Postgresql Memory Problem

Hi all..

i've got a problem with the memory that postgresql 7.3.2 used in RedHat 9.
When the memory usage reached 80% or more of the total memory in the
system..
the memory just keep stuck there..The thing did not reduce bt itself...

Then i decided to stop the postgresql service. But ...the memory that
postgresql used still stuck...I used top to check the memory...

The only way to solved it now...is via rebooting the machine..But that
is ridiculous..
Need Help....maybe i miss configured the postgresql.conf.

But give me something to tinker with..

thank in advanced...

--azman--