S C H E D U L I N G IN C O N C U R R E N T P A S C A L * F. B. S c h n e i d e r A. J. B e r n s t e i n D e p a r t m e n t of C o m p u t e r S c i e n c e S . U . N . Y . at S t o n y B r o o k S t o n y B r o o k , N e w York, 1 1 7 9 4 I n t r o d u c t i o n T h e m o n i t o r c o n s t r u c t [H74] [B75] and the p r o g r a m m i n g l a n - g u a g e C o n c u r r e n t P a s c a l [B75] h a v e p r o v i d e d a b a s i s for r e s e a r c h i n t o o p e r a t i n g systems. B a s e d on the e x p e r i e n c e s g a i n e d by the u s e o f C o n c u r r e n t P a s c a l in the c o n s t r u c t i o n of o p e r a t i n g s y s t e m s
[B76] [G77] t h e r e has b e e n s o m e d i s c u s s i o n a b o u t the m o n i t o r im- p l e m e n t a t i o n s p r o p o s e d b y B r i n c h H a n s e n [B75] a n d H o a r e [H74]. F o r e x a m p l e , t h e (non) p r o b l e m of n e s t e d m o n i t o r c a l l s h a s re- c e i v e d c o n s i d e r a b l e a t t e n t i o n [L77] [H77] [B78] [W78]. In a d d i - tion, t h e l a c k of f a c i l i t i e s for d y n a m i c r e s o u r c e m a n a g e m e n t in C o n c u r r e n t P a s c a l has i n s p i r e d r e s e a r c h p r o p o s a l s w h i c h e x t e n d t h e l a n g u a g e to s o l v e t h o s e p r o b l e m s [$77] [K77] [A78]. It is u n f o r t u n a t e t h a t m u c h of t h i s r e s e a r c h c u l m i n a t e s in n e w m o n i t o r - l i k e o b j e c t s w h i c h c a n o n l y be u s e d to s o l v e p a r t i c u l a r p r o b l e m s . A s p o i n t e d o u t b y P a r n a s [P78] t h i s s e e m s to i n d i c a t e t h a t m o r e p r i m i t i v e c o n s t r u c t s f r o m w h i c h v a r i o u s m o n i t o r - l i k e o b j e c t s m a y be b u i l t , are r e q u i r e d . In t h i s p a p e r s u c h a c o n s t r u c t is in- t r o d u c e d a n d its a p p l i c a b i l i t y in m o n i t o r s u s e d to c o n s t r u c t s y s t e m s w h i c h i m p o s e a s c h e d u l i n g d i s c i p l i n e o n a s h a r e d r e s o u r c e (e.g. disk) is s t u d i e d . T h e S c h e d u l i n g S c e n a r i o C o n s i d e r a s y s t e m in w h i c h a n u m b e r o f c o n c u r r e n t l y e x e c u t i n g p r o c e s s e s a c c e s s some s h a r e d r e s o u r c e s u c h as a disk. D i s k h e a d s e e k t i m e s are u s u a l l y v e r y l o n g c o m p a r e d w i t h the a c t u a l d a t a t r a n s f e r t i m e a s s o c i a t e d w i t h a d i s k a c c e s s . C o n s e q u e n t l y , m o r e e f f i c i e n t d i s k u t i l i z a t i o n , as w e l l as i m p r o v e d a v e r a g e w a i t i n g t i m e f o r p r o c e s s e s a t t e m p t i n g to a c c e s s the d i s k , c a n be r e a l i z e d b y u s i n g a d i s k h e a d s c h e d u l i n g a l g o r i t h m s u c h as t h e one de- s c r i b e d in [H74]. In t h i s a l g o r i t h m r e q u e s t s a r e o r d e r e d so t h a t t h e d i s k h e a d s w e e p s a c r o s s t h e d i s k in o n e d i r e c t i o n , t h e n the ovther, a n a l o g o u s to the o p e r a t i o n of a n e l e v a t o r in a n o f f i c e b u i l d i n g . A n a c c e s s g r a p h [B72] for t h e i m p l e m e n t a t i o n of a s c h e d u l i n g a l g o r i t h m for a d i s k is i l l u s t r a t e d in F i g u r e i. In o r d e r to a c c e s s the d i s k it is r e q u i r e d t h a t a u s e r f i r s t c a l l t h e s c h e d u l - er, w h i c h m a y c a u s e the c a l l e r to b e s u s p e n d e d u n t i l a t i m e w h e n the d i s k t r a n s f e r c a n be p e r f o r m e d e f f i c i e n t l y . U p o n r e t u r n i n g T h i s r e s e a r c h w a s s u p p o r t e d in p a r t b y N S F g r a n t M C S 7 6 - 0 4 8 2 8 .
