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

450 end do
close(20)
end
Performance
To maximize the benefits of asynchronous I/O, you should only use it for large
contiguous data items.
It is possible to perform asynchronous I/O on a large number of small items, but
the overall performance will suffer. This is because extra processing overhead is
required to maintain each item for asynchronous I/O. Performing asynchronous
I/O on a larger number of small items is strongly discouraged. The following are
two examples:
1. WRITE(unit_number, ID=idvar) a1(1:100000000:2)
2. WRITE(unit_number, ID=idvar) (a2(i,j),j=1,100000000)
Performing asynchronous I/O on unformatted sequential files is less efficient. This
is because each record might have a different length, and these lengths are stored
with the records themselves. You should use unformatted direct access or
unformatted stream access, if possible, to maximize the benefits of asynchronous
I/O.
Compiler-Generated Temporary I/O Items
There are situations when the compiler must generate a temporary variable to hold
the result of an I/O item expression. In such cases, synchronous I/O is performed
on the temporary variable, regardless of the mode of transfer that you specified in
the I/O statement. The following are examples of such cases:
1. For READ, when an array with vector subscripts appears as an input item:
a. integer a(5), b(3)
b = (/1,3,5/)
read(99, id=i) a(b)
b. real a(10)
read(99,id=i) a((/1,3,5/))
2. For WRITE, when an output item is an expression that is a constant or a
constant of certain derived types:
a. write(99,id=i) 1000
b. integer a
parameter(a=1000)
write(99,id=i) a
338 XL Fortran Enterprise Edition for AIX : User’s Guide