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Context

Standalone Lavik starts an extra worker with no Meta control service. In DPDK poll mode that worker continues polling: with 16 data shards on 16 vCPUs, 17 runtime workers compete for CPU time.

Adaptive DPDK also silently fell back to polling when RX notifications were unsupported. This PR updates Bycorf to the merged fix in eloqdata/bycorf#10 (7904686).

Behavior before and after

Standalone mode now starts exactly N data workers. Meta-managed mode retains N data workers plus its final control worker. meta-exclusive-cpu has no effect in standalone mode; automatic sizing therefore uses all selected CPUs. Startup logs report control_worker=none when appropriate.

Both DPDK modes remain available with the existing default. An explicitly selected adaptive now requires RX notification support and fails with a BYCORF_DPDK_MODE=poll suggestion instead of falling back. Runtime notification failures stop all workers. The dependency also handles mlx5 RDMA event-channel acknowledgement.

Implementation

Make runtime worker count and CPU placement conditional on Meta management. CPU selection checks configured Meta seeds because CLI automatic sizing occurs before RunServer derives meta_managed_. Data shard IDs and storage layout remain unchanged for an explicit shard count. Update the runtime smoke test to expect two standalone worker rings and two DPDK VNETs when --threads=2.

Design decisions and alternatives

Avoid creating the unused standalone worker instead of assigning it a dedicated CPU. Keep the existing Meta control worker isolation and pinning rules.

Documentation and comments

Updated the architecture overview, tuning guide, public option comments, and CLI help to describe the conditional control worker and CPU reservation.

Test plan

On Ubuntu 24.04 / x86-64, GCC 13, Release with DPDK and SPDK enabled:

• cmake --build build --target lavik lavik_worker_fix_tests --parallel 16 — passed. The temporary focused test target links the existing tests/config_test.cpp and tests/worker_placement_test.cpp against lavik::module and GoogleTest.
• build/lavik_worker_fix_tests — all 44 tests passed, including standalone sizing/pinning and Meta control-worker placement.
• Real DPDK + six-device SPDK startup with 16 shards: log reports workers=16 data_shards=16 control_worker=none cpus=0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15; process inspection confirms 16 worker threads, and Redis PING/SET/GET succeeds.
• Memtier adaptive/VF + six-device SPDK, 16 data workers, 32 client threads, pipeline 1, 1 KiB values: peak-connection repeats (3×60 s each) produced median GET/SET 1.273M/1.376M QPS at 10M keys and 1.279M/1.204M QPS at 1B keys. All completed points had zero connection errors and zero GET misses. GET first repeats were lower (1.184M/1.185M); raw ranges are retained. These measurements use this branch at b83f894, not unrelated main-branch changes merged alongside it.
• python3 tests/runtime_backends_smoke.py build/lavik --dpdk — kernel and DPDK data/recovery/switching/clean-shutdown checks passed.
• Eight Bycorf adaptive failure/compatibility regressions passed; real Azure mlx5 VF startup with 16 adaptive workers and six SPDK devices passed, with idle process CPU around 0.114 CPU after dataset recovery. Throughput measurements are separate.
• clang-format 23.1.1 --dry-run --Werror on all changed C++ files — passed.
• git diff --check — passed.

Risk assessment

Standalone automatic sizing with meta-exclusive-cpu now uses one additional data shard. Existing storage should be restarted with its original explicit shard count. Meta-managed sizing and placement are preserved.

Rollback plan

Revert this commit and retain the explicit shard count used to create storage.

Reviewer guide

Review src/config.cpp for seed-based CPU sizing, src/redis/server.cpp for runtime worker count, and the two placement/config test files for regression coverage.

Follow-up work

Bycorf #10 is merged. The merged commit has the same source tree as the tested Bycorf commit (945e9b4). Benchmark artifacts distinguish original/fixed binaries and explicit poll/adaptive modes; poll used NetVSC while adaptive used the mlx5 VF directly, so this is not a controlled attribution of throughput changes to worker count alone.

[coderabbitai]

Walkthrough

Worker sizing, CPU placement, and control-worker startup now depend on whether Meta seeds are configured. Standalone nodes use only data workers, and the help text and documentation describe the distinction between standalone and Meta-managed modes.

Changes

Meta-Conditional Worker Placement

Layer / File(s)	Summary
Meta-aware sizing and CPU placement include/lavik/config.h, include/lavik/server.h, src/config.cpp, tests/config_test.cpp, tests/worker_placement_test.cpp	CPU reservation, automatic shard sizing, and worker CPU resolution apply Meta-specific behavior only when Meta seeds are configured. Tests cover Meta-seeded and standalone configurations.
Conditional control-worker startup src/redis/server.cpp, app/lavik.cpp, docs/architecture/01-overview.md, docs/operations/quick-start-tuning.md	Standalone nodes start only data workers, while Meta-managed nodes add a control worker. Help text and documentation describe the conditional worker count and CPU placement.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant ValidateServerOptions
  participant SelectedWorkerCpus
  participant RunServer
  participant Workers
  ValidateServerOptions->>SelectedWorkerCpus: resolve CPU requirements based on Meta seeds
  SelectedWorkerCpus->>RunServer: provide worker CPU assignments
  RunServer->>Workers: start shard_count_ workers, plus a control worker only in Meta-managed mode

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Suggested reviewers: liunyl

Merge Risk: 🔵 Low · up to d7846

Operators may avoid a valid standalone configuration because of the guide. Correct the instruction; the remaining risk is bounded.

Pre-merge checks | 4 | 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 16.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 12 functions across 7 files. (2 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check		The title clearly identifies both primary changes: removing the standalone control worker and preventing silent DPDK adaptive fallback.
Description check		The description includes all required template sections. It explains the behavior change, implementation, design decisions, documentation updates, exact test commands and results, risks, rollback, rev…

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Full details: Docstring Coverage

Explanation

Docstring coverage is 16.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 12 functions across 7 files. (2 skipped: 2 unsupported.)

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Actionable comments posted: 1

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Inline comments:
Review comments at @docs/operations/quick-start-tuning.md:
- Around line 64-65: Update the `--meta-exclusive-cpu` guidance in the
quick-start tuning documentation to state that pinning is required only in
Meta-managed mode. Clarify that standalone mode ignores the option and permits
`--no-pin-workers` when no explicit CPU list is set.

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 52845a9 and d78469d.

📒 Files selected for processing (9)

• app/lavik.cpp
• docs/architecture/01-overview.md
• docs/operations/quick-start-tuning.md
• include/lavik/config.h
• include/lavik/server.h
• src/config.cpp
• src/redis/server.cpp
• tests/config_test.cpp
• tests/worker_placement_test.cpp

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