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Subject: Re: [OMPI users] Fortran derived types
From: Richard Treumann (treumann_at_[hidden])
Date: 2010-05-07 09:46:05


If someone is deciding whether to use complex datatypes or stick with
contiguous ones, they need to look at their own situation. There is no
simple answer. The only thing that is fully predictable is that an MPI
operation, measured in isolation, will be no slower with contiguous data
than with discontiguous.

The question for a particular application is:

 in the application context, how does the performance with this
discontiguous datatype compare with the performance I get with other
solutions?"

The other solutions include anything your application must do to allow it
to use contiguous datatypes. (most often, packing & unpacking)

The water gets even more muddy when you consider that each MPI
implementation has differences in how it processes discontiguous data and
even a single MPI (like OpenMPI) could have different underlying trade
offs, depending on the capabilities of the underlying hardware.

It should not matter whether the program is written in C or Fortran. The
cost of processing a discontiguous datatype is tied to the layout of the
data in memory and both languages can produce equally simple or complex
memory layouts.

Dick Treumann - MPI Team
IBM Systems & Technology Group
Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846 Fax (845) 433-8363

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  |"Cole, Derek E" <derek.e.cole_at_[hidden]> |
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  |05/07/2010 08:21 AM |
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  |Re: [OMPI users] Fortran derived types |
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  |users-bounces_at_[hidden] |
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I don't have any hard numbers for fortran, but I do for C structures. Using
C structures with some other C functionality (pointer functions, etc etc)
can yield up to a 3x slowdown at worst, and at best, had a 15% slowdown. I
have seen similar results in fortran, but don't have the benchmark results
for it. In either language, nothing beats raw data types for performance.
Just my .02, I know some out there may not agree.

Derek

-----Original Message-----
From: users-bounces_at_[hidden] [mailto:users-bounces_at_[hidden]] On
Behalf Of Terry Frankcombe
Sent: Thursday, May 06, 2010 12:24 AM
To: Open MPI Users
Subject: Re: [OMPI users] Fortran derived types

Hi Derek

On Wed, 2010-05-05 at 13:05 -0400, Cole, Derek E wrote:
> In general, even in your serial fortran code, you're already taking a
> performance hit using a derived type.

Do you have any numbers to back that up?

Ciao
Terry

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