f r o m the s c h e d u l e r , the d i s k is c a l l e d to p e r f o r m the a c t u a l I/O o p e r a t i o n . L a s t l y , the s c h e d u l e r is c a l l e d to r e p o r t the c o m p l e - t i o n of the t r a n s f e r . T h i s s t r u c t u r e , h o w e v e r , is u n d e s i r a b l e s i n c e it r e v e a l s to h i g h e r l e v e l s of the s y s t e m (the u s e r m o d u l e s in t h i s case) the f u n c t i o n s of s c h e d u l i n g a n d d i s k I/O as s e p a r a t e e n t i t i e s o In a d d i t i o n , a u s e r c o u l d e a s i l y d e f e a t the s c h e d u l i n g a l g o r i t h m by n o t c a l l i n g t h e s c h e d u l e r p r i o r to a c c e s s i n g t h e disk. A m o r e d e s i r a b l e a r r a n g e m e n t w o u l d be to p r o v i d e a s i n g l e c a l l w h i c h i n v o k e d b o t h t h e s c h e d u l i n g and t r a n s f e r f u n c t i o n s , a n d r e t u r n e d to the h i g h e r l e v e l on c o m p l e t i o n . It w o u l d t h e n b e im- p o s s i b l e for a u s e r to d i r e c t l y a c c e s s the r e s o u r c e , s a v e t h r o u g h the " s c h e d u l e r " . A l t h o u g h F i g u r e 2 e x h i b i t s t h i s t y p e of s t r u c - t u r e it is u n a c c e p t a b l e b e c a u s e e n t r y to the s c h e d u l e r is p r e - v e n t e d if an I/O o p e r a t i o n is in p r o g r e s s (due to the m u t u a l ex- c l u s i o n a s s o c i a t e d w i t h t h a t m o n i t o r ) . T h u s no r e a l s c h e d u l i n g can t a k e p l a c e b e c a u s e p r o c e s s e s w o u l d be s u s p e n d e d at the en- t r a n c e to the s c h e d u l e r , i n s t e a d of i n s i d e it w h e r e an o r d e r i n g on t h e s u s p e n d e d p r o c e s s e s w o u l d be i m p o s e d b y s o m e p r i o r i t y w a i t m e c h a n i s m [H74]. T h e N e w C o n s t r u c t To s o l v e t h i s p r o b l e m , w e p r o p o s e a n e w m e c h a n i s m w h i c h c a n be u s e d in d e s i g n i n g a m o n i t o r . O r d i n a r i l y , p r o c e s s e s a t t e m p t i n g to e n t e r m o n i t o r p r o c e d u r e s w h i l e a n o t h e r p r o c e s s is a c t i v e w i t h - in t h e m o n i t o r a r e d e l a y e d b y m u t u a l e x c l u s i o n at m o n i t o r e n t r y . T h e o r d e r in w h i c h t h e s e p r o c e s s e s u l t i m a t e l y e n t e r is u n d e f i n e d . W e p r o p o s e the a d d i t i o n of a f a c i l i t y t h r o u g h w h i c h t h i s o r d e r c a n be e x p l i c i t l y c o n t r o l l e d . T h i s is a c c o m p l i s h e d by a s s o c i a t - ing w i t h e a c h e n t r y p r o c e d u r e an i n t e g e r v a l u e d f u n c t i o n c a l l e d a s c h e d u l i n g d i s c i p l i n e , w h i c h y i e l d s a p r i o r i t y . T h i s p r i o r i t y is u s e d t o o r d e r t h e p r o c e s s e s w a i t i n g to e n t e r t h e m o n i t o r . A s c h e d u l i n g d i s c i p l i n e is a s s o c i a t e d w i t h a p r o c e d u r e e n t r y by t h e u s e c l a u s e in the p r o c e d u r e h e a d i n g . T h e n a m e d