preconditioners are not scalable when using many cores.
%Doing many experiments with many cores is not easy and requires to access to a supercomputer with several hours for developing a code and then improving it.
-In the following we present some experiments we could achieved out on the
-Hector architecture, the previous UK's high-end computing resource, funded by
-the UK Research Councils, which has been stopped in the early 2014.
+In the following we present some experiments we could achieved out on the Hector
+architecture, a UK's high-end computing resource, funded by the UK Research
+Councils~\cite{hector}. This is a Cray XE6 supercomputer, equipped with two
+16-core AMD Opteron 2.3 Ghz and 32 GB of memory. Machines are interconnected
+with a 3D torus.
Table~\ref{tab1} shows the result of the experiments. The first column shows
the size of the 3D Poisson problem. The size is chosen in order to have
