Work out memory usage breakdown instantly with clear inputs, formula shown and shareable results.
Linux deliberately uses spare RAM for page cache, so a low free figure is healthy rather than alarming. The number that matters is available, which counts free memory plus the cache the kernel can reclaim without swapping — roughly 80 percent of it, since some pages are dirty or pinned. Reading used and free without available is the classic misdiagnosis.
Memory accounting
used = application RSS + kernel and slab; free = total - used - page cache; available ~ free + 0.8 x reclaimable cache
Because unused RAM is wasted RAM. The kernel fills it with cached file data, which is discarded instantly when an application needs the memory.
MemAvailable from /proc/meminfo. Alerting on free produces constant false alarms on healthy systems.