s c h e d u l i n g d i s c i p l i n e is t h e n u s e d to c o m p u t e the p r i o r i t y of a p r o c e s s a t t e m p t i n g to e n t e r t h a t p r o c e d u r e . S c h e d u l i n g d i s c i p l i n e s a r e d e f i n e d as P A S C A L f u n c t i o n s , d e c l a r e d g l o b a l to all e n t r y p r o - c e d u r e s . T h e f u n c t i o n m a y r e f e r e n c e p e r m a n e n t m o n i t o r v a r i a b l e s , t h o u g h it m a y n o t a l t e r t h e i r v a l u e s . T h e f u n c t i o n ' s p a r a m e t e r l i s t p r o v i d e s a m e c h a n i s m for u s i n g m o n i t o r p r o c e d u r e e n t r y p a - r a m e t e r s in the p r i o r i t y c o m p u t a t i o n as well. C a l l b y v a l u e is i m p o s e d to e n s u r e t h a t the f u n c t i o n e x e c u t e s w i t h o u t s i d e - e f f e c t s .
is b l o c k e d at m u t u a l e x c l u s i o n . T h e s e p r o c e s s e s are t h e n o r d e r e d on an e n t r y q u e u e , w h i c h is a q u e u e a s s o c i a t e d w i t h the m o n i t o r c o n t a i n i n g e n t r i e s for p r o c e s s e s a r r a n g e d in a s c e n d i n g p r i o r i t y order° The p r o c e s s at the h e a d of the q u e u e is t h e n g r a n t e d e n t r a n c e . T h e e v a l u a t i o n of t h a t s c h e d u l i n g d i s c i p l i n e m a y o n l y be p e r - f o r m e d w h i l e t h e r e is no p r o c e s s a c t i v e l y e x e c u t i n g in the m o n i - tor, b e c a u s e p e r m a n e n t m o n i t o r v a r i a b l e s w i l l be r e f e r e n c e d . It m a y be t h a t a n u m b e r of p r o c e s s e s m u s t be a d d e d to the e n t r y q u e u e e a c h t i m e c o n t r o l is r e l i n q u i s h e d . T h i s is b e c a u s e the t i m e s p e n t at t h e s c h e d u l e d r e s o u r c e (e.g. disk) b y a p r o c e s s c a n be r e l a t i v e l y long, and for t h a t t i m e the m u t u a l e x c l u s i o n at the m o n i t o r w i l l be in e f f e c t due to t h e n e s t e d c a l l (as in F i g u r e 2). F i g u r e 3 i l l u s t r a t e s the u s e of t h i s f e a t u r e by i m p l e m e n t i n g t h e d i s k h e a d s c h e d u l i n g a l g o r i t h m d i s c u s s e d e a r l i e r . It s h o u l d be n o t e d t h a t it is not p o s s i b l e to w r i t e p a t h o - l o g i c a l s c h e d u l i n g d i s c i p l i n e s in w h i c h e a c h t i m e a p r o c e s s re- l i n q u i s h e s c o n t r o l of the m o n i t o r t h e r e l a t i v e o r d e r i n g of the p r o c e s s e s on the e n t r y q u e u e c h a n g e s d u e to the a l t e r e d v a l u e s of the p e r m a n e n t v a r i a b l e s . S u c h a c a p a b i l i t y d o e s n o t a p p e a r to h a v e a n y p r a c t i c a l a p p l i c a t i o n a n d w o u l d i n c u r a h i g h e x e c u t i o n o v e r h e a d for the c o n t e x t s w i t c h e s a s s o c i a t e d w i t h t h e r e p e a t e d e v a l u a t i o n o f t h e p r i o r i t y f u n c t i o n s . T h e c o n s t r u c t , as w e h a v e p r o p o s e d it, r e q u i r e s two c o n t e x t s w i t