Performance degrade running on multicore computer

Hi,

My PostgreSQL server has two CPUs (OS: Fedora 11), each with 4 cores. Total
is 8cores. Now I have several clients running at the same time to do insert
and update on the same table, each client having its own connection. I have
made two testing with clients running in parallel to load 20M data in
total. Each testing, the data is split evenly by the client number such that
each client only loads a piece of data.

1) Long transaction: A client does the commit at the end of loading. Result:
Each postgres consumes 95% CPU. The more clients run in parallel, the slower
the total runing time is (when 8 clients, it is slowest). However, I expect
the more clients run in parallel, it should be faster to load all the data.

2) Short transaction: I set the clients to do a commit on loading every 500
records. Results: Each postgres consumes about 50%CPU. Now the total
running is as what i have expected; the more clients run in parallel, the
faster it is (when 8 clients, it is fastest).

Could anybody help to why when I do the long transaction with 8 clients, it
is slowest? How can I solve this problem? As I don't want to use the 2), in
which I have to set the commit size each time.

Thanks a lot!!

-Afancy