SICP Exercise 3.40 parallel-execute

Exercise 3.40.  Give all possible values of x that can result from executing

(define x 10)

(parallel-execute (lambda () (set! x (* x x)))
                  (lambda () (set! x (* x x x))))

Which of these possibilities remain if we instead use serialized procedures:

(define x 10)

(define s (make-serializer))

(parallel-execute (s (lambda () (set! x (* x x))))
                  (s (lambda () (set! x (* x x x)))))
SOLUTION

 When there is no serialization:

 1000000: P1 executes fully first, then P2 executes
1000000: P2 executes fully first, then P1 executes
10000: P2 changes x from 10 to 1000 between the two times that P1 accesses the value of x during the evaluation of (* x x)
100000: P1 changes x from 10 to 100 between the first and second access of x in P2 during the evaluation of (* x x x)
10000: P1 changes x from 10 to 100 between the second and third access of x in P2 during the evaluation of (* x x x)
1000: P2 accesses x, then P1 sets x to 100, then P2 sets x
100: P1 accesses x, then P2 sets x to 1000, then P1 sets x

 When there is serialization:

 1000000: P1 executes fully first, then P2 executes

1000000: P2 executes fully first, then P1 executes

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