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Question of cell placement in P&R (SOC encounter)

bjbit

Hi,

When I try to place standard cells into the floorplan, I notice that the cells are not averagely distributed, no matter how I set the utilization percentage. Please check out the attacked picture. I even tried the utilization of 200%, and there still be some space forced to be empty. Conversely, the placed cells are heavily overlapping with each other in the rest area. I understand that a common setting of 70% utilization has no overlapping issue. But I am confusing with the exist of the empty wasted space. Is there anything I missed? Thanks.

Best regards, 

Parents

  • I'm not in expert in the placement algorithm but the I think the placer will cluster cells from modules that have alot of connectivity or  timing critical paths in order to reduce parasitics or congestion during routing. This is why it will create cell clusters and then leave parts of the floorplan unused. When utilization is over 100%, strange things happen (placer gets confused?) so don't make any assumptions from placement results when the design utilization is over 100%.

     Try using the option: setPlaceMode -modulePadding <module name(s)> <padding factor>

    This is what I typicaly use this to spread out cell placements.

Reply

  • I'm not in expert in the placement algorithm but the I think the placer will cluster cells from modules that have alot of connectivity or  timing critical paths in order to reduce parasitics or congestion during routing. This is why it will create cell clusters and then leave parts of the floorplan unused. When utilization is over 100%, strange things happen (placer gets confused?) so don't make any assumptions from placement results when the design utilization is over 100%.

     Try using the option: setPlaceMode -modulePadding <module name(s)> <padding factor>

    This is what I typicaly use this to spread out cell placements.

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