Technical data

Profiling a Parallel Fortran Program

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After linking, the resulting executable can be run like any standard

executable. Creating multiple execution threads, running and

synchronizing them, and task terminating are all handled automatically.

When an executable has been linked with –mp, the Fortran initialization

routines determine how many parallel threads of execution to create. This

determination occurs each time the task starts; the number of threads is not

compiled into the code. The default is to use the number of processors that

are on the machine (the value returned by the system call

sysmp(MP_NAPROCS); see the sysmp(2) man page). The default can be

overridden by setting the shell environment variable

MP_SET_NUMTHREADS. If it is set, Fortran tasks will use the speciﬁed

number of execution threads regardless of the number of processors

physically present on the machine. MP_SET_NUMTHREADS can be an

integer from 1 to 16.

Proﬁling a Parallel Fortran Program

After converting a program, you need to examine execution proﬁles to judge

the effectiveness of the transformation. Good execution proﬁles of the

program are crucial to help you focus on the loops consuming the most time.

IRIX provides proﬁling tools that can be used on Fortran parallel programs.

Both pixie(1) and pc-sample proﬁling can be used. On jobs that use multiple

threads, both these methods will create multiple proﬁle data ﬁles, one for

each thread. The standard proﬁle analyzer prof(1) can be used to examine

this output.

The proﬁle of a Fortran parallel job is different from a standard proﬁle. As

mentioned in “Analyzing Data Dependencies for Multiprocessing” on page

79, to produce a parallel program, the compiler pulls the parallel DO loops

out into separate subroutines, one routine for each loop. Each of these loops

is shown as a separate procedure in the proﬁle. Comparing the amount of

time spent in each loop by the various threads shows how well the workload

is balanced.