I was processing a big data with Pentaho in last week and I did get the follwing message: "Failed to execute runnable (java.lang.OutOfMemoryError: Java heap space)".
Well, thank God that the "heap space" in Java is configurable. But, how to do this only in the Pentaho?
The solution is: edit the sponn (.bat or .sh) and change the variable PENTAHODIJAVA_OPTIONS.
By default, this variable have the value "-Xmx512m" means 512Mb of space. So just you set "-Xmx2g" to change the heap space for 2Gb.