XL Fortran Enterprise Edition for AIX : User's Guide - IBM

v The result of an application does not depend on the order in which records are
written out or read in.
v Each thread performs I/O on a different file.
In these cases, results of the I/O operations are independent of the order in which
threads execute. However, you might not get the performance improvements that
you expect, since the I/O library serializes parallel access to the same logical unit
from multiple threads. Examples of these cases are as follows:
v Each thread performs I/O on a pre-determined record in a direct-access file:
v
v
do i = 1, 10
write(4, ’(i4)’, rec = i) a(i)
enddo
Each thread performs I/O on a different part of a stream-access file. Different
I/O statements cannot use the same, or overlapping, areas of a file.
do i = 1, 9
write(4, ’(i4)’, pos = 1 + 5 * (i - 1)) a(i)
! We use 5 above because i4 takes 4 file storage
! units + 1 file storage unit for the record marker.
enddo
In the case that each thread operates on a different file, since threads share the
status of the logical units connected to the files, the thread still needs to obtain
the lock on the logical unit for either retrieving or updating the status of the
logical unit. However, the thread-safe I/O library allows threads to perform the
data transfer between the logical unit and the I/O list item in parallel. If an
application contains a large number of small I/O requests in a parallel region,
you might not get the expected performance because of the lock contention.
Consider the following example:
program example
use omp_lib
integer, parameter :: num_of_threads = 4, max = 5000000
character*10 file_name
integer i, file_unit, thread_id
integer, dimension(max, 2 * num_of_threads) :: aa
call omp_set_num_threads(num_of_threads)
!$omp parallel private(file_name, thread_id, file_unit, i) shared(aa)
thread_id = omp_get_thread_num()
file_name = ’file_’
file_name(6:6) = char(ichar(’0’) + thread_id)
file_unit = 10 + thread_id
open(file_unit, file = file_name, status = ’old’, action = ’read’)
do i = 1, max
read(file_unit, *) aa(i, thread_id * 2 + 1), aa(i, thread_id * 2 + 2)
end do
close(file_unit)
!$omp end parallel
end
The I/O library synchronizes retrieving and updating the status of the logical
units while performing data transfer in parallel. In order to maximize the
parallelism the I/O library provides, it is recommanded to increase the size of
data transfer per I/O request. The do loop, therefore, should be rewritten as
follows:
342 XL Fortran Enterprise Edition for AIX : User’s Guide