scalablility problem

Hi all,

When I run multiple TPC-H queries (DBT3) on  postgresql, I found the system
is not scalable.  My machine has 8GB memory, and 4 Xeon Dual Core processor
( 8 cores in total). OS kernel is linux 2.6.9. Postgresql is 7.3.18. I
run multiple
q2 queries simultaneously. The results are:

1 process takes 0.65 second to finish.
2 processes take 1.07 seconds.
4 processes take 4.93 seconds.
8 processes take 16.95 seconds.

For 4-process case and 8-process case, queries takes even more time than
they are executed serially one after another. Because the system has 8GB
memory, which is much bigger than the DB size(SF=1), and I warmed the cache
before I run the test, I do not think the problem was caused by disk I/O.

I think it might be caused by some contentions. But I do not know postgresql
much. May anybody give me some clue to find the reasons?

Thanks!

Xiaoning