free from the terminal or Reports > Server Info within the control panel reports a small sliver of total system memory as free or available. For example, below is the output from
free on a 4th generation server:
total used free shared buffers cached Mem: 16540796 13011700 3529096 0 135404 10609292 -/+ buffers/cache: 2267004 14273792 Swap: 2104496 56884 2047612
Servers are very efficient at managing memory. Memory that is not occupied by a process may be utilized to store file data (buffers) and metadata (location on disk, size, owner, etc reflected in cached). If there is unbound memory available on a server, it will be used by the server to boost local filesystem performance (buffers and cached as seen above). Newer platforms will also donate free memory to other cloud instances on a server to boost performance. Likewise when memory is needed by a process, memory is donated from first cached storage, then buffers. On newer platforms, if sibling clouds have memory to donate, then that memory is absorbed. Lastly, if memory cannot be converted from buffers, cached, or neighboring clouds, then the out-of-memory killer is invoked to randomly terminate a process with the highest resident stack size until sufficient memory is available to run the process.