Congestion in the Router Log - 2021.2 English

Versal ACAP System Integration and Validation Methodology Guide (UG1388)

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2021.2 English

The router issues additional messages depending on the congestion level and the difficulty to route certain resources. The router also prints several intermediate timing summaries. The first one comes after routing all the clocks and usually shows WNS/TNS/WHS/TNS numbers similar to post-place timing analysis. The next router intermediate timing summary is reported after initial routing. If the timing has degraded significantly, the timing QoR has been impacted by hold fixing and/or congestion.

When congestion level is 4 or higher, the router prints an initial estimated congestion table which gives more details on the nature of the congestion:

  • Global Congestion is similar to how the placer congestion is estimated and is based on all types of interconnects.
  • Long Congestion only considers long interconnect utilization for a given direction.
  • Short Congestion considers all other interconnect utilization for a given direction.

Any congestion area greater than 32x32 (level 5) will likely impact QoR and routability (highlighted in yellow in the table below). Congestion on Long interconnects increases usage of Short interconnects which results in longer routed delays. Congestion on Short interconnects usually induce longer runtimes and if their tile % is more than 5%, it will also likely cause QoR degradation (highlighted in red in the table below).

Figure 1. Initial Estimated Congestion Table

During Global Iterations, the router first tries to find a legal solution with no overlap and also meet timing for both setup and hold, with higher priority for hold fixing. When the router does not converge during a global iteration, it stops optimizing timing until a valid routed solution has been found, as shown on the example below:

Phase 4.1 Global Iteration 0
 Number of Nodes with overlaps = 1157522
 Number of Nodes with overlaps = 131697
 Number of Nodes with overlaps = 28118
 Number of Nodes with overlaps = 10971
 Number of Nodes with overlaps = 7324
WARNING: [Route 35-447] Congestion is preventing the router from routing all nets. 
The router will prioritize the successful completion of routing all nets over timing 

After a valid routed solution has been found, timing optimizations are re-enabled.

The route also flags CLB routing congestion and provides the name of the top most congested CLBs. An Info message is issued and the congested CLBs and nets are written to the text file listed in the message body. You can examine the text file for the list of CLB tiles and congested nets that are involved in the CLB pin-feed congestion, and use the congestion alleviation techniques listed in the Addressing Congestion section to resolve the CLB congestion before routing the design.

INFO: [Route 35-443] CLB routing congestion detected. Several CLBs have high routing utilization, which can impact timing closure. Congested CLBs and Nets are dumped in: iter_200_CongestedCLBsAndNets.txt
Tip: Localized CLB routing congestion can lead to routing failures even when the reported congestion levels for Global, Long, or Short congestion are within the acceptable range (less than 5). Look for the message above and in generated text files for localized congestion hotspots.

Finally, when the router cannot find a legally routed solution, several Critical Warning messages, as shown below, indicate the number of nets that are not fully routed and the number of interconnect resources with overlaps.

CRITICAL WARNING: [Route 35-162] 44084 signals failed to route due to routing 
congestion. Please run report_route_status to get a full summary of the design's 
CRITICAL WARNING: [Route 35-2] Design is not legally routed. There are 91566 node 
Tip: During routing, nets are spread around the congested areas, which usually reduces the final congestion level reported in the log file when the design is successfully routed.