From fe756bb4cbd824ce3c16401e6aca7b0ae23a0f74 Mon Sep 17 00:00:00 2001
From: Timo Koch <timo.koch@iws.uni-stuttgart.de>
Date: Tue, 12 Nov 2019 18:03:39 +0100
Subject: [PATCH] [doc][handbook] Add information on AMG coarse grid solver for
parallel runs
---
doc/handbook/6_parallel.tex | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/doc/handbook/6_parallel.tex b/doc/handbook/6_parallel.tex
index 6fb1907eab..28b1edc4f1 100644
--- a/doc/handbook/6_parallel.tex
+++ b/doc/handbook/6_parallel.tex
@@ -26,6 +26,13 @@ In order for \Dumux simulation to run in parallel, an
MPI library (e.g. OpenMPI, MPICH or IntelMPI) implementation
must be installed on the system.
+Furthermore, we note that the parallel AMG preconditioner of \texttt{dune-istl}
+defaults to an iterative SSOR coarse grid solver if no direct solver is found on your system. Unfortunately,
+the iterative solver has a very high and hard-coded tolerance as a termination criterion, which will not solve
+the coarse grid system with sufficient accuracy for typical problems in \Dumux. We therefore recommend
+to install one of the direct solver libraries supported by \texttt{dune-istl}. This is either UMFPack contained
+in SuiteSparse, or SuperLU, see Section~\ref{sec:listofexternallibs}.
+
\subsection{Prepare a Parallel Application}
Not all parts of \Dumux can be used in parallel. In order to switch to the parallel \texttt{Dumux::AMGBackend}
solver backend include the respective header
--
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