How to make a fast multi-core UDP generator ??

Hi there:
I'm trying to make a UDP generator by using udpgen.click but single thread
kernel click seems not fast enough(around 1.4Mpps with 64B packet), so I try
to configure and run click as multi-threaded version, However it does not
work for me when I'm using udpgen.click. once I type *click-install -t 2*,
it tells me ignore the parameter, while other click configuration file works
fine, I don't know why this udpgen.click can't work with multi-thread.
Another question is which is the possible bottleneck, NIC or CPU? Thanks for
your help.

Best
shule