BPEL Threading Model

Question

I have three BPEL Processes: BPEL1, BPEL2 and BPEL3 invoking each other in a synchronous fashion as shown below:
Synchronous flow - BPEL1 -> BPEL2 -> BPEL3
I have a java web service which sends in the request to the BPEL1. I would like to increase the no of threads to handle more requests from the Java web service. I looked up four parameters that can be tuned:
Dispatcher Invoke Threads
Dispatcher Engine Threads
Dispatcher System Threads
Dispatcher Maximum Request Depth
As per my understanding Dispatcher Invoke Threads comes into the picture when a synchronous transaction is involved. The question I have is:
Is the value of 20 per instance of BPEL Process or per synchronous transaction? With the above flow stated, BPEL1 invoking BPEL2, BPEL2 invoking BPEL3 and response flow from BPEL3 to BPEL2 and BPEL2 to BPEL1, all being synchronous calls, is this parameter for every instance of BPEL1 alone (per synchronous transaction) or per BPEL instance of BPEL1,BPEL2 and BPEL3?*

Answer 1

No, "Dispatcher Invoke Threads" come into play with async (oneway / possible callbacks). This setting controls the threadpool size to create new process instances, after the originator thread saved the instance data and returned.
Engine threads handle callbacks and expirations
System threads are cleanup threads