There is a recent article on lwn.net discussing this issue. The theory being that most application writers prefer to ask for large amounts of memory in one go, to avoid overhead, and may not actually use all that memory. In a big corporate, where uptime is key and MTTR has to be as low as possible this is where commercial grade RHEL comes to the fore. The operating system is at liberty to promise more memory to the application than it has actually supplied. Why Red Hat Linux is not free Well, the not free part is for officially supported updates and support for your OS.Memory can be shared between various processes, through Copy-On-Write memory allocation, memory mapped IO and shared dynamic libraries.If you attempted this you would quickly discover that you applications appear to be using more memory than actually exists on the machine. Something else that may be confusing you is you cannot simply add up the memory in use by all running processes to get a total memory in use figure. Instead they use memory which is not actively mapped to processes in the running OS for things like file cache and buffers for various IO transfer operations. Linux (and all Unix OS'es) try to have as little free memory as possibly. Unused memory does have some data mapped to it, but it is currently not in active use by a running process. Free memory, in the unix world is a page of physical memory that has no logical data mapped to it. Don't confuse free memory with unused memory.
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