Reconvergence Debug: Interview Drills

Interview drills

Interview Drills for Reconvergence Debug focuses on hazard reproduction success, debug cycle time, recurrence rate. The goal is to convert issue observations into mechanism-backed closure decisions.

PROMPT
You see hazard reproduction success, debug cycle time, recurrence rate in Reconvergence Debug. Explain root cause and signoff decision.

STRONG ANSWER
1. Names crossing/reset class and context.
2. Explains Reconvergence failures are intermittent and mode-dependent; debugging requires path correlation, event alignment, and protocol state reconstruction.
3. Requests trace alignment workbook, hazard waveform snippets, corrective action log.
4. Chooses fix/waiver/escalation with regression plan.

WEAK ANSWER
Lists tool commands or generic advice without mechanism and risk framing.

Diagram to draw on whiteboard

Intermittent hazard debug

1) isolate mode + seed
2) align async event traces
3) map reconvergent fanin
4) prove legal protocol state
5) patch and stress regress

Debug tree to narrate

ROOT-CAUSE TREE — Reconvergence Debug

crossing failure observed
        |
   reproducible?
     /        \
   no          yes
   |            |
stress mode   classify issue
expansion       /      |      \
            synchronizer protocol reset/reconvergence
                 |         |           |
            MTBF fit    liveness    release ordering

CDC/RDC deep dive

Reconvergence failures are timing + logic coupling hazards.

Concept diagram

RECONVERGENCE

branch A sync --                 -> merge logic -> illegal combination window
branch B sync --/

Metric graph

HAZARD REPRODUCTION RATE

before fix: 1/2000 runs
after fix: 0/100000 runs

Reports and artifacts

• reconvergence warnings

• gray transition checks

• hazard replay traces

• multi-bit strategy map

Mini case study

Gray pointer was legal, but downstream merge decoded mixed-era values during burst stress.

Debug branches

• Trace fanin cones

• align event windows

• verify decode assumptions

Senior review question

Ask: what evidence proves this risk is closed for silicon, not just tool-clean?

Key takeaways

• State crossing class, assumptions, and owner with every issue.

• Run structural and dynamic regressions after each fix.

Common pitfalls

• Treating all warnings as equivalent risk.

• Waiving issues without containment evidence.

• Skipping reset and reconvergence stress after CDC fixes.

Principal CDC/RDC review addendum

Reconvergence failures are intermittent and mode-dependent; debugging requires path correlation, event alignment, and protocol state reconstruction.

Metric: hazard reproduction success, debug cycle time, recurrence rate