How does OpenROAD compute the floorplan size while using core_utilization

[lustefan97]

Hello,

I have a quick question out of pure curiosity : How does OpenROAD compute the die area when using the core_utilization argument in the initialize_floorplan?

[maliberty]

The code starts at

OpenROAD/src/ifp/src/InitFloorplan.cc

Line 126 in 2bdae1c

utilization /= 100;

In short we take the area of the instances coming from synthesis and convert that to a core area based on the utilization. We then shape that core area based on the aspect_ratio. Finally we expand it by the spacing on each edge to get the die area.

[lustefan97]

I see, thanks for the explanation !

[lustefan97]

@maliberty I have an issue that I think is linked to the floorplanning step, but reported in the global placement step. I use the ASAP7 PDK, I have a dummy design for which I specify a target design utilization during the floorplanning step.

Essentially I try to increase the design utilization, here I target a DU of 70% thus I have set core_utilization 70 (and I am trying to go to 80%). The problem is that during the global placement step, this information is reported :

[INFO GPL-0019] Util(%): 115.63

Which lead to this error :

[ERROR GPL-0301] Utilization exceeds 100%.

I don't understand how, based off of how the floorplan size is computed, this utilization value is obtained since it should also be based off of the sum of the cells area, and for below 70 (tried with target DU of 60 and 65) it was fine...

[maliberty]

My guess is it relates to padding. Can you try setting

export CELL_PAD_IN_SITES_GLOBAL_PLACEMENT = 0
export CELL_PAD_IN_SITES_DETAIL_PLACEMENT = 0

and see if that explains the issue?

[lustefan97]

It did indeed explains the issue, GPL do now have a reported DU that is similar to the target one. May I ask why did padding the design left and right cause such an issue ? I thought that padding would add to the originally computed die size, not "reduce" the available space for placement. The padding argument are not explained in the GPL documentation either...

[rovinski]

Padding makes cells appear larger to GPL (and DPL) in order to prevent placing cells too close to each other. These larger cell sizes will cause it to look like the utilization is higher because there is less free space.