Only a single instance of PFClean can run on a given machine at any one time. The reason for this is that PFClean processing can be very resource heavy and it is assumes that the application has sole access to the CPUs, GPU, memory, disk bandwidth etc... If PFClean has to share these resources with other programs, even another copy of PFClean, then performance can significantly suffer. Put another way, running 2 PFClean jobs in parallel is often slower than running them sequentially since you risk memory swapping, disk access contention and the general overhead of context switches. As the number of concurrent jobs rise this problem escalates and so it is expressly disallowed. If you run PFClean while another copy is already running you will see this error message:
ERROR: already running
and the application will immediately exit. If you specify the
-retry command line argument then rather than immediately exiting
PFClean will wait until the other running copy completes.