Postgres memory usage

Hi:

Our postgres database has tables with several million rows in a server
running Red Hat 8.0 with 1GB of memory. Recently we are experiencing a
low performance in the access to the server via HTTP, after rebooting
the server the speed is the same.

I have noticed that available memory is aparently too low, according to top:

12:58pm up 1:28, 3 users, load average: 0,00, 0,01, 0,09
94 processes: 91 sleeping, 3 running, 0 zombie, 0 stopped
CPU states: 0,0% user, 0,0% system, 0,0% nice, 100,0% idle
Mem: 1031012K av, 1021440K used, 9572K free, 0K shrd, 62864K
buff
Swap: 2040244K av, 14960K used, 2025284K free 876808K
cached

Is it normal for Postgres to allocate almost all the memory in the computer?

Thanks in advance.
Ruben.