The following are quoted from: http://www.eygle.com/digest/2007/07/linux_memory_management_or_why.html
Traditional Unix tools like 'top' often report a surprisingly small amount of free memory after a system has been running for a while. For instance, after about 3 hours of uptime, the machine I'm writing this on reports under 60 MB of free memory, even though I have 512 MB of RAM on the system. Where does it all go?
The biggest place it's being used is in the disk cache, which is currently over 290 MB. This is reported by top as "cached". Cached memory is essentially free, in that it can be replaced quickly if a running (or newly starting) program needs the memory.
The reason Linux uses so much memory for disk cache is because the RAM is wasted if it isn't used. Keeping the cache means that if something needs the same data again, there's a good chance it will still be in the cache in memory. Fetching the information from there is around 1,000 times quicker than getting it from the hard disk. If it's not found in the cache, the hard disk needs to be read anyway, but in that case nothing has been lost in time.
To see a better estimation of how much memory is really free for applications to use, run the command:
free -m
The -m option stands for megabytes, and the output will look something like this:
total used free shared buffers cached
Mem: 503 451 52 0 14 293
-/+ buffers/cache: 143 360
Swap: 1027 0 1027
The -/+ buffers/cache line shows how much memory is used and free from the perspective of the applications. Generally speaking, if little swap is being used, memory usage isn't impacting performance at all.
Notice that I have 512 MB of memory in my machine, but only 503 is listed as available by free. This is mainly because the kernel can't be swapped out, so the memory it occupies could never be freed. There may also be regions of memory reserved for/by the hardware for other purposes as well, depending on the system architecture.
