Bill, The concern that was raised to me (I have not followed the standard, but rely on other folks in the forum for this) is that there is potential for incorrect behavior if a single data object is being used both for computation and for asynchronous communication (disjoint portions of the object are involved in each). The specific concern, if I understood things correctly - others, please correct me if I am wrong - that compilers may respect the ASYNCHRONOUS only with respect to other fortran libraries, such as fortran async I/O, and feel free to modify portions of the buffer being used for communications, only later to restore the parameters. If this is indeed what could happen, this would clearly be a correctness problem for MPI. Seems to me that if this is indeed an issue, MPI will not be the only thing impacted, but also code using other languages, async libraries, etc. It is possible that there is a misunderstanding here, but with the concern about potential correctness issues, want to raise this earlier rather than later. Is there something to be concerned about here ? Thanks, Rich On 3/30/11 11:24 PM, "Bill Long" <[email protected]> wrote:

Hi Rich and others,

On 3/30/11 11:51 AM, Graham, Richard L. wrote:

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Dear member of the WG5 and J3 Fortran standardization committee,

As I understand the core of this thread, the problem is that some vendors don't actually implement anything for ASYNCHRONOUS because their libraries do not support actual asynchronous I/O operations, which makes the ASYNCHRONOUS semantics moot. If all of the vendors supported the semantics as expected, there would be no need for any changes.

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We recommend the following change of the first two paragraphs in this section of the Fortran 2008 standard:

5.3.4 ASYNCHRONOUS attribute

1 An entity with the ASYNCHRONOUS attribute is a variable that may be subject to asynchronous input/output or other asynchronous access by means other than Fortran.

The "or other asynchronous access be means other than Fortran" seems semantically identical to "an object may be referenced, defined, or become undefined, by means not specified by the program.". The second phrase is, of course, the definition of the VOLATILE attribute. The proposed change, then, seems to have the effect of making ASYNCHRONOUS the same as VOLATILE. Some comments on that possibility:

1) It is probably true that some vendors implement ASYNCHRONOUS that way already.

2) The vendors that are more precise in their ASYNCHRONOUS implementation would not be in favor of this change.

3) If this is the only solution, then using VOLATILE directly, and forgetting about ASYNCHRONOUS, would be a solution for the MPI non-blocking transfer routines. The implementation of VOLATILE is likely more uniform and widespread.

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This should be treated only as a wording proposal. We provide the above text only as a suggestion; J3/WG5 are certainly free to adjust it as they feel necessary. The key idea is that we need ASYNCHRONOUS to also include potential memory modifications from agents outside the scope of the Fortran standard.

Right. Sounds like VOLATILE to me.

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The following snipit is a correct code that allows the MPI library to internally use DMA, communications offload (e.g., to a NIC), or other method to asynchronously communicate parts of the array while the user application/main CPU simultaneously reads and writes to ***another*** part of the same array. Specifically, this code snipit is is free of race conditions.

USE mpi_f08 REAL, ASYNCHRONOUS :: buf(100,100) ! communicating a boundary of the array: CALL MPI_Irecv(buf(1,1:100),...req,...) DO j=1,100 DO i=2,100 buf(i,j)=.... ! work only on the inner area END DO END DO CALL MPI_Wait(req,...)

It is not clear to me how this program example would show any difference in performance or semantics if ASYNCHRONOUS were replaced by VOLATILE. Even with the current definitions of the attributes, it is extremely unlikely that a compiler applies the asynchronous attribute on an element by element basis for the array buf.

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With the current status of Fortran 2008 + TR 29113 (Version N1845), a compiler can ignore the ASYNCHRONOUS attribute. For example, a compiler may choose to not implement Fortran asynchronous I/O in an asynchronous way, meaning that there is no need for the compiler to make any restrictions on optimization.

To be clear: with the current Fortran 2008, this optimization is still allowed. With our proposal, this optimization will be prohibited.

We want to mention that the use of VOLATILE would solve the problem, but at a high performance price because it would also prohibit any optimization of the "work" part (e.g., automatic cache optimization, instruction reordering, etc.). Especially since Fortran is used with MPI in high-performance, numerically intensive applications, we feel that VOLATILE cannot be recommended as a solution.

So, it appears that you are saying that the compiler cannot make the element-by-element distinction for the VOLATILE attribute, but it could for ASYNCHRONOUS (in order for your example to optimize the way you hope). Seems like a weak point in the argument.

I agree that VOLATILE has the issues you state. But I don't see how the new definition of ASYNCHRONOUS avoids the same issues.

The MPI standard can state what is required of compiler implementations for them to be compatible with MPI. I think a more direct solution would be to require that the compiler vendor supports the ASYNCHRONOUS semantics if it is to be used with MPI.

Cheers, Bill

-- Bill Long [email protected] Fortran Technical Support & voice: 651-605-9024 Bioinformatics Software Development fax: 651-605-9142 Cray Inc./Cray Plaza, Suite 210/380 Jackson St./St. Paul, MN 55101