[AMPL 2970] how to turn Cplex presolve off in AMPL

[SORA-7]

Hello,

I am using AMPL v10 together with CPLEX 11.
I meet the following problem:
I want to use CPLEX barrier algorithm without presolve in AMPL. I used
the commands as below

option solver cplexamp;
option presolve 0;               <--- turn off AMPL presolve.
option show_stats 1;
option cplex_options ' baropt presolve=0 crossover=0 prestats=1
lpdisplay=1 ';
solve;

Then,  I get the following output:

5255 variables, all linear
2626 constraints, all linear; 15689 nonzeros
1 linear objective; 5 nonzeros.

ILOG CPLEX 10.000, licensed to "university-edinburgh", options: e m b
q use=5
CPLEX 10.0.0: baropt
presolve=0
crossover=0
prestats=1
lpdisplay=1
Reduced presolve eliminated 72 rows and 2697 columns.
Reduced LP has 2554 rows, 2558 columns, and 7751 nonzeros.
Presolve time =    0.01 sec.
CPLEX 10.0.0: optimal solution; objective 5.494632119
7 barrier iterations; no basis.

Hence, it seems presolve still exists in CPLEX. I wonder how to turn
off it

Thank you very much in advance.

best regards,

Naiyuan

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[Paul-432]

I'm not sure, but try adding prereduce=0 to your solver options.  (You
may also want aggregate=0 in the options.)

/Paul

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[SORA-7]

umm, it doesn't work. I have tried to put crossover=0, presolve=0,
prereduce=0, aggregate=0, aggfill=0 and dependency=0 in the options.
But the output
is still the same,

Here is the output:
5255 variables, all linear
2626 constraints, all linear; 15689 nonzeros
1 linear objective; 5 nonzeros.

ILOG CPLEX 10.000, licensed to "university-edinburgh", options: e m b
q use=5
CPLEX 10.0.0: baropt
crossover=0
presolve=0
prereduce=0
aggregate=0
aggfill=0
dependency=0
prestats=1
lpdisplay=1
Presolve time =    0.00 sec.
LP Presolve eliminated 72 rows and 2697 columns.
Reduced LP has 2554 rows, 2558 columns, and 7751 nonzeros.
Presolve time =    0.01 sec.
CPLEX 10.0.0: optimal solution; objective 5.494632119
7 barrier iterations; no basis.

Thank you very much indeed.

Naiyuan




On 11月8日, 下午3時55分, Paul <ru...@...> wrote:
> I'm not sure, but try adding prereduce=0 to your solver options.  (You
> may also want aggregate=0 in the options.)
>
> /Paul

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[Paul-432]

I asked someone at CPLEX.  Turns out you cannot turn off presolve
completely when using the barrier optimizer -- it will do certain
presolve reductions regardless of the settings.  (Partly this has to
do with ensuring that the LP feasible region has an interior before
trying to use an interior point method.)

Bottom line:  you can either turn off all presolving and use a
different algorithm, or use the barrier algorithm and live with some
presolving.

/Paul

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[SORA-7]

Thank you very much for your kind help.

Sincerely,

Naiyuan

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