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

\

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/

–~–~---------~–~----~------------~-------~–~----~

To post to this group, send email to gamsworld@googlegroups.com

To unsubscribe from this group, send email to gamsworld-unsubscribe@googlegroups.com

For more options, visit this group at http://groups.google.com/group/gamsworld?hl=en

-~----------~----~----~----~------~----~------~–~—

\