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