Two engineers report different results for the same floorplan. How do you isolate the cause?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
"Same floorplan" is a claim, not a fact — a screenshot looking identical tells you nothing about whether the underlying analysis state (tool version, options, scenario, library) actually matches. Start with provenance before comparing any numbers: was the floorplan written out from the same checkpoint, using the same tool version, and loaded with the same app options on both sides?
Technical Explanation
- Check scenario and library setup next — if one engineer is analyzing a different active scenario, or pulling from a different reference library revision, congestion and timing numbers will diverge even though the "floorplan" geometry looks the same.
- Compare constraints and placement seed — a different SDC revision, or a placer run with a different random seed/effort setting, can legitimately produce different placement results from the "same" starting floorplan.
- Check PG and blockage assumptions too — if the power grid or blockage set differs between the two runs (even subtly, like a halo change that didn't make it into both setups), congestion and legality results will diverge for a completely mundane reason.
- Don't chase the discrepancy by comparing final numbers — find the first point of divergence (one named object, one hot spot, one report setting) and reproduce both results starting from an identical, clean checkpoint; that's the only way to be sure you're comparing apples to apples.
Formula Or Decision Rule
Decision rule: do not choose an alternative until both runs share the same controlled experiment.
What To Check
- Warning sign: One result is accepted because it looks more reasonable.
- Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
- Correct: Align inputs and settings, rerun, and archive the reproducer.
Command Checks & Actions
report_design-floorplan: reports the floorplan state understood by the toolreport_congestion-mode summary: reports routing demand, capacity, overflow, or hot spotsreport_timing: reports timing paths in the active analysis contextwrite_floorplan-output <output_dir>: writes reproducible floorplan information to files
Run only the commands needed for this question. Save the report with the floorplan version and analysis context.
Healthy, Suspicious & Hard-stop Results
- Healthy: Compare floorplan write-out, checkpoint, tool version, app options, scenarios, libraries, constraints, placement seed, PG/blockage assumptions, and report settings before comparing numbers.
- Hard stop: required legality or physical feasibility is missing or unexplained.
Common Mistake
The Trap: Do not hide the symptom with an arbitrary utilisation, halo, channel, blockage, pin move, or die-size change.
What The Interviewer Is Testing
The interviewer is testing whether you can connect “Two engineers report different results for the same floorplan. ” to measurable physical evidence and an owned correction.
Follow-up Question & Model Response
"* Candidate Model Response: Model response: “I would change it if the same controlled rerun shows that one result is accepted because it looks more reasonable. ”
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