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CIS 720. Message Passing. Message passing systems. Send statements Receive statements. Process naming. P ! m ( para_list ): send message m with parameters para_list to P. send(m( para_list , P)
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CIS 720 Message Passing
Message passing systems • Send statements • Receive statements
Process naming • P ! m(para_list): send message m with parameters para_list to P. • send(m(para_list, P) • P ? m(para_list): receive message m from P, and assign the received parameters to variables in para_list. • receive(m(para_list), P)
Channel naming • send(m(para_list), ch) or ch!m(para_list) • receive(m(para_list), ch) or ch?m(para_list)
Minimal synchronization: 1 • Non-blocking send: 1 + 6 • Blocking send: 1 + 2 + 6 • Reliable blocking send: 1 + 2 + 5 + 6 • Synchronous send: 1 + 2 + 3 + 4 + 5 + 6
Synchronous communication • Send statement: • A ! m(expr): the send statement blocks until the receive statement is also enabled. • Receive statement • B ? m(var): the receive statement blocks until the send statement is also enabled. • When matching send and receive statements are enabled, then var is assigned the value of expr
A pair of send and receive statements are matching if • The send statement appears in the process named in the receive statement. • The receive statement appears in the process named in the send statement. • var = expr is a valid assignment statement • The type of the messages match
P1: P2: y := 2 z := 1 P2 ! y P1 ? w P2 ? y P1 ! w + z
P1: P2: P3: y := 2 z := 1 x = 1 P2 ! y P1 ? W P2 ? x P3 ? y P3 ! w + z P1 ! x
P1: P2: y := 2 z := 1 P2 ! y P1 ! w P2 ? y P1 ? w + z Deadlock
Array copying program A[0..N-1] B[0..N-1] i = 0; P1: P2: do do i < n P2 ! A[i]; i < n P1 ? B[i] i = i + 1 od od
Array copying program A[0..N-1] B[0..N-1] i = 0; P1: P2: do do i < n P2 ! A[i]; i < n P1 ? B[i] i = i + 1 od od This could deadlock because when i = n-1, P2 could check “i < n” before i is incremented.
Array copying program A[0..N-1] B[0..N-1] i = 0; j = 0 P1: P2: do do i < n P2 ! A[i]; j < n P1 ? B[j]; i = i + 1 j = j + 1 od od
Satisfaction condition • one must show the following satisfaction condition for each pair of matching send and receive statements: P1 P2 : : {P}{Q} P2!expr P1?var {U}{V} : :
P1: P2: y := 2 z := 1 P2 ! y P1 ? w P2 ? y P1 ! w + z
P1: P2: {true} {true} y := 2 z := 1 {y = 2} {z = 1} P2 ! y P1 ? w {w = 2} {z = 1 /\ w = 2} P2 ? y P1 ! w + z {y = 3} {y = 3} { y = 2 /\ z = 1} w = y { w = 2 /\ z = 1} { w = 2 /\ z = 1} y = w + z { y = 3}
Guarded Communication • P ? x /\ bool action • Guard evaluates to true if • bool is true • executing P ? x will not cause delay • Guard evaluates to false if bool is false • Guard blocks if bool is true but executing P ? x will cause delay
Mutual exclusion • C: P1: P2: do do do P1?req P1?rel C!req; C!req [] cs cs P2?req P2?rel C!rel; C!rel od od od
Example…. • P1: P2: v1 = 1; v2 = ? v3 = ?; v4 = 2 if if fi fi
Example…. • P1: P2: v1 = 1; v2 = ? v3 = ?; v4 = 2 if if P2 ! v1 P2 ? v2 P1 ! v4 P1 ? v3 [] [] P2 ? v2 P2 ! v1 P1 ? v3 P1 ! v4 fi fi
Computing the minimum value D: num=0; m = Max; A: B: C: do A ? v m = min(m,v);num++ D!a; D!b D!c [] D?a D?b D?c B ? v m = min(m,v);num++ [] C ? v m = min(m,v);num++ [] num = 3 A!m; B!m; C!m od
Computing the minimum value B: b=initial value; num2 = 0 do A ? v b = min(b,v);num2++ [] C ? v b = min(b,v);num2++ [] A ! a skip; num2++ [] C ! a skip; num2++ [] num2 = 4 exit; od A: a=initial value; num1 = 0 do B ? v a = min(a,v);num1++ [] C ? v a = min(a,v);num1++ [] B ! a skip; num1++ [] C ! a skip; num1++ [] num1 = 4 exit; od
Centralized Semaphore • C: A: B: do do do A ? p A ? v C!p; C!p [] cs cs B ? p B ? v C!v; C!v od od od
Centralized Semaphore • C: sem =1A: B: do do do sem =1; A ? p sem-- C!p; C!p [] cs cs sem=1; B ? p sem-- C!v; C!v [] od od sem=0; A ? v sem++ [] sem=0; B ? v sem++ od
Dining philosophers problem P0: do hungry acquire left and right fork eat release forks sleep od
