[HPGMG Forum] [EXTERNAL] Re: Acceptable rounding errors

Jed Brown jed at jedbrown.org
Mon Aug 3 16:20:26 UTC 2015

Brian Van Straalen <bvstraalen at lbl.gov> writes:

> Once you reach the bottom solver for full multigrid you should not be
> in the network at all anymore.  You would be computing a dot product
> over a few hundred doubles in L1 memory.  

And the order of those doubles doesn't depend on the
network/fine-grid/number of processors (at least not in FE), so it's
reproducible in any precision and thus I don't know why we're having
this particular discussion about dot products that occur only on the
coarsest (serial) grid.

The summation order during restriction might not be reproducible
(depends on implementation details including whether you use
MPI_Waitsome versus MPI_Waitall), but can be made reproducible and
parallel-invariant if desired.
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