Parallel Programming Guide for HP-UX Systems

Parallel synchronization

Synchronizing code

Chapter 8152

The ordered section containing the A(I) assignment executes in

iteration order. This ensures that the value of A(I-1) used in the

assignment is always valid. Assuming this loop runs on four threads, the

synchronization of statement execution between threads is illustrated in

Figure 8-2.

Figure 8-2 LOOP_PARALLEL(ORDERED) synchronization

As shown by the dashed lines between initial iterations for each thread,

one ordered section must be completed before the next is allowed to begin

execution. Once a thread exits an ordered section, it cannot reenter until

all other threads have passed through in sequence.

Overlap of nonordered statements, represented as lightly shaded boxes,

allows all threads to proceed fully loaded. Only brief idle periods occur on

1, 2, and 3 at the beginning of the loop, and on 0, 1, and 2 at the end.

Example 8-7 Ordered section limitations

Each thread in a parallel loop containing an ordered section must pass

through the ordered section exactly once on every iteration of the loop. If

you execute an ordered section conditionally, you must execute it in all

possible branches of the condition. If the code contained in the section is

not valid for some branches, you can insert a blank ordered section, as

shown in the following Fortran example:

THREADS

I=1

I=5 I=9 I=13 I=17

I=2 I=6

I=10 I=14

I=18

I=3 I=7 I=11 I=15 I=19

I=4 I=8 I=12 I=16 I=20

Order of statement execution

Statements contained within ordered sections

Nonordered section statements