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
Last updated:
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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