Subject: Re: parallel ccm result verfication
From: Jim Rosinski (rosinski@cgd.ucar.edu)
Date: Fri Sep 17 1999 - 15:39:30 MDT
On Fri, 17 Sep 1999, Akshara Kaginalkar wrote:
> Can anyone help in result verfication between sequential and parallel
> runs of atmospheric models on distributed memory parallel machine
> using MPI.
> 1. does the result from sequential code and parallel version of the
> same code match perfectly ?
In most cases, yes. Defining the cpp token "SPMD" results in different
memory allocation schemes for some variables however, as well as slightly
different code sequences in a few locations. This can result in roundoff
differences in the (SPMD vs. serial) simulation on some machines.
> 2. If the deviation exists, what is the metorological acceptance
> criteria for it ?
See "The Accumulation of Rounding Errors and Port Validation for Global
Atmospheric Models", Rosinski and Williamson, SIAM J. Sci. Comput, vol 18,
no. 2, pp 552-564, March, 1997.
> 3. does the result from sequential runs from two different
> architectures e.g cray and DEC alpha match exactly ?
No. Different compilers will always generated different instruction
sequences for a code this large. Internal floating point representation and
arithmetic differences will also cause answers to differ.
Regards,
Jim Rosinski
CCM Core Group
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