Exercises: Functions

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Exercises identified as mini projects are more extensive and may take considerably longer than the typical exercise.


A Priority Serving System

  1. The system administrators of a mainframe computer have devised of method of partitioning processing time. Every job submitted to the mainframe is assigned a priority: urgent and not urgent. When two jobs have the same priority, the first one submitted is processed first. However, when two jobs are of different priority, the job with the higher priority is always processed first. Urgent jobs arrive at a rate uniformly distributed between 3 and 5 hours and have required processing times uniformly distributed between 1 and 10 hours. Not urgent jobs arrive at a rate uniformly distributed between 30 minutes and 1 hour and have processing times uniformly distributed between 15 minutes and 2 hours. Model the mainframe queue assuming every job, once it is started, is finished without preemption.
  2. Now model the mainframe queue where jobs can be pre-empted by priority. In other words, if a job is submitted to the mainframe while a job of lower priority is processing, the lower priority job is discontinued until the job of higher priority finishes. When a pre-empted job returns to the mainframe, it does not need to be entirely reprocessed. It is processed only for the required time remaining when it was pre-empted. Use the PUT and GET functions in modeling this system.

Empirical Distributions

How can the function, DISK{FILE;I}, be used to generate a random variable from an empirical distribution function with data in the disk file, FILE? (Hint: Consider a random index, I.)

Condensing Events

Use pre-emptive event execution (edge delays equal to * to "condense" several event vertices into a single event) to make the simulation, NETWORKR.MOD, run as fast as possible without changing the output. Be careful about using * too often and causing a stack overflow as explained in the text.

Worker Assignment Problem

In the model, TWOQUEUE.MOD, there are 2 servers assigned to a constant time set-up operation followed by 6 workers assigned to a random time processing operation. Is this a good assignment of the 8 workers? Experiment with the model to determine an optimal allocation of the 8 workers, assuming that all of the workers are equally skilled and paid the same amount. Justify your answer (how do you define "optimal" and why?) as well as the way you designed your experiment.

Execution Priorities

Look at the model, TWOQUEUE.MOD. This is a model of two multiple-server queues in a series, where a constant time setup operation is followed by a random duration processing operation. The first set of servers has a constant service time of 5 minutes. The second set of servers has a random service time with an Erlang(1) (i.e., exponential) distribution with a mean of 4 minutes.

  1. Click on the double edge between the STRT1 and LEAV1 vertices. Change the execution priority on the edge from the LEAV1 vertex to the STRT1 vertex from 3 to a 5. Notice that QUEUE[1] goes negative before time 60. Why?
  2. Change the execution priority on the self-scheduling edge from STRT1 to STRT1 from 1 to 6. Notice that the number of idle servers for queue 0 goes negative right away. Why?

Welding Line (Mini Simulation Project)

The following system is used to weld hoods to automobiles. There are several parallel welding robots. The cars pass by the robots on conveyor lines. Transfers between lines are done using cranes that run north and south on fixed tracks. A schematic of the system for four welding robots is given below.

Type	Percent	Average Weld Times
A	31%	50 (sec.)
B	16	63
C	16	25
D	16	41
E	12	155
F	 9	155

The input rate is 93 jobs per hour. There are six types of automobiles produced at this plant. The average welding times and percentages for each type of car are given below the figure. The time it takes for the crane to pick up a car or release a car is 8 seconds. For simplicity we consider only an approximation to the effects of crane acceleration. The first section of track traveled takes 9 seconds and every additional section traveled without stopping takes 6.5 seconds. A section of track is between each of the loading or unloading points on a crane track. For example, the crane takes 15.5 seconds to travel two sections of the track without stopping.

Cars must leave the welding operation in the same order that they enter it since components added later on in the assembly process must be for the right car. Set-up times for some of these later operations are very large, thus, requiring that set-ups start before cars arrive. The last crane in the schematic above is to sort the cars as they come out of the hood welding operation.

Write a simulation program for this system. For now, consider each of the times to be deterministic. Your model should allow up to three cranes per track and up to six parallel robots. You may use any computer language that you wish for this assignment. Flesh out your model and translate it to C to fill in the details (see Linking). Use "entities" and the PUT and GET functions.

