RE: Workstation Computers for FLUKA

From: Chris Theis <>
Date: Fri, 26 Nov 2010 15:31:53 +0000

Hi Nicholas,

there is a lot of hype recently regarding GPUs (some of it certainly justified), but there are some subtle issues especially
in connection with MC sim=ulations, which I would like to comment on.

> I personally would love to see a version of Fluka take advantage of modern
> graphics processors that have a large number of cores, and are now showing
> good performance for double precision operations.

It is true that support for double precision has become a lot better lately.
However, in comparison to native single precision calculations the performance
benefit is currently significantly lower. The benchmarks GPU vs. CPU usually shown are based on single precision!
The probably more important point is that different vendors have allowed themselves a bit of freedom with
respect to the implementation of the IEEE floating point standard. It is not guaranteed that the
GPU that you are running your program on will yield the same result as the one of your colleague because it's from a different
vendor. In terms of debugging& quality control this is a very important
aspect that is commonly overlooked!

Furthermore, using proprietary platforms like CUDA limits the program to one vendor,
like Nvidia GPUs in this case. Having portability in mind one will
probably have to go for something like OpenCL in the end where you will sacrifice
(a bit) of speed on the altar of portability.

> There are some utilities to convert Fortran 95 code to CUDA, obviously with a lot of manual work to
> specify what resources to allocate for particular code segments, unfortunately I have yet to find anything that can do this for Fortran 77.

I'm afraid that the chance of finding something suitable for F77 are realistically below zero.
The F77 standard is more than 30 years old which renders it practically a dinosaur, looking at the short turnover times seen in
the recent years. Furthermore, maintenance of the popular G77 compiler was stopped many years ago, so I would not count
on anybody making the effort to provide support for modern GPUs with respect to F77.

Received on Fri Nov 26 2010 - 17:18:10 CET

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