perf(sidecar): send engine-batched extract items one per RunBatch, as wide as the adapter dispatches

[svonava]

Summary

On the production path (gateway → NATS → Rust sidecar → sie-server → SGLang), LightOnOCR-3-4B costs about twice as much per page as on the worker's own HTTP path. GLM-OCR costs about 20% more. Three sidecar defaults combine:

• Batching. The scheduler packs up to 12 extract pages into one RunBatch (image extract items cost 1, and the request cap is 12). A batch answers only when its slowest page finishes.
• Pipeline depth. Each model's drain loop keeps 2 batches in flight. SIE_RUST_PIPELINE_DEPTH is clamped to at most 8.
• IPC pool. The pool has 4 single-flight slots, so at most 4 RunBatch calls run at once.

Together they cap LightOnOCR-3-4B at about 24 pages in SGLang, whatever the client concurrency. The sidecar's own timings show it: inference p50 is 20 s while each page waits 60 s end to end at c64. #675 fixed the same head-of-line wait on the worker's direct path. There, the worker serves each extract item of an engine-batched adapter in its own batch and runs up to max_concurrent_dispatch() batches at once.

This PR gives the sidecar the same behaviour:

• Python reports the worker's dispatch width in the EnsureModelReady model descriptor, as a new optional ModelDescriptor.max_concurrent_dispatch. A new ModelWorker.dispatch_width property supplies it. The width is above 1 only for adapters that declare max_concurrent_dispatch() > 1 and have no LoRA state. Today that is the SGLang vision-extract adapter, at 128 for LightOnOCR-3-4B.
• Dispatcher. For a model whose width is above 1:
  • the drain loop sends each formed extract batch as single-item RunBatch calls, each on its own pipeline slot;
  • the model's pipeline depth becomes max(SIE_RUST_PIPELINE_DEPTH, width).
  • Encode and score keep their batches.
• IPC client. When such a model becomes ready, the slot pool grows to its configured size plus the width of every such model. It never shrinks. The multiplexed transport (SIE_IPC_MUX) has no slots and is unchanged.
• Compatibility. Models without a width, and backends that do not send one (older servers, the Candle worker), behave exactly as before.

Measurements

Setup:

• Hardware: one NVIDIA RTX PRO 6000 Blackwell Server Edition (96 GB), 4 vCPU / 16 GiB.
• Image: the released ghcr.io/superlinked/sie-server:v0.10.0-cuda12-sglang-vision-extract (sglang 0.5.10.post1), with sie_server overlaid from main and from this branch.
• Sidecar:
  • the v0.10.0 sie-server-sidecar, whose batching defaults match main;
  • this branch built with cargo build --release.
• Path: the sidecar runs in local-ingest mode (SIE_SIDECAR_INGEST=local), next to sie-server the way Helm wires them, with no env overrides except where noted.
• Clients: closed-loop clients send one-page publish_work extract operations shaped like the gateway's work items.
• Direct: the worker's HTTP extract path on the same image.
• Pages:
  • LightOnOCR-3-4B uses the 65 public pages from perf(server): give each SGLang OCR item its own worker batch #675;
  • GLM-OCR uses the 24 public photos from perf(server): give each SGLang OCR item its own worker batch #675;
  • each request changes one pixel, so no request reuses another's cached prefill.
• GPU-s/page = wall time from the first send to the last answer ÷ pages.

LightOnOCR-3-4B (325 pages at c64, 455 at c128):

Concurrency	Direct	Sidecar, main	Sidecar, this PR
64: GPU-s/page	0.503	0.985	0.493
64: p50 / p95 latency (s)	25.1 / 63.3	60.7 / 80.3	24.4 / 60.6
128: GPU-s/page	0.445	0.985	0.479
128: p50 / p95 latency (s)	45.5 / 104.4	119.9 / 141.9	44.8 / 106.4

GLM-OCR (504 photos per level):

Concurrency	Direct	Sidecar, main	Sidecar, this PR
32: GPU-s/photo	0.136	0.151	0.135
64: GPU-s/photo	0.069	0.083	0.066

Notes

• Run-to-run spread. GLM-OCR at c64 lands a few percent below its direct baseline, within the spread between runs.
• RunBatch budget in local-ingest runs. Queued work carries the gateway's deadline: 120 s by default, SIE_GATEWAY_REQUEST_TIMEOUT. The sidecar waits that long for a RunBatch. Local-ingest work carries none, so a RunBatch waits only the 60 s IPC timeout.
  • The LightOnOCR-3-4B rows for this PR therefore set SIE_IPC_REQUEST_TIMEOUT_S=120.
  • With the 60 s default, pages that run longer than 60 s at this concurrency time out and are retried. That gives 0.553 at c64, and 0.772 at c128 with 25 of 455 pages failing.
  • At c128 even 120 s leaves 14 retried pages, and that is the remaining gap to direct.
  • Behind the gateway, a page slower than 120 s times out on either path.
• Env knobs alone don't recover it. On main, SIE_RUST_PIPELINE_DEPTH=8 with SIE_BATCHER_MAX_BATCH_REQUESTS=16 reaches only 0.898 and 0.711 for LightOnOCR-3-4B, and makes GLM-OCR worse (0.107 at c64). The 4-slot pool still binds and every batch still waits for its slowest page. SIE_BATCHER_MAX_BATCH_REQUESTS=1 makes it worse still: GLM-OCR drops to 0.154, because depth 2 then means 2 pages in flight.
• Production in-flight cap. On the NATS path a sidecar admits at most SIE_PULL_LOOP_MAX_OUTSTANDING items (default: the 64-item fetch budget), so the c64 row is what a default deployment sees.

