[Help-gnucap] Re: [Gnucap-devel] very large pure linear circuits

[walter steffè]

This is a copy without annexes of a message I have sent a few minutes ago.
It is sent in case that the difficoulties I have experienced in posting to the 
gnucap mailing lists was originated by the annexes.

Dear Al Davis

The present message is related to my previous message sent to the
''gnucap-devel'' list and to your reply (thanks for it). I was not able to
post it on the same list so I am trying with the ''help-gnucap'' list which
probably is more appropriate for this topic.

On Tuesday 18 October 2005 18:56, Al Davis wrote:
> Have you benchmarked it as it is?  It is a lot faster than Spice
> for that kind of circuit.  The biggest circuit in my test suite
> is about 35000 nodes.  When the options are set for speed, it
> takes about 15 seconds on a 850 MHz AMD.  It scales roughly
> linearly.  "A few hours" is very slow.  You can do better.

yes I have benchmarked it and here is the gnucap output:

/**************************************************************************
Cube Impedence
cube.ckt
time=    0.00
time=    0.20
time=   89.37
Gnucap   System status
command      --------  last  --------    --------  total  --------
               user      sys    total       user      sys    total
       get     2.53     0.07     2.60       2.53     0.07     2.60
        op     0.00     0.00     0.00       0.00     0.00     0.00
        dc     0.00     0.00     0.00       0.00     0.00     0.00
      tran     0.00     0.00     0.00       0.00     0.00     0.00
   fourier     0.00     0.00     0.00       0.00     0.00     0.00
        ac    89.26     0.11    89.37      89.26     0.11    89.37
function     --------  last  --------    --------  total  --------
               user      sys    total       user      sys    total
     setup     0.01    -0.00     0.01       0.01    -0.00     0.01
     order    -0.00    -0.00    -0.00      -0.00    -0.00    -0.00
function     --------  last  --------    --------  total  --------
               user      sys    total       user      sys    total
   advance     0.00     0.00     0.00       0.00     0.00     0.00
  evaluate     0.00     0.00     0.00       0.00     0.00     0.00
      load    13.88     0.01    13.89      13.88     0.01    13.89
        lu    72.01     0.03    72.04      72.01     0.03    72.04
      back     2.28     0.01     2.29       2.28     0.01     2.29
    review     0.00     0.00     0.00       0.00     0.00     0.00
    accept     0.00     0.00     0.00       0.00     0.00     0.00
    output     1.18     0.06     1.24       1.18     0.06     1.24
  overhead    -0.09     0.00    -0.09      -0.09     0.00    -0.09
      aux1     0.00     0.00     0.00       0.00     0.00     0.00
      aux2     0.00     0.00     0.00       0.00     0.00     0.00
      aux3     0.00     0.00     0.00       0.00     0.00     0.00
     total    89.26     0.11    89.37      89.26     0.11    89.37
iterations: op=0, dc=0, tran=0, fourier=0, total=746
transient timesteps: accepted=0, rejected=0, total=0
nodes: user=264, subckt=116, model=0, total=380
devices: diodes=0, bjts=0, mosfets=0, gates=0, subckts=7
commons: diodes=0, bjts=0, mosfets=0, gates=0, subckts=4
models:  diodes=0, bjts=0, mosfets=0, gates=0, subckts=4
dctran density=58.1%, ac density=58.1%
************************************************************************/


The reported results are obtained on a pentium xeon 2 GHz and 1GB RAM.
I did not set the options for speed because I were nor aware of them.
In the annexes you will find the input files of this benchmark.
I can confirm that gnucap is a lot faster than (the original) Spice for that
kind of circuit. In fact I had done a Spice benchmark a lot of time ago and
it was much worse then the gnucap results.

As I have told in the present  simulatiions the number of nodes (380) is not
large because the they are related to a single subdomain. Nevertheless the
computation time is not small (90s) probably because the ``coupling matrix is
allmost full'' (there are many voltage controlled current generators
connecting the circuital elements). The matrix sparsity will come into play
when I will form the global circuit which will include the subcircuits
associated to many subdomains. Indeed in the global circuit only the
subcircuits related to adjacient subdomain will be coupled.

> As it is now, when a circuit is purely linear, gnucap solves the
> big matrix only once.

Is it true also for the ac analisys?
It seems to me that the total time is proportional to the number of frequency
points.

> If you are willing to give up some accuracy, a completely
> different method such as a moment-matching method, will give
> you huge speedup.

I hope to be able to deal with the exact solvers. A motivaton for this choice
(apart from accuracy) is that I know that the traditional approach allows a
fast computation of the responce sensitivity versus the variation of 
 ircuital parameters and this is of great importance for the circuit
 optimization.  May be that the same can be achieved also with the
 moment-matching method but I not sure of that and I do not know of any open
 software that implement this method. On the other side I have to say that my
 circuit is already an approximation and that the subdomain related
 subcircuits are synthetized starting from the moments sequence associated
 with the electromagnetic responce. So it could be that going back to the
 moments for the simulation of global circuit is a good way to proceed (if I
 can find an available moments based code that is able to analyze my
 circuit).

In your reply you have introduced many other points and to me it is not clear
which are the most important to improve the simuation of my circuit (node
ordering, iterations related to the nonlinearity, memory usage...). May you
please help me to explain the time figures. I see that the most of the time
is used by ''lu'' but I think that 72s for the lu decomposition of a 380x380
matrix is a long time (if it is done only once).


Best regards
Walter Steffe

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cube.ckt
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cube132.ana
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