# [Coin-ipopt] Question about time efficiency in really big problem and variable sensitive

Thu Jun 9 11:08:37 EDT 2005

```Dear Andreas,

I am using Ipopt as a new hope for my really big problem of soil mechanics.
met difficulty with memory allocation.  I have also read messages from
Ipopt's mail-list. But I have many questions about it.  Could you explain
them to me?

1.  Did you make any comparision of time between Ipopt and Lancelot in
really big large-scale problem?  ( Your paper showed, the maximum variables
is 125.000 variables)
Variables:  960 000 + 640 000 slack variables
Constraints:  640 000 (Inqualities in my problem is transformed)
I used Lancelot, its solving time is not even practical in case of one
half of the above problem.
Up to now how many variables can Ipopt solve ?

constraints and Hessian of Lagrangian in "dense" way for problem of 162
variables.  I used "imerit 4", "iquasi 6". It worked 2 times longer than
Lancelot:  Ipopt 152s / Lancelot 21 s.  My computer's structure is AMD 64
processor 3200, 512Mb.  Due to your mail-list, I am  now rewriting gradient
of constraints and Hessian of Lagrangian in "sparse" way.  But I do not know
if my "sparse" storage is correct:

For example:  Hessian of Lagragian in 5 variable problem:

a
0    b
0    0     c
0    d     0     e
f     0     0     0     g

Then:
K = / 1   2   3  4  5  6  7/  or K = /1  3   6   8  10  11 15/  is correct?
HESS(K)= / a   b   c   d   e  f   g/
IRNH(K)= / 1   2   3   4   4  5   5/
ICNH(K)= / 1   2   3   2   4  1   5/

In your example, it is clear that you used K in the second way.  If it is
true, it turns out that total number of HESS in "sparse" way and "dense" way
are equal due to declaration of static array.

3.  About the termination of the problem, it is proved that my
differentiable convex optimization has always a solution.  By using
Lancelot, I got the solution.  But Ipopt stopped with IERR=1, it means I
must increase IMAXITER.  However the solution of final step is acceptable
(means it is approximately to Lancelot's one).

My PARAM.DAT is as follows:
dtol = 1d-4
imerit = 4  (as default)
iquasi = 6
iprint =1
ifile = 1
ioutput = 1
imaxiter = 10000

If I had increased "maxiter", IERR was always 1.  I saw a vibration of
solution in range of 1d-1.  But If I  had made choice of dtol=1d-1.  Ipopt
sttoped at point very far from my Lancelot's solution.  Please show me how
to avoid IERR=1.  Lancelot use Augemented Lagrange method. So muss I change
any parameters?  (Parameters in Ipopt are too much for me to understand all
!)

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