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docs(dogstatsd): document dogstatsd_mem_based_rate_limiter.* as architectural divergence #1775

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27 changes: 25 additions & 2 deletions docs/agent-data-plane/configuration/dogstatsd.md
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@@ -1,6 +1,6 @@
# Configuring DogStatsD on Agent Data Plane

<!-- Last updated: 2026-05-19 -->
<!-- Last updated: 2026-05-29 -->

The DogStatsD implementation on ADP has been redesigned in Rust for better resource guarantees and
efficiency. Because the architecture is different from the original implementation, certain
Expand Down Expand Up @@ -48,7 +48,7 @@ architecture is fundamentally different or the feature is platform-specific.
| Config Key | Description | Reason |
| ---------------------------------------------- | ---------------------------------- | ------------------------------------------------------------ |
| `dogstatsd_host_socket_path` | Host UDS socket dir for DSD | Not read by DSD server; admission controller only |
| `dogstatsd_mem_based_rate_limiter.enabled` | Enable memory rate limiter | Go GC specific; use `memory_limit` |
| `dogstatsd_mem_based_rate_limiter.*` | Memory-based rate limiter (11 keys) | Go GC–specific; ADP uses `memory_limit` (see below) |
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| `dogstatsd_no_aggregation_pipeline_batch_size` | No-aggregation pipeline batch size | Fixed in ADP topology |
| `dogstatsd_packet_buffer_flush_timeout` | Packet buffer flush timeout | ADP decodes inline |
| `dogstatsd_packet_buffer_size` | Datagrams per packet buffer | ADP decodes inline |
Expand All @@ -59,6 +59,29 @@ architecture is fundamentally different or the feature is platform-specific.
| `dogstatsd_workers_count` | Number of DSD processing workers | ADP uses async tasks |
| `use_dogstatsd` | Master DogStatsD enable toggle | Core Agent evaluates and sets `data_plane.dogstatsd.enabled` |

### Memory-based rate limiter (`dogstatsd_mem_based_rate_limiter.*`)

The core agent exposes 11 keys under this prefix to apply backpressure when the Go process
approaches its memory limit. They work by manipulating Go's garbage collector
(`debug.SetGCPercent`, `debug.FreeOSMemory`), allocating a large heap ballast to adjust GC
heuristics, and blocking goroutines to slow packet ingestion. None of these mechanisms have
an equivalent in Rust, and ADP does not use a Go runtime.

ADP takes a different approach to the same problem:

- **Static bounds**: components declare their memory footprint at startup via `MemoryBounds`.
ADP refuses to start if declared bounds exceed the configured `memory_limit`, preventing
over-commitment before any traffic arrives.
- **Dynamic limiting**: a `MemoryLimiter` polls the process RSS every 250 ms. When usage
exceeds 95 % of the effective limit it applies proportional async backpressure (1–25 ms)
to ingestion tasks.
- **Structural backpressure**: bounded channels between components provide back-pressure
independently of memory monitoring.

To set a process memory limit in ADP, use `memory_limit` (bytes). The `memory_slop_factor`
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P2 Badge Document the required memory mode

When a user follows this replacement guidance and only sets memory_limit, ADP does not actually enforce the static bounds or start the RSS-based limiter: memory_mode defaults to Disabled and initialize_memory_bounds returns a no-op limiter in that branch (lib/saluki-app/src/memory.rs lines 91-93 and 157-177). This section should mention that the described refusal/backpressure behavior requires memory_mode: strict or permissive (and enable_global_limiter for dynamic backpressure), otherwise the ignored DogStatsD limiter keys are not replaced by active memory limiting.

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setting reserves a headroom fraction to account for allocations not tracked by ADP's internal
accounting. All 11 `dogstatsd_mem_based_rate_limiter.*` keys are ignored by ADP.
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## Behavioral Differences

<!-- section:behavioral-differences -->
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