markdown/gh_discussions/Runtime/2889.md

[INFO GRT-0101] Running extra iterations to remove overflow - taking a long time

Tool: Global Routing

Subcategory: Global routing issue

Conversation

oharboe

What does this step actually do?

Any hints on how to figure out what is going on here, what to look at?

When I zoom in on the design, I see an ocean of thinly spread fiddlybits:

rovinski

GRT is the global router. It means your design is very dense (at least within some areas). It is attempting to assign nets to routing tracks, but there are too many nets within an area (overflow) so it is trying to reroute nets to avoid this.

GRT runs for a fixed number of iterations before giving up and erroring if it can't resolve the overflow.

The best first thing to check is the global routing heatmap to see congestion hotspots. In your case, it is likely that the density is simply too high and you need to reduce the placement density %.

maliberty

You should also check how many buffers are being inserted as it possible that can cause extra congestion if you have very bad timing.