Parallel Computer Architecture

2.3 Parallelization of an Example Program 107

Pi P

2

A = 1;

b:

flag = 1;

a: while (flag is 0) do nothing;

print A;

FIGURE 2.14 Point-to-point event synchronization using flags. Suppose we want to

ensure that a process Pj does not get past a certain point

(say,

a) in the program until some

other process P

2

has already reached another point (say, b). Assume that the variable flag

(and A) was initialized to 0 before the processes arrived at this scenario. If

P

1

gets to state-

ment a after P

2

has already executed statement b, Pi will simply pass point a. If, on the

other hand, P

2

has not yet executed b, then P

1

will remain in the "idle" while loo ...