add comment about numdiff in docu

git-svn-id: https://svn.dealii.org/trunk@28828 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/doc/development/testsuite.html b/deal.II/doc/development/testsuite.html
index 8676f4f3305a389699da8518b91227a35d921dc5..0b0df74bb8977d743f9b8827a101c7e144bec4b3 100644 (file)
--- a/deal.II/doc/development/testsuite.html
+++ b/deal.II/doc/development/testsuite.html
@@ -161,6 +161,16 @@
     CMake to install deal.II into previously.
     </p>
 
+    <p>
+    Not all tests will succeed on your machine, because you might not have
+    configured with all the required packages (for example PETSc or Trilinos)
+    or, because your machine has slightly different floating point outputs.
+    To improve the number of tests that work correctly, install the tool
+    numdiff and export the environment variable
+    <code>export DEALII_DIFF="numdiff -a 1e-6 -q"</code> 
+    before running the testsuite.
+    </p>
+
     <p>
     Once you have done this, you may simply type
     <code>make</code>. This runs all the tests there are, but stops at