Hi Hubert, We discussed adding an alternative representation for manipulating window offsets, that would be overflow-safe (e.g. unsigned or "big enough"). The user would have to convert to this representation and then back to MPI_Aint in order to pass the displacement to an RMA communication routine. So, the proposed solution will work with the existing interface at the expense of a few extra calls. The fundamental problem is that in MPI-2, the window size if given by an (signed) MPI_Aint argument, which must be positive. Thus, in MPI-2, the window size and offset are both bounded at MPI_AINT_MAXVAL. Dynamic windows are not bounded in this way, so we need some safe way to do offset arithmetic on the full range of addresses. AFAIK, C doesn't define a signed representation and it doesn't define what will happen when you overflow a signed integer. So, I don't any solution where a user can do arithmetic on an MPI_Aint will work. Does Fortran define this behavior? Here's another proposal for safe displacement arithmetic would be to define a new routine: MPI_Address_add(MPI_Aint base, MPI_Aint disp, MPI_Aint new_base) Which returns new_base = base + disp. This would not require the addition of any new types and would make it MPI's job to do the arithmetic in a safe and portable way. This should also be Fortran-friendly. ~Jim. On 9/18/12 5:32 AM, Hubert Ritzdorf wrote:

Hi,

the MPI_Auint has the problem that this doesn't work for the RMA communication interfaces and that you have to create new interfaces for this.

I think another possibility would be to change the base address for dynamic windows to MPI_WIN_BASE_DYNAMIC which should be a predefined address and which could be MPI_MAX_AINT + 1, where MPI_MAX_AINT is the maximal positive value of an MPI_Aint..

Advantages: (*) You can address the entire memory range with the MPI_Aint (positive and negative) displacement No differences between Fortran and C. No integer overflow (*) Same value for Fortran and C Currently, you have to differentiate between dynamic windows created by C and Fortran with all the possible problems in mixing C and Fortran routines/functions since the base addresses are different. (*) Improved scalability Fixed predefined value which will be (probably) the same in all processes. In Fortran, the value of MPI_BOTTOM (address of a global variable) can be different for different processes (example: MPMD applications).

Address calculations would simple (ptr = ((char *) MPI_WIN_BASE_DYNAMIC) + displacement + 1) and could be placed into a macro for C. A function could work for Fortran. What you need is a single additional function MPI_Get_address_dynamic() which returns the displacement relative to MPI_WIN_BASE_DYNAMIC and not relative to MPI_BOTTOM.

Hubert

PS: I know its late. In the errata, we could replace MPI_BOTTOM by MPI_WIN_BASE_DYNAMIC and can add the function. MPI_Get_address_dynamic() in addition to the non-negative displacements. The MPI implementers would have the possibility to realize this and we to discuss it further. If we stay with MPI_BOTTOM as base for dynamic windows, it's fixed.

________________________________________ From: [email protected] [[email protected]] on behalf of Jim Dinan [[email protected]] Sent: Monday, September 17, 2012 11:12 PM To: MPI 3.0 Remote Memory Access working group Subject: [Mpi3-rma] Target displacement in dynamic windows

Hi All,

In today's meeting, we arrived at the following approach to addressing the target displacements issue for dynamic windows:

The MPI 3.0 errata should include the following two statements:

1. For dynamic windows, the target_disp argument to RMA communication operations is not restricted to non-negative values.

2. Users are cautioned that displacement arithmetic can overflow in variables with MPI_Aint type and result in unexpected values on some platforms. This issue will be addressed in a future version of MPI.

In MPI 3.1, we will look at two possible mechanisms for providing safe offset arithmetic:

1. MPI_Get_pointer(MPI_Aint address, void *ptr), which will allow a user to convert between addresses and pointers.

2. MPI_Auint -- an address integer which can represent the full range of MPI_Aint as a positive integer. This could be an unsigned integer, but for Fortran compatibiliy, it may need to be a singed integer that's larger than MPI_Aint. The following conversion functions would be provided:

MPI_Get_auint(MPI_Aint addr, MPI_Auint *uaddr) MPI_Get_aint(MPI_Auint uaddr, MPI_Aint *addr)

Please send comments. Thanks,

~Jim. _______________________________________________ mpi3-rma mailing list [email protected] http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma

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