Substract cache from memory usage.

[maleadt]

Including it seemed a bit misleading to me, as it doesn't reflect actual memory usage and wouldn't trigger eg. OOM. It also makes it hard to compare different container's behavior, where one might be leaking memory and the other just caching a lot.

Ref moby/moby#10824

[scanterog]

Hi @maleadt! Thanks for reporting this. I took a look and I've found that rss stat inside memory.stat always matches usage_in_bytes - cache. In case this is correct, we might open just one file and get rss out of it rather than opening two files and executing the subtraction operation. We might look into runc project in order to understand how rss (resident set size) is defined.

According to this https://docs.docker.com/engine/admin/runmetrics/#memory-metrics-memorystat, rss should work, but I think it would be better total_rss because it includes sub-cgroups.