[MPI3 Fortran] Fwd: Serious problem/bug in MPI libraries with the alignment of MPI_DOUBLE_PRECISION
Supalov, Alexander
alexander.supalov at intel.com
Wed Sep 28 10:48:09 CDT 2011
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: mpi3-fortran-bounces at lists.mpi-forum.org [mailto:mpi3-fortran-bounces at lists.mpi-forum.org] 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-report-F2008-2011-09-08-changeonlyplustickets.pdf
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" <ftillier at microsoft.com>
> To: "Rolf Rabenseifner" <rabenseifner at hlrs.de>, "Jeff Squyres" <jsquyres at cisco.com>, "Shinji Sumimoto"
> <s-sumi at labs.fujitsu.com>, "Hubert Ritzdorf" <hubert.ritzdorf at emea.nec.com>, "Howard Pritchard" <howardp at cray.com>,
> "Brian Smith" <smithbr at us.ibm.com>, "Charles J Archer" <archerc at us.ibm.com>, "Rajeev Thakur" <thakur at mcs.anl.gov>,
> "Bill Long" <longb at cray.com>, "Bill Gropp" <wgropp at uiuc.edu>, "Richard Graham" <rlgraham at ornl.gov>
> Cc: "N.M. Maclaren" <nmm1 at cam.ac.uk>, "Iain Bason" <iain.bason at oracle.com>
> 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.
> >
> >
--
Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner at hlrs.de
High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530
University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832
Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner
Nobelstr. 19, D-70550 Stuttgart, Germany . (Office: Allmandring 30)
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