[MPIWG Fortran] Profiling for MPI_SIZEOF?

[Jeff Squyres (jsquyres)]

On May 15, 2014, at 4:26 PM, Bill Long <longb at cray.com> wrote:

>> 2. Without support for TYPE(*), DIMENSION(..) (which no compiler supports yet), an MPI implementation has to provide N overloaded versions of MPI_SIZEOF to be able to determine the type of the first argument.
> 
> “no compilers” is a bit strong.  Cray and gfortran both support this.  However, “only a minority of compilers” is certainly true.

I thought gfortran supported TYPE(*) DIMENSION(*), not DIMENSION(..).  Is that incorrect?

My apologies about incorrectly citing the Cray compiler!  :-(

> That said, the optimal implementation would be to write all of the specific versions (I’m seeing 10) for a generic named MPI_SIZEOF.  For each specific version, the executed code reduces to
> 
>   size = storage_size(x)/8
> 
> which the compiler will evaluate at compile time based on the type of x. 

No, there's 10 * 7 versions, right?  One for each type (10), and then 1 for each rank.

The language in the MPI spec explicitly allows you to pass an array as the argument of x (although it does say that it will return the size of the base element -- not the array).

>> 3. However, unlike how we specified the Fortran routine names in the MPI-3.0 errata so that they could be intercepted by PMPI, there are no specified specific subroutine names for the overloaded MPI_SIZEOF routines.
> 
> Right.  Those got made up internally.
> 
> % grep -i sizeof mpi_f08_types.f90
> ! MPI_Sizeof in 17.1.9
> interface MPI_Sizeof                                                                               
>   module procedure MPI_Sizeof_xint8
>   module procedure MPI_Sizeof_xint16
>   module procedure MPI_Sizeof_xint32
>   module procedure MPI_Sizeof_xint64
>   module procedure MPI_Sizeof_xreal32
>   module procedure MPI_Sizeof_xreal64
>   module procedure MPI_Sizeof_xreal128
>   module procedure MPI_Sizeof_xcomplex32
>   module procedure MPI_Sizeof_xcomplex64
>   module procedure MPI_Sizeof_xcomplex128
>   private :: MPI_Sizeof_xint8
>   private :: MPI_Sizeof_xint16
>   private :: MPI_Sizeof_xint32
>   private :: MPI_Sizeof_xint64
>   private :: MPI_Sizeof_xreal32
>   private :: MPI_Sizeof_xreal64
>   private :: MPI_Sizeof_xreal128
>   private :: MPI_Sizeof_xcomplex32
>   private :: MPI_Sizeof_xcomplex64
>   private :: MPI_Sizeof_xcomplex128
> 
> …..
> 
> 
>> In reality, I'm assuming no tool will ever care about intercepting MPI_SIZEOF (at least, they haven't complained in the several years that Open MPI/MPICH have provided it).  But it's an inconsistency in the standard.
> 
> We also don’t intercept the comparison routines that implement the == and /= operators for the new and old handles.  (Again a case of trivial code where all the timing would be in the interceptor overhead.) 

Oh -- good point; forgot about those...

> Actually, I’d make the same argument for the conversion routines between the various forms of MPI_Status.  

The c2f/f2c functions are defined to be not profile-able, so we're ok there.

>> 
>> I see two obvious solutions -- perhaps there are more:
>> 
>> 1. Define that MPI_SIZEOF is not able to be profiled.
>> 2. For compilers that do not support TYPE(*), DIMENSION(..), define some kind of pattern of specific procedure names for the overloaded instances of MPI_SIZEOF.
>> 
>> I'm in favor of #1.
> 
> Definitely like #1.
> 
> Cheers,
> Bill
> 
>> 
>> Thoughts?
>> 
>> -- 
>> Jeff Squyres
>> jsquyres at cisco.com
>> For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/
>> 
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> 
> Bill Long                                                                       longb at cray.com
> Fortran Technical Suport  &                                  voice:  651-605-9024
> Bioinformatics Software Development                     fax:  651-605-9142
> Cray Inc./ Cray Plaza, Suite 210/ 380 Jackson St./ St. Paul, MN 55101
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-- 
Jeff Squyres
jsquyres at cisco.com
For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/