A multi CUP performance problem

I found  postgresql running on the multi CPU machine doesn't have better performance than single CPU machine. Can
youexplain the reason? And how to resolve the problem?
 
  In the Todo list, I read the below message:
Add code to detect an SMP machine and handle spinlocks accordinglyfrom distributted.net,
http://www1.distributed.net/source,inclient/common/cpucheck.cpp 
 
  I want to know how to handle spinlocks even if it can detect an SMP machine.
  Thank you!