Hi,
As you mentioned, it should be theoretically possible to mix 32 bit
machines with 64 bit machines if you don't use conflicting data type.
But, here are few points why this is not possible in LAM and our
position on them.
LAM does not handle mixed sizes in its communication. The issue is that
there is no sensible thing for the MPI to do in this situation. What
*should* happen if a 64 bit app sends a 64 bit long to the 32 bit
machine? The MPI could throw an error and then your program which
doesn't use conflicting types should run fine. But we're not setup to
handle that mainly because nobody has asked for this feature before.
Pointers to address space of different machines are used in LAM, for
example in gm RPI for RDMA kinds of operation, as well as using remote
pointers as unique identifiers. We *could* always assume 64 bits for
pointer sizes (although it's not quite as easy as that -- 32 bit
machines would have to look at the entire 64
bits for uniqueness and whatnot), but it hasn't been a priority and
therefore we never setup to do it.
We would appreciate any patches from you for adding these as long as
they don't affect performance. We will be happy to answer all your
questions on lam-devel list.
Thanks.
--
Shashwat Srivastav
LAM / MPI Developer (http://www.lam-mpi.org)
Indiana University
http://www.cs.indiana.edu/~ssrivast
On Thursday, Nov 13, 2003, at 11:46 America/Chicago, Praveen Agrawal
wrote:
> Hi,
> The LAM site states that mixing 32 bit machines with
> 64 bit machines is not compatible.
>
> I have been wondering why should this be. Below are my
> reasons to beleive so:(corresponding sizes)
> ILP32 ILP64
> char 8 8
> short 16 16
> int 32 32
> long 32 64<different>
> long long 64 64
> pointer 32 64<different>
> enum, float 32 32
> double 64 64
> long double 128 128
>
> So, in our mpi pgm if we just don't use long then
> there shouldn't be any problem. As sharing address
> across machines doen't make sence.
>
> In my institute we have got many sunfire machines(64)
> & intel clusters, we want to integrate them together.
> Can anyone please throw me light on this(pro/cons),
> specially the LAM developers.
>
> Cheers,
> Praveen.
>
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