Re: gnucap speed

Hello Al

Thank you very much for your detailed and superb explanation about the 
gnucap solver.
It has remembered me the time at University at Engineering School. Great 
to see that gnucap uses a better and fastest solver algoritm than 
ngspice and qucs. I understand that QUCS uses a brute force one, gnucap 
a clever one and ngspice something in between.

I started with ngspice several years ago during my final Engineering 
degree. I choosed ngspice at that moment cause it was an open source 
simulator and I needed to simulate some circuits for my final project. 
It was my own choice because in the University nobody encouraged the use 
of free software at all. Only close and propietary simulators were 
available there under windows. I use Debian GNU/Linux from year 2000 and 
I am happy with it.

After several years not using any simulation software, I started to 
enjoy electronics again (RF radio receivers and transmiters are my 
passion  due to my amateur radio hobby)  I discovered QUCS. I has nice 
features and a great looking and easy to use GUI. But after that great 
GUI there is a very slow simulator (which is some times frustrating) 
with good  RF design capabilities (filters, transmision strip lines, 
etc..). I have had a bad experience with a transformer. The transformer 
even transforms the DC! I also had problems with a simple flip flop 
circuit simulated with transient analysis, not having the expected results.

Andy Fierman recommended me having a look at gnucap. At first glance I 
was reluctant but now I am convinced that it is a very good simulator 
that can serve me to solve many problems, with very good speed. I like 
very much the improvements made over ngspice.

If someone (I am not able to do it) could make work QUCS GUI with gnucap 
will be a great improvement. The only problem with gnucap and ngspice is 
the usability. It takes some time to get used to them, but once you 
undertand them you can simulate very quickly.

Many thanks Al

Ramiro.



El 22/07/11 16:43, al davis escribió:
> On Friday 22 July 2011, Ramiro Aceves wrote:
>> On gnucap: 0.719 seconds
>> Same circuit on QUCS: 65 seconds.
>>
>> I do not understand why there is such tremendous  time
>> difference. Same  stop time, same step size. Gnucap goodness
>> or Qucs badness....
>
> I knew Qucs was slow on large circuits, but that circuit is
> pretty small.
>
> There is a big difference in the algorithms.
>
> Qucs uses very simple text-book algorithms, without
> optimization.  Gnucap is highly optimized.  NGspice is somewhere
> in between.
>
> Qucs is really designed for adademic size problems where speed
> is not an issue.  Gnucap is really designed for very large
> problems, where even Spice is too slow.
>
> To Qucs credit ..  If you try to read the code, as a beginner,
> you are likely to understand it.  I think they considered this
> to be important.
>
> Typically, speed benchmarks show Gnucap speed to be a linear
> function of circuit size, NGspice speed to be a quadratic
> function of circuit size, Qucs speed to be a cubic function of
> circuit size.  I have experimented with some algorithms with
> speed as bad as an exponential function of circuit size.
>
> To look deeper, let's look at the matrix solver.  All of them
> use a similar method, based on LU decomposition or Gauss
> Elimination.
>
> The Qucs solver is classic.  Look at the code and you will see.
> It's the classic LU solver, straight out of any text on
> numerical analysis.  If you have every studied numerical
> analysis, you would know that LU decomposition speed is
> "O(n^3)", proportional to the size of the matrix cubed.  Look at
> the code, you will see the classic 3 nested loops, the classic
> square array.  (storage needs are quadratic .. "O(n^2)".
>
> Some "homework grade" simulators use "expansion by minors" to
> solve the matrix .....  exponential time.
>
> Real circuits result in most of the matrix entries being zero.
> If you care about space or speed, storing and manipulating these
> zero entries is a waste, so Spice uses a sparse matrix solver,
> that only stores and solves non-zero numbers.  This cuts both
> time and storage way down.  It is heuristic, so in theory it
> could be as bad as O(n^3), but usually isn't.  That difference
> in time you see is in all of those operations that multiply by
> zero or add zero.  Typically, it ends up being about O(n^2)
> because of the analysis time.
>
> Gnucap carrys this further, by a simpler analysis that takes
> linear time, and further heuristics to not solve the whole
> matrix every time, and not dealing with all components every
> time.  One example of this is that resistors are solved only
> once, and never looked at again for the entire transient run.
>
> I would love to have a way to run gnucap under the Qucs GUI.
> The Qucs GUI shows where they put their effort.  It's really
> nice!
>
> This might happen.  ..  A Gnucap plugin to read and write the
> Qucs format, and a wrapper to use Qucs devices (the special RF
> ones). ....  and there it is.
>
>
> _______________________________________________
> Help-gnucap mailing list
> [email protected]
> https://lists.gnu.org/mailman/listinfo/help-gnucap
>