readme notes

README.md

@@ -26,3 +26,30 @@ All examples run (including the parallel), however it may be needed to manually
 Mac OS X
 --------
 Tested to work with both llvm-gcc-4.2 and clang. Fortran compiler [libraries](http://cran.r-project.org/bin/macosx/tools) must be installed. According to [R admin manual](http://www.cran.r-project.org/doc/manuals/R-admin.html#OS-X) "the OpenMP support in this version of gcc is problematic, and the alternative, clang, has no OpenMP support". So, parallel templates with OS X will require a different compiler installed.
+
+Linux specific notes
+====================
+
+Metis orderings
+---------------
+For large 3D random field models the ordering algorithms shipping with R's Matrix package are far from optimal. To get better orderings available run the following in the terminal:
+
+* sudo apt-get install libsuitesparse-metis-3.1.0
+
+This will install a more complete version of CHOLMOD with more orderings available. Then install TMB like this:
+
+* make install-metis
+
+For a quick example of how to use it start R, load TMB and run
+
+* runExample("ar1xar1")
+
+Issues with library unloading
+-----------------------------
+On recent versions of gcc the following problem may be encountered: When the user cpp file is changed, re-compiled and re-loaded, the changes does not take place. To see if you are affected by this issue, assuming your compiled DLL is called "mymodel.so", try running:
+
+* readelf -s mymodel.so | grep UNIQUE
+
+If this gives a lot of output it is not possible to unload the library, and R will have to be restarted every time the model is re-compiled.
+A workaround is to use clang++ instead of gcc.
+