Yes, that is what I meant.
Enclosed is a C example.
The point is that the code would logically make sense for task 0, but since it
asks for a segment of size=0, it only gets a null pointer, which cannot be used
to access the shared parts.
Peter
----- Original Message -----
> I think Peter's point is that if
> - the windows uses contiguous memory
> *and*
> - all tasks knows how much memory was allocated by all other tasks in the
> window
> then it could/should be possible to get rid of MPI_Win_shared_query
> that is likely true if no task allocates zero byte.
> now, if a task allocates zero byte, MPI_Win_allocate_shared could return a
> null pointer and hence makes MPI_Win_shared_query usage mandatory.
> in his example, task 0 allocates zero bytes, so he was expecting the returned
> pointer on task zero points to the memory allocated by task 1.
> if "may enable" should be read as "does enable", then returning a null
> pointer can be seen as a bug.
> if "may enable" can be read as "does not always enable", the returning a null
> pointer is compliant with the standard.
> I am clearly not good at reading/interpreting the standard, so using
> MPI_Win_shared_query is my recommended way to get it work.
> (feel free to call it "bulletproof", "overkill", or even "right")
> Cheers,
> Gilles
> On Thursday, February 11, 2016, Jeff Hammond < jeff.scie...@gmail.com >
> wrote:
> > On Wed, Feb 10, 2016 at 8:44 AM, Peter Wind < peter.w...@met.no > wrote:
>
> > > I agree that in practice the best practice would be to use
> > > Win_shared_query.
> >
>
> > > Still I am confused by this part in the documentation:
> >
>
> > > "The allocated memory is contiguous across process ranks unless the info
> > > key
> > > alloc_shared_noncontig is specified. Contiguous across process ranks
> > > means
> > > that the first address in the memory segment of process i is consecutive
> > > with the last address in the memory segment of process i - 1. This may
> > > enable the user to calculate remote address offsets with local
> > > information
> > > only."
> >
>
> > > Isn't this an encouragement to use the pointer of Win_allocate_shared
> > > directly?
> >
>
> > No, it is not. Win_allocate_shared only gives you the pointer to the
> > portion
> > of the allocation that is owned by the calling process. If you want to
> > access the whole slab, call Win_shared_query(..,rank=0,..) and use the
> > resulting baseptr.
>
> > I attempted to modify your code to be more correct, but I don't know enough
> > Fortran to get it right. If you can parse C examples, I'll provide some of
> > those.
>
> > Jeff
>
> > > Peter
> >
>
> > > > I don't know about bulletproof, but Win_shared_query is the *only*
> > > > valid
> > > > way
> > > > to get the addresses of memory in other processes associated with a
> > > > window.
> > >
> >
>
> > > > The default for Win_allocate_shared is contiguous memory, but it can
> > > > and
> > > > likely will be mapped differently into each process, in which case only
> > > > relative offsets are transferrable.
> > >
> >
>
> > > > Jeff
> > >
> >
>
> > > > On Wed, Feb 10, 2016 at 4:19 AM, Gilles Gouaillardet <
> > > > gilles.gouaillar...@gmail.com > wrote:
> > >
> >
>
> > > > > Peter,
> > > >
> > >
> >
>
> > > > > The bulletproof way is to use MPI_Win_shared_query after
> > > > > MPI_Win_allocate_shared.
> > > >
> > >
> >
>
> > > > > I do not know if current behavior is a bug or a feature...
> > > >
> > >
> >
>
> > > > > Cheers,
> > > >
> > >
> >
>
> > > > > Gilles
> > > >
> > >
> >
>
> > > > > On Wednesday, February 10, 2016, Peter Wind < peter.w...@met.no >
> > > > > wrote:
> > > >
> > >
> >
>
> > > > > > Hi,
> > > > >
> > > >
> > >
> >
>
> > > > > > Under fortran, MPI_Win_allocate_shared is called with a window size
> > > > > > of
> > > > > > zero
> > > > > > for some processes.
> > > > >
> > > >
> > >
> >
>
> > > > > > The output pointer is then not valid for these processes (null
> > > > > > pointer).
