Adding CPUs to SMP is not always advantageous

Hi,

I was going through the  best practices for using vmware virtual SMP technology.

However, i came across the fact that adding cpus to the smp system does not always increase the throughput. i could read the reasons explained why it is so.

But i could not quite understand it as it is completely new for me,

Please could some one explain me the same in simple words why is it not always advantageous to keep adding more CPUs to SMP system.

Thanks.