Alexander and Charles, thank you for your lists. I updated the summary_out.txt . It has now three bad categories: A) 4 / 8 / 4 and B) 4 / 8 / 8 and C) 4 / 4 / 8 which is new and very strange because MPI's alignment has nothing to do with the compilers alignment. Alexander,
Why don't we leave this to the implementation and the compiler to decide?
The reason is simple: It is not possible to write portable code according to the rule in MPI-2.2 p79:7-9 "The definition of extent is motivated by the assumption that the amount of padding added at the end of each structure in an array of structures is the least needed to fulfill alignment constraints." The goal of this standardization is to be able to write portable application code. And the goal of this MPI sentence is, to be able to write such portable code without use of MPI_TYPE_CREATE_RESIZED. In C, this should work, in Fortran there are incompatibilities that should be resolved with MPI-3.0. Best regards Rolf ----- Original Message -----
From: "Alexander Supalov" <[email protected]> To: "MPI-3 Fortran working group" <[email protected]> Sent: Wednesday, September 28, 2011 5:48:09 PM Subject: Re: [MPI3 Fortran] Fwd: Serious problem/bug in MPI libraries with the alignment of MPI_DOUBLE_PRECISION Hi,
Why don't we leave this to the implementation and the compiler to decide? Our testing results (see attachment) are all over the map on the platforms we currently support. I'd not like to deal w/ any of this specifically in the future product. What happens is platform specific and looks acceptable to the users. And the standard example may just show one possible resolution. Why bother?
Best regards.
Alexander
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Rolf Rabenseifner Sent: Wednesday, September 28, 2011 9:14 AM To: MPI-3 Fortran working group Subject: Re: [MPI3 Fortran] Fwd: Serious problem/bug in MPI libraries with the alignment of MPI_DOUBLE_PRECISION
Fab and all,
your idea seems optimal, but ... (see later)!!!!
In detail: We have the problem that the Intel ***Fortran*** compiler answers with two different values a=4 and b=8. Both are Fortran values, a=4 for Fortran SEQUENCE derived types and b=8 for Fortran BIND(C) derived types. (Fortran non-sequence and non-bind(C) derived types are not really supported by MPI). The MPI Forum has to decide which value should be used in the example on page 581 of https://svn.mpi-forum.org/trac/mpi-forum-web/attachment/ticket/229/mpi-repor... i.e., whether Alternative 1 or 2 is the correct one.
Your proposal (I name it Alternative 3) is that the user has to choose the mapped C basic type. This is a non-trivial mapping. For MPI_INTEGER it is MPI_INT or MPI_LONG? For MPI_INTEGER8 it is MPI_LONG_LONG? What are we doing with the unnamed predefined datatypes, produced by MPI_TYPE_CREATE_F90_COMPLEX / _REAL / _INTEGER?
But your proposal inspires me to propose an Alternative 4.1 and 4.2:
We need a routine that converts a given Fortran basic named or unnamed datatype with
4.1) by default "SEQUENCE alignment" into an unnamed predefined datatype with "BIND(C) alignment"; this works together with Alternative 1 for page 581. MPI_F2003_CONVERT_SEQ_TO_BIND_C(IN seq_datatype, OUT bind_c_datatype)
4.2) by default "BIND(C) alignment" into an unnamed predefined datatype with "SEQUENCE alignment"; this works together with Alternative 2 for page 581. MPI_F2003_CONVERT_BIND_C_TO_SEQ(IN bind_c_datatype, OUT seq_datatype)
Best regards Rolf
----- Original Message -----
From: "Fab Tillier" <[email protected]> To: "Rolf Rabenseifner" <[email protected]>, "Jeff Squyres" <[email protected]>, "Shinji Sumimoto" <[email protected]>, "Hubert Ritzdorf" <[email protected]>, "Howard Pritchard" <[email protected]>, "Brian Smith" <[email protected]>, "Charles J Archer" <[email protected]>, "Rajeev Thakur" <[email protected]>, "Bill Long" <[email protected]>, "Bill Gropp" <[email protected]>, "Richard Graham" <[email protected]> Cc: "N.M. Maclaren" <[email protected]>, "Iain Bason" <[email protected]> Sent: Wednesday, September 28, 2011 12:35:53 AM Subject: RE: Serious problem/bug in MPI libraries with the alignment of MPI_DOUBLE_PRECISION Hi Rolf,
It seems that all the a == 4 results come from using the Intel Fortran compiler suite. Given that Microsoft doesn't ship a Fortran compiler, mandating a == 8 doesn't put us in a position to succeed. Therefore, I cannot support such a proposal.
however, shouldn't a DOUBLE PRECISION BIND(C) derived type map to MPI_DOUBLE, not MPI_DOUBLE_PRECISION?
