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35 lines
1.8 KiB
Markdown
35 lines
1.8 KiB
Markdown
# `vm-monitor`
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The `vm-monitor` (or just monitor) is a core component of the autoscaling system,
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along with the `autoscale-scheduler` and the `autoscaler-agent`s. The monitor has
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two primary roles: 1) notifying agents when immediate upscaling is necessary due
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to memory conditions and 2) managing Postgres' file cache and a cgroup to carry
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out upscaling and downscaling decisions.
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## More on scaling
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We scale CPU and memory using NeonVM, our in-house QEMU tool for use with Kubernetes.
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To control thresholds for receiving memory usage notifications, we start Postgres
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in the `neon-postgres` cgroup and set its `memory.{max,high}`.
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* See also: [`neondatabase/autoscaling`](https://github.com/neondatabase/autoscaling/)
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* See also: [`neondatabase/vm-monitor`](https://github.com/neondatabase/vm-monitor/),
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where initial development of the monitor happened. The repository is no longer
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maintained but the commit history may be useful for debugging.
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## Structure
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The `vm-monitor` is loosely comprised of a few systems. These are:
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* the server: this is just a simple `axum` server that accepts requests and
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upgrades them to websocket connections. The server only allows one connection at
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a time. This means that upon receiving a new connection, the server will terminate
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and old one if it exists.
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* the filecache: a struct that allows communication with the Postgres file cache.
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On startup, we connect to the filecache and hold on to the connection for the
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entire monitor lifetime.
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* the cgroup watcher: the `CgroupWatcher` manages the `neon-postgres` cgroup by
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listening for `memory.high` events and setting its `memory.{high,max}` values.
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* the runner: the runner marries the filecache and cgroup watcher together,
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communicating with the agent throught the `Dispatcher`, and then calling filecache
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and cgroup watcher functions as needed to upscale and downscale
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