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Memory Usage shows the amount of RAM and swap in use by the system:
Memory Usage
total used free shared buffers cached
Mem: 496 486 10 0 252 27
-/+ buffers/cache: 206 290
Swap: 1014 0 1014
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A common misconception is that the amount of free RAM shown by the
Mem row indicates that the system
is running in low-memory situation. In the above example, the amount of free RAM is
only 10 megabytes. This is, however, actually the amount of unallocated RAM. Linux
will use as much RAM as is possible, partly becuase it will only unallocate RAM
when it runs out of free (unallocated) memory, and partly because Linux will try to
cache and buffer as much data as possible. While Linux does need to actively use
some buffers and cache, much of this can be allocated directly to a running process.
As a result, to get an idea of how much used and available memory is allocated
on a given system, it is best to look at the used and free columns of the
-/+ buffers/cache row because this takes into account the buffers
and cache in use by the system. In general, the best indicator of a low memory
situation is the amount of swap in use. If this increases above 10% as noted by the
Memory Usage Graph you may wish to
consider adding RAM, although other factors can contribute to this such as a memory
leak in a running process, etc.
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