On Sunday, August 21, 2016, Ben Menadue <ben.mena...@nci.org.au
<mailto:ben.mena...@nci.org.au>> wrote:
Hi,
In Fortran, using uninitialised variables is undefined behaviour.
In this case, it’s being initialised to zero (either by the
compiler or by virtue of being in untouched memory), and so
equivalent to MPI_COMM_WORLD in OpenMPI. Other MPI libraries don’t
have MPI_COMM_WORLD .eq. 0 and so the same program would fail.
Similarly, if the same memory has previously been stored to (and
so non-zero) and the compiler doesn’t zero-initialise the variable
(most won’t unless you explicitly ask for it), it will fail with
OpenMPI.
Such false successes are a (the?) reason why MPI libraries should not
define valid handles to default initializers, whether they
be standardized or implementation-specific...
Jeff
Just keep in mind that the same is true for *all* variables in
Fortran; even integers and reals have undefined value until
they’re first stored to. This can be quite annoying as specifying
the initial value when declaring them also gives them the SAVE
attribute…
Cheers,
Ben
*From:*users [mailto:users-boun...@lists.open-mpi.org
<javascript:_e(%7B%7D,'cvml','users-boun...@lists.open-mpi.org');>]
*On Behalf Of *Matt Thompson
*Sent:* Sunday, 21 August 2016 3:07 AM
*To:* Open MPI Users <users@lists.open-mpi.org
<javascript:_e(%7B%7D,'cvml','users@lists.open-mpi.org');>>
*Subject:* Re: [OMPI users] mpi_f08 Question: set comm on
declaration error, and other questions
On Fri, Aug 19, 2016 at 8:54 PM, Jeff Squyres (jsquyres)
<jsquy...@cisco.com
<javascript:_e(%7B%7D,'cvml','jsquy...@cisco.com');>> wrote:
On Aug 19, 2016, at 6:32 PM, Matt Thompson <fort...@gmail.com
<javascript:_e(%7B%7D,'cvml','fort...@gmail.com');>> wrote:
> > that the comm == MPI_COMM_WORLD evaluates to .TRUE.? I
discovered that once when I was printing some stuff.
>
> That might well be a coincidence. type(MPI_Comm) is not a
boolean type, so I'm not sure how you compared it to .true.
>
> Well, I made a program like:
>
> (208) $ cat test2.F90
> program whoami
> use mpi_f08
> implicit none
> type(MPI_Comm) :: comm
> if (comm == MPI_COMM_WORLD) write (*,*) "I am MPI_COMM_WORLD"
> if (comm == MPI_COMM_NULL) write (*,*) "I am MPI_COMM_NULL"
> end program whoami
> (209) $ mpifort test2.F90
> (210) $ mpirun -np 4 ./a.out
> I am MPI_COMM_WORLD
> I am MPI_COMM_WORLD
> I am MPI_COMM_WORLD
> I am MPI_COMM_WORLD
>
> I think if you print comm, you get 0 and MPI_COMM_WORLD=0
and MPI_COMM_NULL=2 so...I guess I'm surprised. I'd have
thought MPI_Comm would have been undefined until defined.
I don't know the rules here for what happens in Fortran when
comparing an uninitialized derived type. The results could be
undefined...?
> Instead you can write a program like this:
>
> (226) $ cat helloWorld.mpi3.F90
> program hello_world
>
> use mpi_f08
>
> implicit none
>
> type(MPI_Comm) :: comm
> integer :: myid, npes, ierror
> integer :: name_length
>
> character(len=MPI_MAX_PROCESSOR_NAME) :: processor_name
>
> call mpi_init(ierror)
>
> call MPI_Comm_Rank(comm,myid,ierror)
> write (*,*) 'ierror: ', ierror
> call MPI_Comm_Size(comm,npes,ierror)
> call
MPI_Get_Processor_Name(processor_name,name_length,ierror)
>
> write (*,'(A,X,I4,X,A,X,I4,X,A,X,A)') "Process", myid,
"of", npes, "is on", trim(processor_name)
>
> call MPI_Finalize(ierror)
>
> end program hello_world
> (227) $ mpifort helloWorld.mpi3.F90
> (228) $ mpirun -np 4 ./a.out
> ierror: 0
> ierror: 0
> ierror: 0
> ierror: 0
> Process 2 of 4 is on compy
> Process 1 of 4 is on compy
> Process 3 of 4 is on compy
> Process 0 of 4 is on copy
That does seem to be odd output. What is the hostname on your
machine?
Oh well, I (badly) munged the hostname on the computer I ran on
because it had the IP address within. I figured better safe than
sorry and not broadcast that out there. :)
FWIW, I changed your write statement to:
print *, "Process", myid, "of", npes, "is on",
trim(processor_name)
and after I added a "comm = MPI_COMM_WORLD" before the call to
MPI_COMM_RANK, the output prints properly for me (i.e., I see
my hostname).
> This seems odd to me. I haven't passed in MPI_COMM_WORLD as
the communicator to MPI_Comm_Rank, and yet, it worked and the
error code was 0 (which I'd take as success). Even if you
couldn't detect this at compile time, I'm surprised it doesn't
trigger a run-time error. Is this the correct behavior
according to the Standard?
I think you're passing an undefined value, so the results will
be undefined.
It's quite possible that the comm%mpi_val inside the comm is
(randomly?) assigned to 0, which is the same value as mpif.f's
MPI_COMM_WORLD, and therefore your comm is effectively the
same as mpi_f08's MPI_COMM_WORLD -- which is why MPI_COMM_RANK
and MPI_COMM_SIZE worked for you.
Indeed, when I run your program, I get:
-----
$ ./foo
[savbu-usnic-a:31774] *** An error occurred in MPI_Comm_rank
[savbu-usnic-a:31774] *** reported by process [756088833,0]
[savbu-usnic-a:31774] *** on communicator MPI_COMM_WORLD
[savbu-usnic-a:31774] *** MPI_ERR_COMM: invalid communicator
[savbu-usnic-a:31774] *** MPI_ERRORS_ARE_FATAL (processes in
this communicator will now abort,
[savbu-usnic-a:31774] *** and potentially your MPI job)
-----
I.e., MPI_COMM_RANK is aborting because the communicator being
passed in is invalid.
Huh. I guess I'd assumed that the MPI Standard would have made
sure a declared communicator that hasn't been filled would have
been an error to use.
When I get back on Monday, I'll try out some other compilers as
well as try different compiler options (e.g., -g -O0, say). Maybe
this is just an undefined behavior, but it's not one I'm too
pleased about. I'd have expected the result you got. Now I'm
scared that somewhere in my code, in the future, there could be a
rogue comm declared and never nulled out, so I think it's
executing on some subcomm, but it runs on MCW.
Welp, maybe for safety it's time to make a vim macro that does:
type(MPI_Comm) :: comm
comm = MPI_COMM_NULL
I'm pretty sure that will *never* execute anything on that comm
until I fill it with what I want later on. Just wish I could do
that in one statement.
--
Matt Thompson
Man Among Men
Fulcrum of History
--
Jeff Hammond
jeff.scie...@gmail.com <mailto:jeff.scie...@gmail.com>
http://jeffhammond.github.io/
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