Timing changed dramatically between two runs, but the netlist is provably identical. What do you compare first?
From PDVerse MMMC Interview Masterclass · pdVerse Mentor Series
Direct answer
The run environment, before you ever suspect tool instability. Compare SDC, library versions, parasitic extraction/revision, derating and SI settings, clock definitions, and tool version — in that order — looking for the first thing that actually differs.
Mentor explanation
An identical netlist rules out one entire category of explanation, which is genuinely useful — but it does not mean the environment producing the timing number was identical. Timing results depend on far more than just the netlist: which SDC was loaded, which library revision, which SPEF, what derating and SI settings were active, what tool version and analysis mode were used. The senior move is to freeze both run's manifests side by side and walk down the list systematically rather than guessing. Nine times out of ten, something in that list quietly changed, and once you find the first divergent input, the "dramatic" timing change usually stops being mysterious.
Interview trap
Jumping to "the tool must be inconsistent" before comparing a single input file between the two runs.
Key takeaways
- The run environment, before you ever suspect tool instability.
- Compare SDC, library versions, parasitic extraction/revision, derating and SI settings, clock definitions, and tool version — in that order — looking for the first thing that actually differs.
Self-check: can you answer this aloud?
Try a 45-second answer using this structure:
- State the direct answer.
- Explain the timing or physical reason.
- Name one caveat.
- Say how you would verify it in a real flow.
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