So memory used by the file cache is free (from the point of view of being available for allocation to programs) but serving a useful purpose until it is needed by a program.Įven if all Linux memory is used, and very little is free or in use as a file cache – that can be OK. But if programs need that physical memory, the kernel will reallocate the file cache memory to the programs. Linux uses all physical memory that is not needed by running programs as a file cache, for efficiency. The file cache does what it sounds like – caches in memory recently accessed files – meaning that if a program requests access to a file that is in the file cache, no disk access is required. Almost half the memory on this system is in use: but by the file cache. Think of ‘free memory’ in Linux as ‘wasted memory’ (or better ‘memory the operating system has not yet been able to take advantage of’). Some people may see the output of free, or the above graph, and react with “1G on my 48G system is all that’s free? I’m out of memory!” This is a natural reaction – but wrong.
Or you can use your handy monitoring system to view this over time: There are three things that may spring to mind when you think of measuring your memory system:įor physical memory usage, you can run top or free from a shell: $ free -g Now I’ll talk about how to monitor virtual memory, on Linux (where it’s easy) and, next time, on Solaris (where most people and systems do it incorrectly.) Running programs that require more memory than the system has will mean pages (or processes) are moved to/from disk and memory in order to get enough physical memory to run – and system performance will suck.) (TL DR: swapping is moving an entire process out to disk paging is moving just specific pages out to disk, not an entire process.
In a prior blog post, I talked about what virtual memory is, the difference between swapping and paging, and why it matters.
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