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 reports 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 that 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).

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