Alternative 3:
Can't we simply document that MPI_DOUBLE_PRECISION (and any Fortran types that need similar treatment) apply only to SEQUENCE derived types, while BIND(C) derived types should use the C types?
-Fab
Rolf Rabenseifner wrote on Tue, 27 Sep 2011 at 14:38:45
Dear all,
thank you very much for the fast answers. The result is maximal bad, i.e., about 50% of the MPIs use SEQUENCE alignments and other about 50% use BIND(C) based calculation of the alignment.
Here the results (also attached as summary_out.txt). Please read carefully my proposal after this summary sheet.
In all protocols: ----------------- Size of one Fortran REAL = 4 Size of one Fortran DOUBLE PRECISION = 8 Results: --------
a / b / c = Alignments of: - DOUBLE PRECISION within a SEQUENCE derived type = a - DOUBLE PRECISION within a BIND(C) derived type = b - MPI_DOUBLE_PRECISION (= k_i in MPI-2.2 p.78:45) = c
A) The 4 / 8 / 4 group = alignment of MPI_DOUBLE_PRECISION is correct for (old-style) SEQUENCE derived types
JeffSquyres_OpenMP_linux-x86-64-ifort.txt: Intel + OpenMPI on x86-64 Results: 4 / 8 / 4 FabTillier_Intel+MicrosoftMPI_out.txt: Intel + MS-MPI on ? Results: 4 / 8 / 4
B) The 4 / 8 / 8 group = alignment of MPI_DOUBLE_PRECISION is correct for (modern) BIND(C) derived types
Rajeev_mpich_out.ifort.x86-64.txt: Intel + mpich2 on x86-64 Results: 4 / 8 / 8 Rolf_asama+cray_output.txt: Intel + NEC-MPI? on asama Results: 4 / 8 / 8 BillLong_Cray_5compiler_out.txt: Intel + CrayMPI on Cray XE6 Results: 4 / 8 / 8 BrianSmith-IBM_outlog-bgp-xl.txt: Intel + IBM-XL on BGP Results: 4 / 8 / 8 BrianSmith-IBM_outlog-xlf.txt: xlf + IBM-XL on ? Results: 4 / 8 / 8
C) No problems because all DOUBLE PRECISION alignments are identical = 8
BillLong_Cray_5compiler_out.txt: PGI + CrayMPI on Cray XE6 Results: 8 / 8 / 8 BillLong_Cray_5compiler_out.txt: Cray + CrayMPI on Cray XE6 Results: 8 / 8 / 8 BillLong_Cray_5compiler_out.txt: gfortran + CrayMPI on Cray XE6 Results: 8 / 8 / 8 BillLong_Cray_5compiler_out.txt: pathscale + CrayMPI on Cray XE6 Results: 8 / 8 / 8 BrianSmith-IBM_outlog-gcc.txt: gfortran + IBM-XL on BGQ Results: 8 / 8 / 8 JeffSquyres_OpenMP_linux-x86-64-gfortran.txt: gfortran + OpenMPI on x86-64 Results: 8 / 8 / 8 JeffSquyres_OpenMP_osx-x86-84-gfortran.txt: gfortran + OpenMPI on osx-x86-64 Results: 8 / 8 / 8 Rajeev_gnu_laptop_out.txt: gfortran + mpich2 on laptop Results: 8 / 8 / 8 Rajeev_mpich_out.gfortran.x86-64.txt: gfortran + mpich2 on x86-64 Results: 8 / 8 / 8
D) No problems because all DOUBLE PRECISION alignments are identical = 4
Rajeev_mpich_out.gfortran.ia32.txt: gfortran + mpich2 on ia32 Results: 4 / 4 / 4 Rajeev_mpich_out.ifort.ia32.txt: Intel + mpich2 on ia32 Results: 4 / 4 / 4
Proposal: ---------
The goal of a standard is the possibility to write portable code. Therefore the answer should be standardized and not implementation- specific, or in other words, not defining it is the worst solution.