c h e s for e a c h p r o c e s s t h a t e n t e r s the m o n i t o r ; o n e f o r p r i o r i t y e v a l u a t i o n , and a s e c o n d a t m o n i t o r entry. In c e r t a i n s i t u a t i o n s e v e n t h i s m a y b e d e e m e d e x c e s s i v e o v e r h e a d . C o n c l u s i o n A g e n e r a l i z a t i o n of t h e m o n i t o r [H74] [B75] h a s b e e n p r e - s e n t e d w h i c h p e r m i t s a n a t u r a l s o l u t i o n to s c h e d u l i n g p r o b l e m s . The c o n s t r u c t is a n a l o g o u s to a p r i o r i t y w a i t m e c h a n i s m [H74] f o r m o n i t o r entry. P r e s e n t m o n i t o r i m p l e m e n t a - ~ n s u s u a l l y i m p o s e a f i r s t c o m e f i r s t s e r v e d d i s c i p l i n e o n m o n i t o r e n t r y w h i c h p r e - c l u d e s t h e i r use in c e r t a i n s i t u a t i o n s a n d l e a d s to a w k w a r d s y s t e m c o n s t r u c t i o n s . A c k n o w l e d g e m e n t s T h e a u t h o r s a r e g r a t e f u l to A. M a h j o u b , P. H a r t e r , a n d K. E k a n a d h a m for d i s c u s s i o n a b o u t t h e m a t e r i a l p r e s e n t e d here.
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t y p e d i s k s c h e d = m o n i t o r ( d i s k d r i v e : d i s k ) ; c o n s t d i s k s i z e = D ; ( * n u m b e r o f t r a c k s o n d i s k *) v a t i n c r , l a s t i n c r : b o o l e a n ; t r a c k s s c a n n e d , c u r a d d r , l a s t a d d r : i n t e g e r ; f u n c t i o n p r i o r i t y b e g i n if i n c r a n d e n d ; ( t r k n o : i n t e g e r ) : i n t e g e r ; ( t r k n o > c u r a d d r ) t h e n p r i o r i t y -- t r k n o + t r a c k s s c a n n e d e l s e i f i n c r t h e n p r i o r i t y := d i s k s i z e - t r k n o + d i s k s i z e + t r a c k s s c a n n e d e l s e i f n o t i n c r a n d t r k n o < c u r a d d r t h e n p r i o r i t y := d i s k s i z e - t r k n o + t r a c k s s c a n n e d e l s e p r i o r i t y := d i s k s i z e + t r k n o + t r a c k s s c a n n e d (* s c h e d u l e d i s c i p l i n e *) p r o c e d u r e entr_____~y a c c e s s d i s k ( t r k a d d e r : i n t e g e r ; v a r b l o c k : p a g e ; i o t y p e : ( r e a d , w r i t e ) ) u s e p r i o r i t y ( t r k a d d e r ) ; b e g i n l a s t a d d r := c u r a d d r ; c u r a d d r := t r k a d d e r ; l a s t i n c r := i n c r ; if c u r a d d r > l a s t a d d r t h e n i n c r := t r u e e l s e i n c r := f a l s e ; if n o t ( l a s t i n c r = i n c r ) t h e n t r a c k s s c a n n e d := t r a c k s s c a n n e d + d i s k s i z e ; c a l l d i s k d r i v e ( b l o c k , i o t y p e ) ; e n d ; b e g i n (* i n i t i a l i z a t i o n *) i n c r := t r u e ; c u r a d d r := 0 ; t r a c k s s c a n n e d := 0 e n d F i g u r e 3 - E x a m p l e 19
A78 B72 B75 B76 G77 H74 H77 K77 L77 P78 S77 W78 R e f e r e n c e s
Andrews, G.R., McGraw, J.R.r "Language F e a t u r e s for
P r o c e s s I n t e r a c t i o n " , O_perating S_j{stems Reviewr Vol llr No. 2 (April 1977) pp 114-127.
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