Oracle using all the ram on my server

Question

Hi,
I have oracle 11g r2 installed on red hat linux 5 with 8 GB of RAM. I need to specify that only oracle is installed on this machine, and even in idle (absolutely no one connected on the database) all the memory is used
total used free shared buffers cached
Mem: 8175452 8127612 47840 0 11504 6787324
The performance of this machine is really poor although the hardware is supposed to be more than sufficient to the task its supposed to do.
This is the value of sga
Total System Global Area 3574284288 bytes
Fixed Size 2218512 bytes
Variable Size 2650801648 bytes
Database Buffers 905969664 bytes
Redo Buffers 15294464 bytes
And
select * from v$pgastat order by 2 desc;
gives the following result
NAME VALUE UNIT
PGA memory freed back to OS 1643595431936 bytes
bytes processed 1395680836608 bytes
extra bytes read/written 1166086521856 bytes
aggregate PGA target parameter 1241513984 bytes
aggregate PGA auto target 988563456 bytes
maximum PGA allocated 730347520 bytes
maximum PGA used for manual workareas 392226816 bytes
maximum PGA used for auto workareas 377039872 bytes
total PGA allocated 298163200 bytes
total PGA inuse 143101952 bytes
total freeable PGA memory 126287872 bytes
global memory bound 124149760 bytes
recompute count (total) 593752
max processes count 113
process count 88
cache hit percentage 54.48 percent
total PGA used for manual workareas 0 bytes
over allocation count 0
total PGA used for auto workareas 0 bytes

Answer 1

You may not expressly be using all the RAM in your machine but OS X will, whether you know it or not.
Each time you read a file from disk, whatever was read is cached by the system; when the file or program is no longer in use, the memory is marked free for the system to recover for other purposes but as long as it remains in memory the next time you need to access that disk information it's simply read from memory.
The more RAM, the more disk data is kept around and the faster your machine will operate until that memory needs to be recovered or you reboot (or otherwise flush the disk cache.)