gfortran documentation fix: replace getpid() with THIS_IMAGE() in init_random_seed()

[Janne Blomqvist]

On Tue, May 17, 2016 at 2:30 AM, Damian Rouson
<damian@sourceryinstitute.org> wrote:
> All,
>
> Would THIS_IMAGE() be a reasonable, standard-conforming replacement for
> the gfortran extension “getpid()” in the following documentation page?
>
> https://gcc.gnu.org/onlinedocs/gfortran/RANDOM_005fSEED.html
>
> From the comment a few lines above the call, it seems the primary motivation
> for generating a process ID (PID) is for each parallel instance to receive a
> unique integer result.  In my very limited testing with gfortran/OpenCoarrays
> in shared memory on Linux and OS X, the getpid() result differs from that of
> THIS_IMAGE() by an offset that is constant across all images.
>
> THIS_IMAGE() might be superior in that it guarantees each image a
> unique result even in distributed memory, which I assume getpid() does
> not guarantee.  If others agree, I’ll submit a documentation patch.  I will
> also add a comment suggesting that the user replace THIS_IMAGE()
> with the appropriate procedure for the user’s chosen parallelization
> technology (e.g., MPI process rank or OpenMP thread number).

I can't really make heads or tails of the description at
https://gcc.gnu.org/onlinedocs/gfortran/THIS_005fIMAGE.html, but from
looking at the example and some other examples I found with google, it
seems that THIS_IMAGE() is mostly equivalent to MPI_Comm_Rank() in the
MPI world? That is, it's an integer in the range [1, NUM_IMAGES()]?

If so, then replacing getpid() with THIS_IMAGE() might not be such a
good idea, what if a user launches several instances of a serial
program on the same machine (which I think is a vastly more common
occurrence than somebody actually using co-arrays)? Then THIS_IMAGE()
will return 1 for all processes?

If you're worried about getpid() returning the same value on multiple
machines in the distributed memory case (unlikely, but entirely
possible), you could amend the example to additionally XOR
THIS_IMAGE() in addition to the current time and getpid(), instead of
replacing the usage of getpid().

As for threads or OpenMP, currently the GFortran random number
generator is a single serial generator protected by a mutex, it
doesn't make sense to initialize it separately in each thread. I
submitted a RFC patch a while ago to fix this (
https://gcc.gnu.org/ml/fortran/2015-12/msg00119.html ), I was planning
to dust if off and submit it properly for gcc 7 at some point, but
we'll see if I find the time..
-- 
Janne Blomqvist