Output agreement

LightOnOCR-3-4B samples at temperature 0.1. Each sidecar run is compared with the direct run on identical request bytes:

Run	Concurrency	Exact match vs direct	Mean normalized edit distance
Sidecar, main	64	0.785	0.0092
Sidecar, this PR	64	0.782	0.0119
Sidecar, main	128	0.778	0.0112
Sidecar, this PR	128	0.785	0.0078

#675 measured a band of 0.774 / 0.0091 between two direct runs.

GLM-OCR decodes greedily. Compared byte for byte with direct, at c32 and c64:

• this PR matches 94.0% and 94.8%;
• main's sidecar matches 93.3% and 94.0%.

Tests

• Rust, dispatcher:
  • an engine-batched model's 6 extract items reach the backend as 6 one-item RunBatch calls, all in flight before any finishes;
  • without a width they still share batches and stay within the default depth;
  • only extract on an engine-batched model is split, and a reload without a width restores batching.
• Rust, IPC client:
  • the pool grows by each ready model's width and never shrinks, and every slot of the grown pool can be checked out at once;
  • ensure_model_ready grows the pool only for a ready model that reports a width.
• Python:
  • the descriptor carries the worker's dispatch width;
  • ModelWorker.dispatch_width follows _dispatch_width, including 1 for LoRA adapters.
• tools/check_ipc_types_parity.py passes. For the sidecar crate, cargo test, cargo clippy --all-targets -- -D warnings and cargo fmt --check pass on Rust 1.98.1.

Relation to #628

#628 adds per-model batch cost caps and a runs-alone lane for long audio. The two compose, and I left a note there.

• This PR reports the width on ModelDescriptor, not on EnsureModelReadyResponse where fix(sidecar): serve long-form audio in its own lane, with per-model batch cost caps #628 adds max_batch_tokens.
• It changes nothing under scheduler/. The only shared file is dispatcher.rs, in the drain loop and the readiness handling.
• Once both land, the split could move onto fix(sidecar): serve long-form audio in its own lane, with per-model batch cost caps #628's runs-alone lane.

Summary by CodeRabbit

• New Features
  • Models can now report their supported concurrent processing capacity. When a model supports higher concurrency, extract items can be processed individually in parallel, with processing capacity adjusted to match.
  • Models that do not report a higher concurrency limit continue to use the existing batching behavior. Models without the new setting retain the default behavior, preserving compatibility with older servers. Processing capacity can grow when higher limits are reported, while existing capacity is not reduced.

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📝 Walkthrough

Walkthrough

The server reports each model’s dispatch width in its descriptor. The sidecar uses the reported width to adjust non-multiplexed IPC pool capacity and to schedule extract items individually when the width exceeds 1.

Changes

Model dispatch width

Layer / File(s)	Summary
Report dispatch width packages/sie_server/src/sie_server/core/worker/model_worker.py, packages/sie_server/src/sie_server/ipc_types.py, packages/sie_server/src/sie_server/queue_executor.py, packages/sie_server/tests/core/test_worker_concurrent_dispatch.py, packages/sie_server/tests/test_queue_executor_stage1d.py, packages/sie_server_sidecar/src/protocol/ipc_types.rs, packages/sie_server_sidecar/docs/architecture-guide.md	The worker exposes its dispatch width, and ModelDescriptor carries the optional max_concurrent_dispatch value. Descriptor construction and tests cover valid widths and missing values.
Grow IPC capacity from readiness packages/sie_server_sidecar/src/ipc_client.rs	For non-multiplexed clients, model readiness updates per-model width tracking. The IPC pool grows to include tracked widths, and its capacity does not shrink. Tests cover additive sizing and slot availability.
Schedule extract items by model width packages/sie_server_sidecar/src/dispatcher.rs	The dispatcher records model widths and adjusts pipeline depth. For extract operations on models with width greater than 1, it schedules individual RunBatch calls. Tests cover concurrency, unchanged batching, shutdown, and model-specific width changes.

Sequence Diagram(s)

sequenceDiagram
  participant ModelWorker
  participant QueueExecutor
  participant IpcClient
  participant Dispatcher
  participant RunBatch
  ModelWorker->>QueueExecutor: expose dispatch_width
  QueueExecutor->>IpcClient: provide ModelDescriptor with max_concurrent_dispatch
  Dispatcher->>IpcClient: ensure_model_ready
  IpcClient-->>Dispatcher: return readiness descriptor
  Dispatcher->>Dispatcher: record model width and set pipeline depth
  Dispatcher->>RunBatch: dispatch individual extract items when width exceeds 1

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No confirmed issue remains that should block merging after normal checks.

Pre-merge checks | 4 | 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 55.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 60 functions across 8 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main sidecar change: dispatching engine-batched extract items as separate RunBatch calls according to adapter dispatch width.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Inline comments:
Review comments at @packages/sie_server_sidecar/src/dispatcher.rs:
- Around line 6133-6135: Update the active pipeline in dispatch_decoded when a
later EnsureModelReady descriptor increases the model’s effective width: add
permits to the existing pipeline semaphore by the width increase, without
restarting the drain loop. Keep the configured depth unchanged whenever it
already exceeds the reported width; use pipeline_depth_for to preserve that
depth when determining the effective width.

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• packages/sie_server/src/sie_server/core/worker/model_worker.py
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