Using up memory and swap space

[Margaret Doll]

I have a ROCKS cluster running RedHat  2.6.18-53.1.14.e15 on the kernel.

 From time to time the memory and swap space on the system is almost  
completely used and the system hesitates.  The users logged onto the  
system are not taking up the memory.

How do I find which process is using the memory?
If the memory is no longer being used by a process, how do I "free" it  
up.

Same question with the swap space.

  free
              total       used       free     shared    buffers      
cached
Mem:       2054132    2026008      28124          0       1816     
1348832 -/+ buffers/cache:     675360   1378772
Swap:      1020116     983328      36788

Thanks.