In the automobile production sub-system simulation model, add 33 seconds to each of the average welding times given. Now make the welding times uniformly distributed with a range of 20% of the mean. All confidence intervals are to be at the 90% level.

  1. Run the simulation of the automobile production system five times for 200 cars each. Use independently selected pseudo-random number generator seeds for each replications. Compute a confidence interval for the mean throughput rate of the system (rate that cars exit).
  2. Run the simulation for one run of 1000 automobiles. Compute confidence intervals for the mean throughput rate using the batched means method with 5 batches.)
  3. Comment on the above experiment, including identification of possible sources of error.
  4. Given that there are spaces for 15 automobiles in the system, decide how these 15 buffer spaces should be allocated. Justify your design and give confidence intervals for the throughput rate.

A Barge Unloading System (Mini Simulation Project)

River barges arrive at a warehouse carrying "piggyback" truck trailers mounted on flatcars. These flatcars are to be unloaded and mounted on railroad flatcars that will be taken away by trains. There are 4 trailers on each barge. The time between barge arrivals has a uniform distribution between 3 and 5 hours. A barge must wait for a single berth before it can dock. A single crane is used to move the trailers from the barge to the warehouse and from the warehouse to the flatcars. After being unloaded from the barges and mounted on flatcars, the trailers are taken to a large rail yard where they are assigned to trains according to their destination.

Once a barge has docked, 4 spaces in a warehouse must be free in order for unloading to start. There are 20 trailer spaces in the warehouse. The time to unload a barge is uniformly distributed between 1 and 3 hours. A train arrives at the warehouse to collect the waiting trailers. The time between train arrivals has a uniform distribution between 3 and 7 hours. The train has between 6 and 12 empty flatcars when it arrives. It takes 30 minutes to load each trailer onto a flatcar.

  1. Develop a resident entity simulation model that will keep track of the lengths of all important queues and utilizations of the warehouse space, berths, and crane. Clearly state any assumptions you are making in your model. Run this model for a time of 20 (hours) and trace the number of trucks unloaded. Run the model for 100 hours in High Speed mode and use the output file see how many trucks were unloaded from the barges. What is the major bottleneck: the berth, number of available flatcars, warehouse space, the crane?
  2. Consider the purchase of an automatic crane that cuts the barge unloading time by 50%. Model this (reduce the delay time between the start unload event and the end unload event). Run the new model for 100 time units and observe (from the output trace) the number of trucks unloaded. Do not create any new state variables.
  3. Add the necessary (two) event vertices that permit the crane to shut down every 10 hours for 1 hour of preventive inspection/maintenance. The crane will finish what it is doing before undergoing inspection.
  4. Eliminate the inspection/maintenance procedure and have the automatic crane break down every 8 to 12 hours (uniformly distributed). Have the repair time be uniformly distributed between 0.5 and 1.5 hours. If the crane is busy when it breaks, the current unloading operation must be restarted from scratch (use a cancelling edge). Run the simulation for 100 hours and compare with the systems in parts (a), (b), and (c). (This problem was suggested by G. Samorodnitsky.)

Buffer Capacities

In the model, NETWORKR.MOD, assume that each machine group has a finite capacity queue. For machine group, G, no more than B[G] jobs can be waiting. If the next queue for a job is filled, that job does not free its machine; it is blocked. Enrich your model to include this constraint. Be careful not to send two parts to a queue that only has space for one; when the second part arrives, it will find the queue full! (Hint: Reserve space in the next queue before freeing the current machine.) Assume that B[G] is 4 for all machine groups, and run at least 20 jobs through your model. Debug your model.

A Layout (Mini Simulation Project)

Using NETWORKR.MOD as a starting place, include the transit times between the various machines. Assume the job transit data is deterministic (conveyors?) and is provided in an array called TRANS(I,J), which is the time it takes to travel from machine group I to machine group J. The transit times are proportional to the distance between machine groups (state assumptions you are making). What data structures in C could be used effectively in this model? (Hint: The ENTITY structure). Tell how the structures will be used. How is your model improved with these structures (if at all)?

