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reductions: add note about FP associativity #16

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Oct 9, 2024
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reductions: add note about FP associativity #16

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reductions: add note about FP associativity
davidozog Oct 9, 2024
c299db1
reductions: revert small whitespace change
davidozog Oct 9, 2024
11c76e0
Update content/shmem_reductions.tex
davidozog Oct 9, 2024
d7a4aa5
reductions: revert back to associative binary ops
davidozog Oct 9, 2024
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12 changes: 12 additions & 0 deletions content/shmem_reductions.tex
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In the API note, the proposal is going to be silent on statements like the following

  • that the result should be the same on all PEs
  • that the result should be the same if the operation is called multiple times on the same data during the same run
  • that the result should be the same from run to run
  • that the result may be different on different architectures, due to differing arithmetic
  • that the result on IEEE-754 compatible platforms should always be the same everywhere and everywhen
  • that we might consider new APIs or environment variables that enforce more consistent results

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This is a nice summary of the open issues. Can you capture it on the follow-up issue for OpenSHMEM 1.7 (and create said issue if we don't have one already)?

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See openshmem-org#550

Original file line number Diff line number Diff line change
Expand Up @@ -376,6 +376,18 @@ \subsubsubsection{PROD}
Zero on successful local completion. Nonzero otherwise.
}

\apinotes{
The binary operations performed by \openshmem reductions are intended to be
associative and commutative.
However, floating point arithmetic is not associative or commutative due to
the inherent inaccuracies of floating-point representations caused by
rounding errors and finite precision.
This can lead to variations in the result of \openshmem arithmetic
reduction operations on floating-point datatypes, including NaN values.
A future version of the \openshmem specification may clarify the behavior
of reductions on floating point datatypes.
}

\begin{apiexamples}

\apicexample
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