No clarification necessary. Standard is not user guide. Semantics are clear from what is defined. Users who don't like the interface can write a library that does what they want.
Jeff On Thursday, February 11, 2016, Nathan Hjelm <hje...@lanl.gov> wrote: > > I should also say that I think this is something that may be worth > clarifying in the standard. Either semantic is fine with me but there is > no reason to change the behavior if it does not violate the standard. > > -Nathan > > On Thu, Feb 11, 2016 at 01:35:28PM -0700, Nathan Hjelm wrote: > > > > Jeff probably ran with MPICH. Open MPI's are consistent with our choice > > of definition for size=0: > > > > query: me=1, them=0, size=0, disp=1, base=0x0 > > query: me=1, them=1, size=4, disp=1, base=0x1097e30f8 > > query: me=1, them=2, size=4, disp=1, base=0x1097e30fc > > query: me=1, them=3, size=4, disp=1, base=0x1097e3100 > > query: me=2, them=0, size=0, disp=1, base=0x0 > > query: me=2, them=1, size=4, disp=1, base=0x109fe10f8 > > query: me=2, them=2, size=4, disp=1, base=0x109fe10fc > > query: me=2, them=3, size=4, disp=1, base=0x109fe1100 > > query: me=2, them=PROC_NULL, size=4, disp=1, base=0x109fe10f8 > > query: me=3, them=0, size=0, disp=1, base=0x0 > > query: me=3, them=1, size=4, disp=1, base=0x1088f30f8 > > query: me=3, them=2, size=4, disp=1, base=0x1088f30fc > > query: me=3, them=3, size=4, disp=1, base=0x1088f3100 > > query: me=3, them=PROC_NULL, size=4, disp=1, base=0x1088f30f8 > > query: me=0, them=0, size=0, disp=1, base=0x0 > > query: me=0, them=1, size=4, disp=1, base=0x105b890f8 > > query: me=0, them=2, size=4, disp=1, base=0x105b890fc > > query: me=0, them=3, size=4, disp=1, base=0x105b89100 > > query: me=0, them=PROC_NULL, size=4, disp=1, base=0x105b890f8 > > query: me=1, them=PROC_NULL, size=4, disp=1, base=0x1097e30f8 > > > > To be portable only use MPI_Win_shared_query and do not rely on the > > return value of base if you pass size = 0. > > > > -Nathan > > > > On Thu, Feb 11, 2016 at 08:23:16PM +0000, Peter Wind wrote: > > > Thanks Jeff, that was an interesting result. The pointers are here > well > > > defined, also for the zero size segment. > > > However I can't reproduce your output. I still get null pointers > (output > > > below). > > > (I tried both 1.8.5 and 1.10.2 versions) > > > What could be the difference? > > > Peter > > > mpirun -np 4 a.out > > > query: me=0, them=0, size=0, disp=1, base=(nil) > > > query: me=0, them=1, size=4, disp=1, base=0x2aee280030d0 > > > query: me=0, them=2, size=4, disp=1, base=0x2aee280030d4 > > > query: me=0, them=3, size=4, disp=1, base=0x2aee280030d8 > > > query: me=0, them=PROC_NULL, size=4, disp=1, base=0x2aee280030d0 > > > query: me=1, them=0, size=0, disp=1, base=(nil) > > > query: me=1, them=1, size=4, disp=1, base=0x2aabbb9ce0d0 > > > query: me=1, them=2, size=4, disp=1, base=0x2aabbb9ce0d4 > > > query: me=1, them=3, size=4, disp=1, base=0x2aabbb9ce0d8 > > > query: me=1, them=PROC_NULL, size=4, disp=1, base=0x2aabbb9ce0d0 > > > query: me=2, them=0, size=0, disp=1, base=(nil) > > > query: me=2, them=1, size=4, disp=1, base=0x2b1579dd40d0 > > > query: me=2, them=2, size=4, disp=1, base=0x2b1579dd40d4 > > > query: me=2, them=3, size=4, disp=1, base=0x2b1579dd40d8 > > > query: me=2, them=PROC_NULL, size=4, disp=1, base=0x2b1579dd40d0 > > > query: me=3, them=0, size=0, disp=1, base=(nil) > > > query: me=3, them=1, size=4, disp=1, base=0x2ac8d2c350d0 > > > query: me=3, them=2, size=4, disp=1, base=0x2ac8d2c350d4 > > > query: me=3, them=3, size=4, disp=1, base=0x2ac8d2c350d8 > > > query: me=3, them=PROC_NULL, size=4, disp=1, base=0x2ac8d2c350d0 > > > > > > > ---------------------------------------------------------------------- > > > > > > See attached. Output below. Note that the base you get for > ranks 0 and > > > 1 is the same, so you need to use the fact that size=0 at rank=0 > to know > > > not to dereference that pointer and expect to be writing into > rank 0's > > > memory, since you will write into rank 1's. > > > I would probably add "if (size==0) base=NULL;" for good measure. > > > Jeff > > > > > > $ mpirun -n 4 ./a.out > > > > > > query: me=0, them=0, size=0, disp=1, base=0x10bd64000 > > > > > > query: me=0, them=1, size=4, disp=1, base=0x10bd64000 > > > > > > query: me=0, them=2, size=4, disp=1, base=0x10bd64004 > > > > > > query: me=0, them=3, size=4, disp=1, base=0x10bd64008 > > > > > > query: me=0, them=PROC_NULL, size=4, disp=1, base=0x10bd64000 > > > > > > query: me=1, them=0, size=0, disp=1, base=0x102d3b000 > > > > > > query: me=1, them=1, size=4, disp=1, base=0x102d3b000 > > > > > > query: me=1, them=2, size=4, disp=1, base=0x102d3b004 > > > > > > query: me=1, them=3, size=4, disp=1, base=0x102d3b008 > > > > > > query: me=1, them=PROC_NULL, size=4, disp=1, base=0x102d3b000 > > > > > > query: me=2, them=0, size=0, disp=1, base=0x10aac1000 > > > > > > query: me=2, them=1, size=4, disp=1, base=0x10aac1000 > > > > > > query: me=2, them=2, size=4, disp=1, base=0x10aac1004 > > > > > > query: me=2, them=3, size=4, disp=1, base=0x10aac1008 > > > > > > query: me=2, them=PROC_NULL, size=4, disp=1, base=0x10aac1000 > > > > > > query: me=3, them=0, size=0, disp=1, base=0x100fa2000 > > > > > > query: me=3, them=1, size=4, disp=1, base=0x100fa2000 > > > > > > query: me=3, them=2, size=4, disp=1, base=0x100fa2004 > > > > > > query: me=3, them=3, size=4, disp=1, base=0x100fa2008 > > > > > > query: me=3, them=PROC_NULL, size=4, disp=1, base=0x100fa2000 > > > > > > On Thu, Feb 11, 2016 at 8:55 AM, Jeff Hammond < > jeff.scie...@gmail.com <javascript:;>> > > > wrote: > > > > > > On Thu, Feb 11, 2016 at 8:46 AM, Nathan Hjelm <hje...@lanl.gov > <javascript:;>> wrote: > > > > > > > > > > > > On Thu, Feb 11, 2016 at 02:17:40PM +0000, Peter Wind wrote: > > > > > I would add that the present situation is bound to give > > > problems for some > > > > > users. > > > > > It is natural to divide an array in segments, each > process > > > treating its > > > > > own segment, but needing to read adjacent segments too. > > > > > MPI_Win_allocate_shared seems to be designed for this. > > > > > This will work fine as long as no segment as size zero. > It can > > > also be > > > > > expected that most testing would be done with all > segments > > > larger than > > > > > zero. > > > > > The document adding "size = 0 is valid", would also make > people > > > confident > > > > > that it will be consistent for that special case too. > > > > > > > > Nope, that statement says its ok for a rank to specify that > the > > > local > > > > shared memory segment is 0 bytes. Nothing more. The standard > > > > unfortunately does not define what pointer value is returned > for a > > > rank > > > > that specifies size = 0. Not sure if the RMA working group > > > intentionally > > > > left that undefine... Anyway, Open MPI does not appear to be > out of > > > > compliance with the standard here. > > > > > > > MPI_Alloc_mem doesn't say what happens if you pass size=0 > either. The > > > RMA working group intentionally tries to maintain consistency > with the > > > rest of the MPI standard whenever possible, so we did not > create a new > > > semantic here. > > > MPI_Win_shared_query text includes this: > > > "If all processes in the group attached to the window specified > size = > > > 0, then the call returns size = 0 and a baseptr as if > MPI_ALLOC_MEM > > > was called with size = 0." > > > > > > > > > > > To be safe you should use MPI_Win_shared_query as suggested. > You can > > > > pass MPI_PROC_NULL as the rank to get the pointer for the > first > > > non-zero > > > > sized segment in the shared memory window. > > > Indeed! I forgot about that. MPI_Win_shared_query solves this > > > problem for the user brilliantly. > > > Jeff > > > -- > > > Jeff Hammond > > > jeff.scie...@gmail.com <javascript:;> > > > http://jeffhammond.github.io/ > > > > > > -- > > > Jeff Hammond > > > jeff.scie...@gmail.com <javascript:;> > > > http://jeffhammond.github.io/ > > > _______________________________________________ > > > users mailing list > > > us...@open-mpi.org <javascript:;> > > > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users > > > Link to this post: > > > http://www.open-mpi.org/community/lists/users/2016/02/28508.php > > > > > _______________________________________________ > > > users mailing list > > > us...@open-mpi.org <javascript:;> > > > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users > > > Link to this post: > http://www.open-mpi.org/community/lists/users/2016/02/28511.php > > > > > > > _______________________________________________ > > users mailing list > > us...@open-mpi.org <javascript:;> > > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users > > Link to this post: > http://www.open-mpi.org/community/lists/users/2016/02/28513.php > > -- Jeff Hammond jeff.scie...@gmail.com http://jeffhammond.github.io/
