Here is a guide on how to achieve this:
On Linux, you can do this with:
taskset --cpu-list 0-1 ./startup.sh --option
This will start the application on cores 0 and 1, and Java will think that it is only running on a 2 core machine.
Replacing "0-1" with "0-3" will use the first 4 cores.
On Windows, you can do this with:
start /affinity 3 startup.bat
This will start up Yellowfin on the first 2 cores (The "3" is a bit mask for 2)
Replacing "3" with "F" will start on the first 4 cores.
Both these methods will restrict performance of Yellowfin as this is the equivalent of running the application on a smaller server.