2. parallel The maximum number of independent subtasks in a
given task at a given point in its execution. E.g. in
computing the expression
(a + b) *
(c + d) the expressions a, b, c and d can all be calculated in
parallel giving a degree of parallelism of (at least) four.
Once they have been evaluated then the expressions a + b and c
+ d can be calculated as two independent parallel processes.
executed in parallel (or in either sequential order) only if:
(i) there is no overlap between the inputs of P and the
outputs of Q and vice versa and
(ii) there is no overlap between the outputs of P, the outputs
of Q and the inputs of any other task.
If process P outputs value v which process Q reads then P must
be executed before Q. If both processes write to some
variable then its final value will depend on their execution
order so they cannot be executed in parallel if any other
process depends on that variable's value.
(1995-05-07)