| # High congestion - asap7 . Many OA and AO combinational cells in the congestion areas. | |
| Tool: Global Placement | |
| Subcategory: Congestion issues | |
| ## Conversation | |
| ### Mudbhatkal | |
| I'm seeing high congestion at global place for a design using asap7 libraries on Openroad. I tried using cell padding with 8 cell sites on the left and right. It helped reduce the congestion a bit, but not enough. I have set a max fanout and tightened the max capacitance in addition to the previous changes, and congestion is still there. I also changed the flow to congestion-driven instead of timing-driven and used the routability check overflow option but still see the congestion although it has decreased slightly. Any suggestions? | |
| ### maliberty | |
| Can you provide a test case or at least some images of the problem? | |
| ### Mudbhatkal | |
| The RTL was generated through [OpenASIP ](http://openasip.org/) | |
| By testcase do you mean odb file ? If not, how do I generate one ? | |
| <img width="751" alt="new_Cong" src="https://github.com/The-OpenROAD-Project/OpenROAD/assets/19781271/f02dbc11-ef8d-464c-b763-647caff65122"> | |
| The image shows estimated congestion and is captured after detailed placement. | |
| ### maliberty | |
| `make global_place_issue` should package the test case. Do not post if you are using any confidential data. | |
| ### Mudbhatkal | |
| Here is the testcase : https://tuni-my.sharepoint.com/:u:/g/personal/pooja_mudbhatkal_tuni_fi/EVDGc1VOxIZNhEfmDVLf1JcB56kh2ipprLFusncKTGhAWA?e=NnEMJh | |
| Please let me know if you have access issues | |
| ### maliberty | |
| This design does have an very unusual distribution of cell types. I think you will need to enlarge the block and lower the placement density to compensate for it. | |
| @gudeh this could be an interesting case to test your routability improvements on. | |
| ### maliberty | |
| Btw, what sort of block is this? | |
| ### gudeh | |
| Hi @Mudbhatkal, I noticed from the log files in the make issue you uploaded that you used yosys version 0.33. Have you tried other Yosys versions? ORFS is currently using 0.38 from a recent update. If you have a local installation of ORFS you can use the 'build_openroad.sh' script. I usually use the following: `./build_openroad.sh --local`. | |
| Maybe this would provide a different set of logic gates and a different routing scenario. For some reason I was not able to run Yosys on my end with your files. | |
| ### maliberty | |
| Have you tried lowering PLACE_DENSITY as I suggested earlier? | |
| ### stefanottili | |
| vliw cores I’ve seen in the past had wide and deep muxes around the registers. If these are not isolated in a separate rtl hierarchy, synthesis tools tend to ignore mux levels and generate ao/oa spagetti netlists that no R&R tool can untangle. | |
| Physical synthesis tools are supposed to be able to give you a better netlist, but since yosys isn’t one of them, you’ll have to rewrite the rtl of the muxes to get a P&R friendly netlist. | |
| Put each mux level in a separate rtl hierarchy and make sure that synthesis doesn’t flatten them. Separate enables per level of muxing. You should be able to identify mux structures in your netlist. | |
| P&R tools are usually bad at visualizing cone connectivity, but OR has timing critically color coded cone viewing as part of the timing debug. Use it after placement to look at your muxes. | |
| ### Mudbhatkal | |
| [big_tta_fixed.zip](https://github.com/The-OpenROAD-Project/OpenROAD/files/14486076/big_tta_fixed.zip) | |
| This RTL has some fixes but I still see congestion issues. Also tried with synth_hierarchial set to 1. Attaching it here in case someone finds it useful. | |