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neon/libs/vm_monitor/README.md
Em Sharnoff 9fe5cc6a82 vm-monitor: Switch from memory.high to polling memory.stat (#5524) 
tl;dr it's really hard to avoid throttling from memory.high, and it
counts tmpfs & page cache usage, so it's also hard to make sense of.

In the interest of fixing things quickly with something that should be
*good enough*, this PR switches to instead periodically fetch memory
statistics from the cgroup's memory.stat and use that data to determine
if and when we should upscale.

This PR fixes #5444, which has a lot more detail on the difficulties
we've hit with memory.high. This PR also supersedes #5488.
2023-10-17 15:30:40 -07:00

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vm-monitor

The vm-monitor (or just monitor) is a core component of the autoscaling system, along with the autoscale-scheduler and the autoscaler-agents. The monitor has two primary roles: 1) notifying agents when immediate upscaling is necessary due to memory conditions and 2) managing Postgres' file cache and a cgroup to carry out upscaling and downscaling decisions.

More on scaling

We scale CPU and memory using NeonVM, our in-house QEMU tool for use with Kubernetes. To control thresholds for receiving memory usage notifications, we start Postgres in the neon-postgres cgroup and set its memory.{max,high}.

Structure

The vm-monitor is loosely comprised of a few systems. These are:

  • the server: this is just a simple axum server that accepts requests and upgrades them to websocket connections. The server only allows one connection at a time. This means that upon receiving a new connection, the server will terminate and old one if it exists.
  • the filecache: a struct that allows communication with the Postgres file cache. On startup, we connect to the filecache and hold on to the connection for the entire monitor lifetime.
  • the cgroup watcher: the CgroupWatcher polls the neon-postgres cgroup's memory usage and sends rolling aggregates to the runner.
  • the runner: the runner marries the filecache and cgroup watcher together, communicating with the agent throught the Dispatcher, and then calling filecache and cgroup watcher functions as needed to upscale and downscale