Memory used but where 315

Linux tries to make the best use of the available memory as it is available. It reserves a small amount of heap (the 300 meg you saw) for Malloc() operations, but the rest is allocated to buffering the disk drives and hard drives.

If your applications have need for more memory, Linux can flush some of the cache and make that extra memory available to the applications as needed.

The main reason for doing this is because the hard drive latency is the biggest performance person in a modern PC. The memory and backplane can handle gigabytes-second, but the hard drive spins at rates between 4500 and 10,000 RPM due to the physics of the spinning drive (heat). A typical 7200 RPM drive with an 8 Mb internal cache can deliver about 40 megabytes-second - if the operating system can request entire tracks or cylinders at a time. If the operating system is going out to the hard drive to flip between directory tracks and multiple application tracks, and reading only 512 bytes per request, the performance can really suffer often something as simple as an FTP file transfer can slow to less than 2 megabytes-second.. By combining the hard drive cache with the Linux buffer cache, the disk drive access performance can come much closer to that 20-40 megabyte-second.

There have been attempts to further increase drive throughput, usually by reading all of the cylinders at once, having multiple heads so that you can read 2, 3, or even 4 heads and capture entire cylinders or tracks in a single rotation (many CD and DVD drives still use this appreach), but to really take advantage of these designs, the information either needs to be organized and stored in large clusters, or you need a really large drive cache so that you can pull down 2-8 megabytes in a single spin and throw away what you don't need. Linux can also improve performance by using it's cache across multiple drives - typically in a RAID 10 environment where parbreastions are striped across multiple drives, and then mirrored. Linux can be used as a SAN storage system, or it can be used with high performance SAN and NAS storage systems.

If you need more refined control over your memory allocation, you can either use virtual machines, or use ulimit to keep an application from grabbing too much memory. If you need the extra memory, Linux will provide it to you, but if you are not using it directly, Linux can make good use of what you aren't using.

It is worth noting that most of the time, when an application is trying to grab very large amounts of memory, it's usually because the application is trying to do it's own memory management. In general this is usually not a really good idea. The problem is that if an application is caching the drive, then there can be conflicts and memory thrashing between the drive cache and the application cache. Some database applications bypbutt the Linux caching by requesting raw or uncached parbreastions.