How is the Linux calculating MemFree

Question

I am trying to understand my embedded linux memory usage.

By using the top utility and the process file /proc/meminfo I can see how much virtual memory a process is using, and how much physical memory is available to the system. But it would seem for any given process the virtual memory can be very much higher than the used physical memory. As this is an embedded system memory swapping is disabled.(SwapTotal = 0)

How is linux calculating the free physical memory? As it doesn't seem to be accounting for everything allocated in the virtual memory space.

Answer 1

MemFree in /proc/meminfo is a count of how many pages are free in the buddy allocator. This buddy allocator is the fundamental unit of physical memory allocation in the kernel; however there are a lot of ways pages can be returned to the buddy allocator in time of need - for example, freeing empty SLABs, discarding cache/buffer RAM (even if this means invalidating PTEs in a running process), or as a last resort, swapping things out.

In fact, MemFree is generally controlled to be only 5-10% of total physical RAM, with any extra free RAM being co-opted into cache as time goes on. As such, MemFree alone is a very incomplete view of the overall memory situation.

As for the virtual memory (VSIZE) of a given process, this refers to the sum total of the sizes of all mapped memory segments in the process's address space. However, not all of these will be physically present - some may be paged in upon first access and as such will not register as memory in use until actually used. The resident size (RSIZE) is a more accurate view, as it only registers pages that are mapped in right now - although this may also not be accurate if a given page is mapped in multiple virtual addresses (which is very common when you consider multiple processes - shared libraries have the same physical RAM mapped to all processes that are using that library)