Linux "free -m": Total, used and free memory values don't add up

Question

On a linux system, while using "free", following are the values:

         total      used      free    shared  buff/cache   available
Mem:  26755612    873224    389320    286944    25493068    25311948
Swap:        0         0         0

The total, used and free values don't add up. I'm expecting total = used + free.

Question: What am I missing here?

Answer 1

For the main memory, the actual size of memory can be calculated as used+free+buffers+cache OR used+free+buffers/cache because buffers/cache = buffer+cache.

The man page of free highlights used as Used memory (calculated as total - free - buffers - cache)

As the man page of free says :-

total Total installed memory (MemTotal and SwapTotal in /proc/meminfo)

used Used memory (calculated as total - free - buffers - cache)

free Unused memory (MemFree and SwapFree in /proc/meminfo)

shared Memory used (mostly) by tmpfs (Shmem in /proc/meminfo, on kernels 2.6.32, displayed as zero if not available)

buffers Memory used by kernel buffers (Buffers in /proc/meminfo)

cache Memory used by the page cache and slabs (Cached and Slab in /proc/meminfo)

buff / cache Sum of buffers and cache

available Estimation of how much memory is available for starting new applications, without swapping. Unlike the data provided by the cache or free fields, this field takes into account page cache and also that not all reclaimable memory slabs will be reclaimed due to items being in use (MemAvailable in /proc/meminfo, available on kernels 3.14, emulated on kernels 2.6.27+, otherwise the same as free)

In your case,

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873224(used) + 389320(free) + 25493068(buff/cache) = 26755612(total)

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Answer 2

Linux likes to cache every file that it opens. Every time you open a file for reading, Linux will cache it but it will drop those caches if it needs the memory for something more important -- like when a process on the system wants to allocate more memory. These caches in memory simply make Linux faster when the same files are used over and over again. Instead of actually going to disk every time it wants to read the file, it just gets it from memory and memory is a lot faster that disk. That is why your system shows 25493068 used in buff/cache but also shows 25311948 available. Much of that cached data can be freed if the system needs it.