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Reviewed-on: #13 Co-authored-by: bQUARKz <bquarkz@gmail.com> Co-committed-by: bQUARKz <bquarkz@gmail.com>
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2.1 KiB
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38 lines
2.1 KiB
Markdown
---
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id: LSN-0026
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ticket: perf-runtime-telemetry-hot-path
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title: Push-based Telemetry Model
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created: 2026-04-10
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tags: [performance, telemetry, atomics]
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---
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# Push-based Telemetry Model
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The PROMETEU telemetry system evolved from an on-demand scan model (pull) to an incremental counter model (push), aiming to minimize the impact on the runtime's hot path.
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## The Original Problem
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Previously, at every host tick, the runtime requested memory usage information from the asset banks. This resulted in:
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- $O(n)$ scans over resource maps.
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- Multiple read lock acquisitions in every tick.
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- Unnecessary overhead on handheld hardware, where every microsecond counts.
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## The Solution: Push Model with Atomics
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The implemented solution uses `AtomicUsize` in drivers and the VM to maintain the system state in real-time with $O(1)$ read and write cost:
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1. **Drivers (Assets):** Atomic counters in each `BankPolicy` are updated during `load`, `commit`, and `cancel`.
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2. **VM (Heap):** A `used_bytes` counter in the `Heap` struct tracks allocations and deallocations (sweep).
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3. **System (Logs):** The `LogService` tracks log pressure emitted in each frame.
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## Two Levels of Observability
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To balance performance and debugging, the collection was divided:
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- **Frame Snapshot (Always):** Automatic capture at the end of each logical frame. Irrelevant cost ($O(1)$). Serves the `Certifier` and historical logs.
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- **Host Tick (On-Demand):** Detailed collection in every tick only occurs if `inspection_active` is enabled (e.g., F1 Overlay on).
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## Lessons Learned
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- **Trigger Decoupling:** We should not use the `Certifier` state to enable visual debugging features (like the overlay), as they have different purposes and costs.
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- **Eventual Consistency is Sufficient:** For telemetry metrics, it is not necessary to lock the system to obtain an exact value every nanosecond. Relaxed atomic reading is sufficient and much more performant.
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- **Cost Isolation:** Moving the aggregation logic to the driver simplifies the runtime and ensures that the telemetry cost is paid only during state mutations, rather than repeatedly during stable execution.
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