c o n s u m e r s ~ w e w o u l d h a v e the u s u a l i n t e r p l a y b e t w e e n p r o d u c e r s and c o n s u m e r s : a s h a r e d b u f f e r . T h e f i r s t p h a s e o f o u r d e s i g n me thod is to d r a w a p i c t u r e o f t a s k s w i t h n a m e s a r r o w s b e t w e e n them. A r r o w s w i t h the s a m e h e a d mus t h a v e the s a m e name. B e c a u s e the A D A e q u i v a l e n t o f a f i r s [ p h a s e p i c t u r e is a l i s t o f p a r t i a l l y d e f i n e d t a s k i n t e r f a c e s w h o s e e n t r i e s c o r r e s p o n d to the h e a d s o f a r r o w s ~ we s h a l l h e r e a f t e r use the w o r d ~ e n t r y H i n s t e a d o f Uname o f a r r o w n.