Well I'm afraid I'm back with my writing problems...
The following program :
program io_try
implicit none
integer :: i, n = 1000000
open(1,file='io_try.out')
do i = 1, n
write(1,'(i7)') i ! just write a lot of junk
enddo
close(1)
end
compiled with g77:
g77 -O3 -ffree-form io_try.f -o io_try
time io_try
real 0m1.048s
user 0m1.002s
sys 0m0.022s
compiled with gfortran
gfortran -O3 -ffree-form io_try.f -o io_try
rm io_try.out
time io_try
real 0m2.041s
user 0m2.007s
sys 0m0.034s
time io_try
real 0m32.868s
user 0m10.892s
sys 0m21.969s
g77 -O3 -ffree-form io_try.f -o io_try
time io_try
real 0m1.010s
user 0m0.991s
sys 0m0.018s
To summarize, gfortran takes twice as much time as g77 writing stuff to
a file (two first examples).
But, on top of that, it is 10 times slower when the result file already
exists (taking in account user time only, but sys time also shows large
differences), whereas g77 is insensitive to the existence of an old
result file...
I'm afraid I have no idea of the reasons for that.
I have also noticed that a program compiled with g77 will not only
overwrite an existing file but erase all previous data (even if the new
file is shorter that the old one), whereas a gfortran compiled program
will only overwrite and, when the new file is shorter leave previous
results. I don't know whether that is relevant.
Regards,
Philippe Depondt dep@ccr.jussieu.fr
# INSP Institut des Nano-Sciences de Paris
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