LOPTIS Frequently Asked Questions

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How many plants, warehouse, products, etc. can be in a LOPTIS model?

Although there is no limit on the number of things (number of plants, number of products, ...), the mathematical model that is created by LOPTIS must fit into your computer. There is one constraint for every product at every plant, warehouse, and market at which it can appear. There are many other constraints as well - primarily associated with physical capacities. The other limit involved is the limit of your patience - the larger the model, the longer it runs.

LOPTIS is data-driven. By our definition, this means that it works with the data that you supply. There are no a priori limits on any data dimensions. Further, if you leave out a whole class of data, LOPTIS doesn't care; it simply leaves out that part of the model. Having said that, I must add that there is a bare minimum of data required: production capacity (provides products at sources), demands (provides sinks to absorb the products), and distribution costs (provides arcs between the sources and sinks).

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How long does it take to run a LOPTIS model?

As for performance, each situation that we model has its own unique performance characteristics. You will have to experiment to see how your "version" is going to do.

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How big a computer do I need to run LOPTIS?

LOPTIS requires that the matrix must fit completely in RAM when the optimizer loads the matrix to solve it. We recommend a minimum of 8 MB. The matrix representation that we use exploits a concept known as supersparsity and packs the most matrix into the least amount of memory. Other RAM requirements aren't as demanding, e.g., the model database is a virtual file and once we get a minimum amount of buffer space, all additional RAM goes to improving performance by reducing I/O. But even then, once we have all we need, more RAM doesn't help.

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I don't plan to model warehouses, can I still use LOPTIS?

The short answer is yes. You mentioned that you are planning to not have warehouses in your model. Does this imply that you will have a single time period model? This is an important point because in a multiple time period model the things that tie the periods together are product inventory at the warehouses and capacity transfer in capital investment scenarios. If you don't have either of these and are still modeling multiple time periods, you will save computer run time if you make a separate model run for each period.

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How does LOPTIS handle intermediate products?

If a product created by one of your plants is used as a raw-material at that plant or at a different plant we refer to that product as an intermediate product. The intermediate product appears in BOTH the raw material and the product tables. It has all of the properties of both raw-materials and finished products, i.e., it can be purchased as a raw-material and sold as a product and shipped as either. In addition, if you ship it from one of your plants to another, you must supply the intermediate product shipping costs data table.

Notice that the shipment of the intermediate product goes from one plant to another plant without first going through a warehouse. If there needs to be an inventory of this product, it will occur in the plant that uses it as a raw-material. If the intermediate product is shipped to a warehouse, the only thing the model can do with it is store it in inventory or ship to another warehouse or market.

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What does the message "DISK IS FULL" mean?

The message "disk is full" tells us that your hard-drive is out of space.

If you look at how much space is unused on your disk after LOPTIS terminates, you will probably see some space available. This is misleading because some the LOPTIS files are workspace files that are deleted when it is finished. The various workspace files used by LOPTIS can become quite large and at any one point during the execution two or more of them could be active. I am currently running a customized version of LOPTIS that creates a model that has 40,000 rows and 143,000 columns; the model database occupies almost 30 megabytes including the data tables, LOPTIS generated control tables, the LP matrix, and the optimal solution. I never start a LOPTIS model run without a lot of space on my hard-drive (100 megabytes or more). We don't necessarily need all of that space, but then I don't worry about running out of space on my disk.

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I noticed that C-WHIZ minimized my LOPTIS objective. How do I force it to maximize?

LOPTIS automatically detects when to maximize and when to minimize. The decision is based on the presence of a PRICE table. If you supply prices, we maximize contribution to profit; otherwise, we minimize variable cost. Ultimately, direction of optimization is controlled by one of the MPSIII variables in the CR, XMINMAX; it is set to MIN or MAX.

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The LOPTIS print file has the full MPS solution output. How can I eliminate it to save run time and file space?

Producing the solution report should not take a lot of time, but if you want to turn it off, you would save some time and would reduce the size of your print file. In the CONTROL table you have entries that say SOLUTION and/or CONTSOL; either delete them or put an asterisk (*) in column 1 of the line.

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The optimizer says that the last iteration is at 150 sec., but it seems to take a long time after that before the job is finished even when I turn off the reports. What's going on?

After C-WHIZ finds the optimal solution, there are quite a few tasks yet to be done including: recording the optimal solution onto the database, printing the standard SOLUTION report (the rows and columns), and in the case of LOPTIS, to call the mixed-integer optimizer to do the branch-and-bound processing, if appropriate. After all of this is finished, the LOPTIS report program is executed to create the tabular reports. You have only turned off the last step.

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I get the message, NO IVL TABLE, THIS MAY NOT BE A MIP PROBLEM. Do I have to make another table called IVL including all the variables I want to be integer?

The LOPTIS matrix generator creates the Integer Variable List (Z:IVL) table. It is not practical for you to add variable names to Z:IVL (even if you could); the matrix must be built with the appropriate structures to support the "integerness" of these variables.

The only integer variables possible in the standard LOPTIS model are the plant and warehouse existence variables, i.e., the variables that determine whether or not a new plant or warehouse is built or purchased or whether or not an existing plant or warehouse is expanded or sold. You provide data for your proposed plant and warehouse expansion and acquisition projects plus fixed costs and the cost of building or acquiring the facility. In the rest of the data tables you supply all necessary data for the proposed facilities just as though they already existed. LOPTIS handles it automatically from there.

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I put small numbers (like 0.0042) into my tables, but when I look at the tables with TABEDIT all I see is 0.00. Do I actually have a number there and, if so, how can I see it?

Yes, you do have a number. In the LOPTIS model database we maintain all of our numeric values as double precision real numbers. We specified a display format that would get the most information on a screen and that would work most of the time. If you are viewing your data tables with the DOS TABEDIT, once you get a table on the screen, look at the function menu across the bottom of the screen. F9 Width can be used to specify the total width of each numeric field and the number of decimal places to the right of the decimal point. What you specify applies only to the current display, so you can't hurt anything - play with it until you get what you want. If you are using the Windows TABEDIT, select the Format menu and follow your nose.

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