Improve error message [ERROR GPL-0305] RePlAce diverged at newStepLength.

[oharboe]

Description

I wanted to have a look at what happens when there are long wires to distribute a signal, so I created a pathological case:

However, I get a mysterious error message in global placement.

I think global placement wants me to tweak some variable or other, but the error message doesn't give me a clue what to fix.

untar and run global_place_skip_io_Bus_asap7_766_2024-12-07_15-05.tar.gz

OpenROAD v2.0-17210-g8300b5293 
Features included (+) or not (-): +Charts +GPU +GUI +Python
This program is licensed under the BSD-3 license. See the LICENSE file for details.
Components of this program may be licensed under more restrictive licenses which must be honored.
global_placement -skip_io -density 0.40 -pad_left 0 -pad_right 0
[INFO GPL-0002] DBU: 1000
[INFO GPL-0003] SiteSize: (  0.054  0.270 ) um
[INFO GPL-0004] CoreBBox: (  2.052  2.160 ) ( 763.992 17.820 ) um
[INFO GPL-0006] NumInstances:              1035
[INFO GPL-0007] NumPlaceInstances:           19
[INFO GPL-0008] NumFixedInstances:         1016
[INFO GPL-0009] NumDummyInstances:            0
[INFO GPL-0010] NumNets:                     22
[INFO GPL-0011] NumPins:                     40
[INFO GPL-0012] DieBBox:  (  0.000  0.000 ) ( 766.000 20.000 ) um
[INFO GPL-0013] CoreBBox: (  2.052  2.160 ) ( 763.992 17.820 ) um
[INFO GPL-0016] CoreArea:             11931.980 um^2
[INFO GPL-0017] NonPlaceInstsArea:       29.627 um^2
[INFO GPL-0018] PlaceInstsArea:           1.823 um^2
[INFO GPL-0019] Util:                     0.015 %
[INFO GPL-0020] StdInstsArea:             1.823 um^2
[INFO GPL-0021] MacroInstsArea:           0.000 um^2
[INFO GPL-0031] FillerInit:NumGCells:     23710
[INFO GPL-0032] FillerInit:NumGNets:         22
[INFO GPL-0033] FillerInit:NumGPins:         40
[INFO GPL-0023] TargetDensity:            0.400
[INFO GPL-0024] AvrgPlaceInstArea:        0.096 um^2
[INFO GPL-0025] IdealBinArea:             0.240 um^2
[INFO GPL-0026] IdealBinCnt:              49757
[INFO GPL-0027] TotalBinArea:         11931.980 um^2
[INFO GPL-0028] BinCnt:      1024     32
[INFO GPL-0029] BinSize: (  0.745  0.490 )
[INFO GPL-0030] NumBins: 32768
[NesterovSolve] Iter:    1 overflow: 0.934 HPWL: 5131
[NesterovSolve] Iter:   10 overflow: 1.190 HPWL: 1460
[NesterovSolve] Iter:   20 overflow: 1.158 HPWL: 1043
[NesterovSolve] Iter:   30 overflow: 1.152 HPWL: 977
[NesterovSolve] Iter:   40 overflow: 1.156 HPWL: 924
[deleted]
[NesterovSolve] Iter: 2200 overflow: 0.670 HPWL: 114440
[NesterovSolve] Iter: 2210 overflow: 0.670 HPWL: 113127
[NesterovSolve] Iter: 2220 overflow: 0.670 HPWL: 110586
[ERROR GPL-0305] RePlAce diverged at newStepLength.
Error: global_place_skip_io.tcl, 21 GPL-0305
openroad> 

Suggested Solution

Make the global placement work or make the error message actionable.

Additional Context

No response

[oharboe]

I heard a remark that a CPU is ca. 1 cycle wide, which is what I wanted to explore a bit out of curiosity.

Not related to this issue as such, but I reduced the width and I got a result.

There is a single wire running across, every 150um or so, there is a buffer and from the timing report, I see something like 50ps per 150um.

430um across. so 50ps * 3 (ca. 430 / 150um is roughly 3), is 150ps. The actual arrival time was 190ps for the worst path.

So 33ps per 100um.

megaboom is ca. 1000x1000um (or should be, not quite there yet).

10 * 33ps = 333ps = 3GHz, which is roughly commercial speeds for such a part.

So the "a CPU is one cycle wide" remark is plausible in a very approximate sense.

[gudeh]

Hi @oharboe, this is where the error report comes from:

I would say this is more then a divergence, it is an actual crash because newStepLength was calculated as NAN or infinity. I will investigate why. Also, I think changing the wording for this report is necessary, since it is an actual crash.

[gudeh]

@oharboe would you happen to know why I was not able to run your issue with debug mode? Furthermore, I noticed that gpl never converged to the 0.10 overflow, I want to investigate that.

[oharboe]

@oharboe would you happen to know why I was not able to run your issue with debug mode?

Don't know. Sorry.

[gudeh]

Using a lower density such as 0.10, gpl converges.

I believe we get stuck in 0.67 overflow when using 0.40 density, as provided by Oyvind, because of the bin area scaling in line

OpenROAD/src/gpl/src/nesterovBase.cpp

Line 892 in 5daa1de

const float scaledBinArea

. However, it would be good to confirm

I wonder how we could avoid this issue while still making it work with a 0.40 density. My understanding is that a lower actual density than the target density should still be acceptable.

[titan73]

I encountered the same message #6338. It takes me some times that the density was too high as the message is not very straigtforwad.
That would be good to have a better message saying which metric is overflowing. Something like "Density is too high (120%), aborting" for example.