> > > > >
> > > >
> > >
> >
>
> > > > > > Did I understood this wrongly? shouldn't the pointers be
> > > > > > contiguous,
> > > > > > so
> > > > > > that
> > > > > > for a zero sized window, the pointer should point to the start of
> > > > > > the
> > > > > > segment of the next rank?
> > > > >
> > > >
> > >
> >
>
> > > > > > The documentation explicitly specifies "size = 0 is valid".
> > > > >
> > > >
> > >
> >
>
> > > > > > Attached a small code, where rank=0 allocate a window of size zero.
> > > > > > All
> > > > > > the
> > > > > > other ranks get valid pointers, except rank 0.
> > > > >
> > > >
> > >
> >
>
> > > > > > Best regards,
> > > > >
> > > >
> > >
> >
>
> > > > > > Peter
> > > > >
> > > >
> > >
> >
>
> > > > > > _______________________________________________
> > > > >
> > > >
> > >
> >
>
> > > > > > users mailing list
> > > > >
> > > >
> > >
> >
>
> > > > > > us...@open-mpi.org
> > > > >
> > > >
> > >
> >
>
> > > > > > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users
> > > > >
> > > >
> > >
> >
>
> > > > > > Link to this post:
> > > > > > http://www.open-mpi.org/community/lists/users/2016/02/28485.php
> > > > >
> > > >
> > >
> >
>
> > > > > _______________________________________________
> > > >
> > >
> >
>
> > > > > users mailing list
> > > >
> > >
> >
>
> > > > > us...@open-mpi.org
> > > >
> > >
> >
>
> > > > > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users
> > > >
> > >
> >
>
> > > > > Link to this post:
> > > > > http://www.open-mpi.org/community/lists/users/2016/02/28493.php
> > > >
> > >
> >
>
> > > > --
> > >
> >
>
> > > > Jeff Hammond
> > >
> >
>
> > > > jeff.scie...@gmail.com
> > >
> >
>
> > > > http://jeffhammond.github.io/
> > >
> >
>
> > > > _______________________________________________
> > >
> >
>
> > > > users mailing list
> > >
> >
>
> > > > us...@open-mpi.org
> > >
> >
>
> > > > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users
> > >
> >
>
> > > > Link to this post:
> > > > http://www.open-mpi.org/community/lists/users/2016/02/28496.php
> > >
> >
>
> > > _______________________________________________
> >
>
> > > users mailing list
> >
>
> > > us...@open-mpi.org
> >
>
> > > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users
> >
>
> > > Link to this post:
> > > http://www.open-mpi.org/community/lists/users/2016/02/28497.php
> >
>
> > --
>
> > Jeff Hammond
>
> > jeff.scie...@gmail.com
>
> > http://jeffhammond.github.io/
>
> _______________________________________________
> users mailing list
> us...@open-mpi.org
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> Link to this post:
> http://www.open-mpi.org/community/lists/users/2016/02/28503.php
#include <mpi.h>
#include <stdio.h>
/* test zero size segment.
run on at least 3 cpus
mpirun -np 4 a.out */
int main(int argc, char** argv) {
// Initialize the MPI environment
MPI_Init(NULL, NULL);
int world_size;
MPI_Comm_size(MPI_COMM_WORLD, &world_size);
int world_rank;
MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
int disp_unit = sizeof(world_rank);
MPI_Aint size;
MPI_Info info=0;
MPI_Win win;
int* ptr;
MPI_Comm comm;
if(world_rank == 0){
size = 0;
}else{
size = 1*disp_unit;
}
MPI_Win_allocate_shared (size, disp_unit, MPI_INFO_NULL, MPI_COMM_WORLD, &ptr, &win);
MPI_Win_fence(0 , win);
printf("rank %d, pointer value %p \n",world_rank,ptr);
if(ptr != NULL){
ptr[0] = world_rank;
MPI_Barrier(MPI_COMM_WORLD);
if(world_rank != world_size-1){
printf("rank %d: first value = %d , value at start of next segment = %d \n",
world_rank,ptr[0],ptr[1]);
}else{
printf("rank %d: first value = %d , value at end of preceding segment = %d \n",
world_rank,ptr[0],ptr[-1]);
}
}else{
MPI_Barrier(MPI_COMM_WORLD);
printf("rank %d: pointer set to NULL\n",world_rank);
}
MPI_Finalize();
}
