Memory accounting code: review

[dranikpg]

I've been re-reading our memory accounting code to add better heuristics:

• max_memory_limit is just a global size_t that can be mutably changed by the config registry 🎖
• used_mem_peak and rss_mem_peak are unnecessarily global
• Deny OOM:
  • GetMemoryUsage updates every 1us, whereas global atomics are updated every ~1/shard_countms (rss even less frequently), i.e. 99% of the calls are ineffective
  • Not sure why we use command timestamps?
• No uniformity in flag access. Some code calls GetFlag on hot paths, server state caches at least a handful of other flags. Once calculated parameters were left forgotten with flag updates (like soft_budget_limit_)
• "Modules" often don't care about centralizing memory calculations. Deny oom has 3 places where it repeats its calculation (using different sources at the same time), tiering calculates it's offload threshold two times - same formula - differently looking code
• No real comments to clarify the meaning or usage of important variables or calculations