The environment variable GMK_THREADS
should be set to the number of threads requested for the compilation.
for small jobs, one thread is far enough. For big jobs (compilation of the whole code or recompilation of many modules), it is worth using two or three threads. The best ratio of performance seems to be obtained with two threads (the elapsed time is almost devided by two). A bigger number of threads makes the compilation a bit faster but it could break your user limit of processes on the machine.