render times question on 2 processor machine?

Hi all, I was doing some render time tests on a machine which has 2 processors both with hyper threading (note… it is not dual core, it is 2 processors). It runs SMP linux kernel 2.4.21. My observations are as follows:

When I run my render on a single processor machine with HT, it takes around 45 seconds. When I run same render on above 2 processor machine, it still takes same time. When I do top, it shows 24.9% of CPU usage. Now I executed 2 processes of render with same blend file. top shows 24.9% for each of the processes (I also used processor affinity to schedule them on different processors) but rendering takes 70 secs for one of them and around 85 secs for other one, when simultaneously rendered. I expected both of them to finish in 45 sec or slightly more time. This sounds strange to me… What am I doing wrong. Some one please help. Thanks much in advance.

Do you have the “thread” buton selected in the render buttons screen? I think it’s in the bottom left corner.

I have a Dual CPU G5 and have notices a big improvement when that is turned on. It isn’t quite 2x as fast, but it’s still a nice improvement.

I have threads button turned on. But I dont see improvement.