Cool things you can do with G95

G95 is much more than just a compiler. It is a system for doing computations. There are several very cool things that you can do with g95:

Runtime configuration
Suspending and resuming program execution

Runtime options

There are many runtime options that can be set by the user. Many of these are left processor-dependent by the standard, but may be set by the end user at runtime. To get a complete list of these options, run any g95-compiled program with --g95. Instead of running your program, you'll get a list of variables and what they are set to.

To modify a setting, set the appropriate environment variable. Under the bash shell this is something like:

export G95_ENDIAN=big

Corefile Resume

The execution of a G95-compiled programs can be suspended and resumed.

If you interrupt a program by sending it the QUIT signal, which is usually bound to control-backslash the program will write an executable file named 'dump' to the current directory.

Running this file causes the execution of your program to resume from when the dump was written.

andy@fulcrum:~/g95/g95 % cat tst.f90
b = 0.0
do i=1, 10
  do j=1, 3000000
    call random_number(a)
    a = 2.0*a - 1.0
    b = b + sin(sin(sin(a)))
  enddo
  print *, i, b
enddo
end

andy@fulcrum:~/g95/g95 % g95 tst.f90
andy@fulcrum:~/g95/g95 % a.out
 1 70.01749
 2 830.63153
 3 987.717
 4 316.48703
 5 -426.53815
 6 25.407673       (control-\ hit)
Process dumped
 7 -694.2718
 8 -425.95465
 9 -413.81763
 10 -882.66223
andy@fulcrum:~/g95/g95 % ./dump
Restarting
............Jumping
 7 -694.2718
 8 -425.95465
 9 -413.81763
 10 -882.66223
andy@fulcrum:~/g95/g95 %

Any open files must be present and in the same places as in the original process. If you link against other languages, this may not work.

While the main use is allowing you to preserve the state of a run across a reboot, other possibilities include pushing a long job through a short queue or moving a running process to another machine.

Automatic checkpointing of your program can be done by setting the environment variable G95_CHECKPOINT with the number of seconds to wait between dumps.

New checkpoint files overwrite old checkpoint files.

A program can cause a checkpoint to be written with the following code:

subroutine dump

  interface
    function getpid() bind(c, name='getpid')
      integer :: getpid
    end function getpid

    subroutine kill(signal, pid) bind(c, name='kill')
      integer, value :: signal, pid
    end subroutine kill
  end interface

  call kill(getpid(), 3)  ! SIGQUIT = 3

end subroutine dump