The manner in which Datadog calculates the system.mem.used metric produces a value that may sometimes be different from what might be displayed by common system resource reporting tools.
For example, running 'free -m' on an Ubuntu machine may produce the following memory breakdown (values represent Megabytes):
A Datadog agent running on this same machine will report a system.mem.used metric with a value of 56856 MB - clearly different from the 'free -m' used memory value of 1203 MB.
The reason for this discrepancy is because Datadog includes cached memory in its formula for used memory, where 'free -m' does not.
Datadog calculates used memory as follows:
* system.mem.used(56856) = system.mem.total(128831) - system.mem.free(71975)
Again, Datadog's system.mem.used metric includes cached memory, so subtracting this cached memory from used memory results in the following value:
* system.mem.used(56856) - system.mem.cached(55653) = 1203
1203 MB - identical to the used memory value reported by 'free -m' in the example above.
The system.mem.usable metric represents free memory plus cached memory plus buffers (on Linux, it reflects "MemAvailable" attribute from /proc/meminfo whenever possible).