which is more efficient

GAMS
1

porque wrote:

I have really in trouble
my model is very big and I have to make it more efficint

You should back up a bit and say what you mean by “very big” and “more efficient”. Is this and LP, MIP, MINLP, etc. Give the number of rows, columns, and nonzeros GAMS reports in the .lst file, for example. Do you mean the model is not efficient in space, so you want it to be smaller? This will probably also make things faster at the same time. But even models that have reasonable size can be very slow to generate or solve. If it is time you are wanting to save, you should say if it is generation time or solution time (both reported in the solve summary).

is fixing variables more efficint than wiriting them in constraints?

Fixing is more efficient, but how much so depends on the model type. It may not matter much.

is preventing gams to calculate the variables which are 0 more
efficient then not using many ord’s and many $ conditions?

Typically the use of ord’s and $ conditions (think about using dynamic sets there) is very efficient.

moreover if I have an equation such that Xij=a*Yij
for the conditions which X is 0, is it enough to fix only x variables
to 0 or is it necessary to fix y variables as well?

It’s enough to fix Xij but if you can generate a model that doesn’t even contain Xij or Yij.

-Steve

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2

thank you
it is MIP problem

presolve eliminated 30730rows and 42950 columns and 313 coefficient modifications
reduced MIP has 1638 rows and 11367 columns
reduced LP has 1400 rows and 23088 columns


and unsolvable because it gives out of memory error
I use 64 bit system to use much morememory but again gives memory error but of course with smaller gap
so I thought it might be because of my model equations

one more question
there is suppliers and transportation modes used.
for some suppliers some modes are not used
I gace zero capacity for them so 0 costs
but if I gave some capacity but huge cost, is it efficient
I do not think so but now I can not really understand the effect of my modeifications








— On Sun, 13/4/08, Steven Dirkse wrote:

From: Steven Dirkse
Subject: Re: which is more efficient
To: gamsworld@googlegroups.com
Date: Sunday, 13 April, 2008, 10:01 PM
porque wrote:

I have really in trouble
my model is very big and I have to make it more
efficint

You should back up a bit and say what you mean by
“very big” and “more
efficient”. Is this and LP, MIP, MINLP, etc. Give
the number of rows,
columns, and nonzeros GAMS reports in the .lst file, for
example. Do
you mean the model is not efficient in space, so you want
it to be
smaller? This will probably also make things faster at the
same time.
But even models that have reasonable size can be very slow
to generate
or solve. If it is time you are wanting to save, you
should say if it
is generation time or solution time (both reported in the
solve summary).

is fixing variables more efficint than wiriting them
in constraints?

Fixing is more efficient, but how much so depends on the
model type. It
may not matter much.

is preventing gams to calculate the variables which
are 0 more
efficient then not using many ord’s and many $
conditions?

Typically the use of ord’s and $ conditions (think
about using dynamic
sets there) is very efficient.

moreover if I have an equation such that Xij=a*Yij
for the conditions which X is 0, is it enough to fix
only x variables
to 0 or is it necessary to fix y variables as well?

It’s enough to fix Xij but if you can generate a model
that doesn’t even
contain Xij or Yij.

-Steve

Yahoo! For Good helps you make a difference

http://uk.promotions.yahoo.com/forgood/


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