I expect only a few Fortran codes are using arrays of derived types in Fortran. The oldest library is mpich. Therefore I would propose to standardize Fortran alignments are based on BIND(C) which should be the same as in C.
This would imply that IBM-XL and mpich2 and derivatives need no change. But it also implies that the younger OpenMPI and Microsoft-MPI need a change. These implementation may add an environment switch to keep the old behavior if this switch is set, but I would expect that there is nearly no user who needs or want to use this switch.
Fab and Jeff, could you live with this proposal?
Best regards Rolf
PS: Additional remarks: Yes, some compiler detect that DOUBLE PR. on 4 byte alignment is inefficient, but this is only an information. Other compilers detect, that we use DOUBLE PRECISION in BIND(C) derived types instead of using the C double through these new intrinsic C types in Fortran. This is also okay and all compiler work as expected, they use the C alignment.
Rolf Rabenseifner wrote on Tue, 27 Sep 2011 at 09:14:33
In my output from Cray, the compilers - gnu - pgi - cray all use 8 byte double alignment in C and Fortran. ==> no problem with those compilers on the tested platform. Maybe we have differences between IA64 and 32 architectures for the same compiler.
Important is to check MPI with those compilers that have Fortran 4 byte double precision alignment (e.g. Intel).
Best regards Rolf
On Sep 27, 2011, at 10:21 AM, Rolf Rabenseifner wrote:
Dear all, (now with attachment)
as far as I know, you represent some MPI libraries that are not directly based on another one in the list: - mpich2: Rajeev - OpenMPI: Jeff - IBM: Rich Treumann - NEC: Hubert - Fujitsu: Shinji Sumimoto - Microsoft: Fab (Which independent library is missing on this list?) (Who of the list uses already mich2 or OpenMPI as bases or does not provide a Fortran binding, and can be therefore removed from this list?)
The problem is simple and has at the end only three numbers:
Alignments of: - DOUBLE PRECISION within a SEQUENCE derived type = 4 (e.g. with Intel) - DOUBLE PRECISION within a BIND(C) derived type = 8 - MPI_DOUBLE_PRECISION (= k_i in MPI-2.2 p.78:45) = 4 or 8 The details: Which is the alignment of Fortran DOUBLE PRECISION according to k_i on MPI-2.2, page 78 line 45 and page 96 line 42.
Since 1977 (Fortran 77) in Fortran COMMON blocks and 1990 (Fortran 90) in Fortran SEQUENCE derived types (which have the same memory layout according to the SEQUENCE-rules), the MPI alignment and the Fortran alignment in such constructs should be the same, e.g., the Intel compiler has alignment 4 !!!
This may be significantly different to C, where Intel uses alignment 8!
With Fortran 2003, we got the Fortran BIND(C) derived types. Here, a Fortran DOUBLE PRECISION has with Intel's compiler the alignment 8.
As far as I understand, mpich2 and Cray produce following results with an Intel compiler:
Alignments of: - DOUBLE PRECISION within a SEQUENCE derived type = 4 - DOUBLE PRECISION within a BIND(C) derived type = 8 - MPI_DOUBLE_PRECISION (= k_i in MPI-2.2 p.78:45) = 8
This is definitely wrong compared to MPI-1.1 and MPI-2.0 but may be helpful for MPI-3.0 when we like to switch to a definition that is based on BIND(C).
Question: What is the output of the attached test program when running with - your MPI library - and your compiler - and with Intel's compiler
Please can you send me the output of such mpiruns?
Thanks in advance and best regards Rolf
PS: We need not to care about 8/8/8. Only 4/8/x causes problems: If all those have x=8 then I'll propose to adopt the MPI-3.0 to the reality. If we have some implementations with 4/8/8 and others with 4/8/4 then we have a serious problem because only one answer can be correct, and the question would be whether the historical answer (x=4) or the modern answer (x=8) should be chosen.
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