Graph-based analysis (GBA) and path-based analysis (PBA) give significantly different slack on the same critical path. What's going on, and which one do you trust?
From PDVerse MMMC Interview Masterclass · pdVerse Mentor Series
Direct answer
GBA propagates conservative, graph-level worst-case assumptions efficiently across the whole design; PBA recalculates a specific path with path-specific context, removing some of that built-in pessimism. Neither one is universally "more correct" — you compare them under aligned settings and understand what each is actually measuring before drawing a conclusion.
Mentor explanation
It's a common shortcut to say PBA is simply the more accurate number and GBA is just conservative noise. That's not the full picture. GBA's pessimism exists on purpose — it lets you screen an entire design efficiently without recalculating every possible path combination, and that conservatism is part of how it protects you from misses. PBA earns its tighter number by focusing narrowly, with path-specific context, on paths you selected — which means it only tells you about the paths you looked at, not the ones you didn't. A GBA-driven implementation risk doesn't disappear just because a handful of PBA-selected paths came back clean; and relying only on exhaustive PBA everywhere isn't practical at design scale either. The professional answer compares both under matched settings and treats them as complementary tools, not a contest with one universal winner.
Interview trap
Using a small, selected-path PBA result as proof that no other path in the design fails. It only proves the paths you actually checked.
Tool check
report_timing -pba_mode path; report_timing -pba_mode exhaustiveCompare selected-path and broader PBA under aligned settings.
Key takeaways
- GBA propagates conservative, graph-level worst-case assumptions efficiently across the whole design; PBA recalculates a specific path with path-specific context, removing some of that built-in pessimism.
- Neither one is universally "more correct" — you compare them under aligned settings and understand what each is actually measuring before drawing a conclusion.
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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