<p>If no cluster is used, R works sequentially. Keep <code>parallel=NULL</code>. No parallelisation takes place and therefore no speed up can be obtained.</p></li>
</ul> 

<p>In general, if <code>parallel=NULL</code>, all information concerning the cluster set-up is taken from commandline, else, it can be specified using the three arguments <code>parallel</code>, <code>clustertype</code>, <code>nodes</code>, and, if necessary, <code>clusterhosts</code>.</p>
<p>sfCluster is a Unix tool for flexible and comfo