( ESNUG 372 Item 4 ) -------------------------------------------- [05/31/01]
Subject: ( ESNUG 371 #2 ) Tricks For Undoing PhysOpt Clustering Congestion
> Though our design's overall row utilization is low, about 55%, PhysOpt
> (since it doesn't have real global routing) keeps clustering cells to
> create very high density areas. This happens with -congestion option,
> even when we leave empty space around the cluster. This congestion makes
> our design unroutable.
>
> - [ Shrek's Donkey ]
From: [ Synopsys R&D ]
John,
Let me be absolutely clear. We have a global router within PhysOpt to
estimate congestion. Our customers have run 170+ designs through PhysOpt
and our congestion analysis correlated very well with IBM, Avanti, and
Cadence detailed routers. To use our global router, set congestion_effort
to [low | medium | high]. The higher you set it, the more detailed (but
more CPU intensive the congestion estimates are.)
As for [ Shrek's Donkey ], I am sorry that he is having so much trouble.
There are a number of reasons this may be happening and various antidotes.
One way to avoid congestion and limit clustering is to use the following
switch:
set_congestion_options -max_utilization 0.85
The default is 0.95 and the user should try some lower values (0.85 is in
the example). Also, there are a number of articles in Solvit on techniques
to avoid congestion which may help this user.
- [ Synopsys R&D ]
---- ---- ---- ---- ---- ---- ----
From: Wei Chen <chenw2@sd.conexant.com>
Hi John,
I am a PhysOpt user, and have seen the same clustering especially on low
utlization floor plans with tight timing constraints. Synopsys does
provide a command called set_congestion_options which is very usefull. This
command has several options: -horizontal, -vertical, and -max_util
I have, so far, only used the -max_util option which turned out to be very
successful. The value of max utilization can be rather delicate, small
variations can cause long run times. The option does not take effect
unless the -congestion option is used during compile.
- Wei Chen
Mindspeed Technologies, a Conexant Business
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