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What's the actual difference between GRLB and RDE for preroute parasitic estimation, and when does each apply?

From PDVerse PnR Interview Handbook · pdVerse Mentor Guide

By Tabish Iqbal · 9+ Years VLSI Physical Design Experience

Short Answer

GRLB (Global-Route-Layer-Based) improves preroute/postroute correlation, controlled by opt.common.use_route_aware_estimation -- auto (enabled only when per-unit resistance varies across layers), true (always), or a command to remove all global-route-based estimation before routing. RDE (Route-Driven Estimation) performs actual global routing plus extraction from those routes, auto-enabled for technologies below 16nm (otherwise opt.common.enable_rde must be set). Critically, when RDE is enabled, the GRLB setting is IGNORED -- they aren't both active at once.

Technical Reference DiagramWhat's the actual difference between GRLB and RDE for preroute parasitic estimation, and when does each apply?
What's the actual difference between GRLB and RDE for preroute parasitic estimation, and when does each apply?, illustrating the physical design concept.

Technical Explanation

  • GRLB (Global-Route-Layer-Based) estimation improves preroute/postroute correlation, controlled by opt.common.use_route_aware_estimation: auto (enabled only when per-unit resistance varies across layers), true (always enabled), or a command to remove all global-route-based estimation before routing.
  • RDE (Route-Driven Estimation) is a heavier technique: it performs actual global routing plus extraction from those real global routes, rather than estimating from layer resistance alone.
  • RDE is auto-enabled for technologies BELOW 16nm; for other technologies, opt.common.enable_rde must be explicitly set to true.
  • Critically: when RDE is enabled, the GRLB setting (opt.common.use_route_aware_estimation) is IGNORED entirely -- the two techniques are not both active simultaneously, RDE supersedes GRLB when both could apply.
  • RDE is applied during the final_opt stage of place_opt/clock_opt, and its parasitics are stored in the design library and reused -- it honors the -early_cap_scale/-late_cap_scale/-early_res_scale/-late_res_scale options of set_extraction_options.
  • Both are applied via set_app_options -name opt.common.use_route_aware_estimation -value auto (GRLB) and set_app_options -name opt.common.enable_rde -value true (RDE) -- the option names alone don't take effect without the real command wrapping them.

Common Mistake

The Trap: Assuming GRLB and RDE settings combine or stack -- when RDE is enabled, GRLB's setting is explicitly ignored, not layered on top of it.

Follow-up Question & Model Response

"Why would RDE be auto-enabled specifically below 16nm rather than at every process node uniformly?"

Candidate Model Response: Because parasitic effects (resistance, coupling) become proportionally more significant relative to intrinsic gate delay as process nodes shrink -- below 16nm, the accuracy gap between a layer-resistance-based estimate (GRLB) and actual global-route-derived extraction (RDE) becomes large enough that the heavier RDE technique is worth its extra cost by default, while at larger, less parasitic-dominated nodes GRLB's lighter estimate remains adequate.

Practical Example

Debug Scenario: A design on a 7nm process shows RDE auto-enabled, and a GRLB setting configured earlier in the flow script has no observable effect -- expected, since RDE being active means the GRLB setting is documented to be ignored entirely at that point.

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