  1. Translate your model to C.. Use this simulation (or your model) to help determine an efficient high-level layout for a jobshop (i.e., the shapes, sizes, and locations for the different machine groups. Each machine takes 16 square yards of space (including space for the job being processed and aisle space). Each job waiting in the queue takes 4 square yards of space. Transit between machine groups is by overhead crane, so travel is rectilinear not line-of-sight. Transit time between machine groups is 5 seconds per yard traveled between the head of each input queue. Write a report that explains and sells your design.
  2. Assume that you will make $10 for each type 1 job completed, $20 for each type 2 job completed, and $30 for each type 3 job completed. You may sequence the jobs in each queue in any manner you see fit. With the machines specified in the text, lay out the factory to maximize the rate that income is generated and minimize the size of the factory.
  3. To help solve this problem, design and run experiments to decide how to allocate waiting space for work-in-process (be creative). Explain the rationale for your experimental strategies: How did you initialize the system? Determine run lengths? Use variance reduction techniques? Compute confidence intervals?

Processing Priorities

Assume in NETWORK.MOD that completed type 1 jobs are worth twice as much money as type 2 jobs, which in turn are worth three times the money as type 3 jobs. It has been suggested that the jobshop operate by priority. The obvious scheme is to process waiting type 1 jobs before waiting type 2 jobs, with waiting type 3 jobs to be processed last. Make the necessary adjustments to your model for priority job processing. (Use the PUT and GET functions in SIGMA.)

  1. Make 5 runs of 8 hours each with the priority service rule and 5 runs of 8 hours with the current FIFO rule (a total of 10 one-day runs). Use independent streams for each run and estimate a confidence interval for the difference in total income for the day.
  2. Repeat this experiment using common seeds for each pair of runs (with and without the priority rule). Use independent seeds for different pairs of runs. Again estimate a confidence interval for the difference in total income for the 8 hour day.
  3. Comment on the differences between parts (a) and (b). What might be wrong with this experiment? Do you recommend the priority processing rule? Do you know a better one?

Motor Vehicles Department (Mini Simulation Project)

A State Motor Vehicles office has three clerks on duty. The office opens at 10:00 A.M. and closes its doors at 5:00 P.M. Customers already inside the office after closing are served. Between the hours of 10 and 12 (noon), walk-in customers arrive according to a Poisson process at a rate of (t-10) customers per minute, where t denotes time in hours. Between the hours of 12 and 13 (i.e., over the noon hour), customers arrive at a constant rate of 2 . Finally, between times 13 (1:00 P.M.) and 17 (5:00 P.M.), the arrival rate is (17 - t)/2.

Clerks process customers according to a uniform distribution between 0.5 and 2 minutes per customer. Occasionally a customer will call on the telephone and one of the clerks will take the call (possibly interrupting service for a customer). The time between phone calls is distributed as an exponential random variable with a mean of 5 minutes. It typically takes half the time to serve a call as to serve a walk-in customer. Every 20 minutes, one of the clerks is due for a ten minute break on a rotating basis (one break per hour for each clerk).

  1. With at most 10 runs of the model give an estimate of the capacity of the system as a function of the demand constant . Use whatever system performance measures you think are reasonable and justify them. Make and carefully state any additional assumptions about the system that you need; be sure to offer some justification for each assumption. A run is for a single day of operation. Carefully plan your experiment and give the rationale for your experimental design.
  2. As an aid in contract negotiations, consider the effect of adding an additional clerk or cutting the amount of break time taken by each clerk.

There is a plan to add a drive-up window to the office. This window is to be serviced by the same clerks that serve walk-in customers. Drive-in customers arrive according to the same pattern as the walk-in customers. Drive-in customers will balk if there are 3 or more cars waiting for service at the drive-in window. Of these balking drive-in customers, 80% will try to park their cars in the parking lot and walk inside for service. Once a former drive-in customer parks and walks inside, s/he will not balk unless the line has more than 10 customers in it. The parking lot has space for 10 cars. Customers desiring parking in a full lot will wait only 30 seconds before balking. Assume that it takes between 2 and 3 minutes (uniformly) for a customer to travel from the lot to the office and vice versa (including time to park).

Enrich you simulation to include the possible drive-in window. Use the your model to advise the State as to what they should do (hire more clerks?, build the drive-up window?, increase the size of the parking lot?, etc.).

A Turnpike Gas Station

A toll road gas station has 2 pumps in tandem but only 1 access lane. Cars arrive only from the left at intervals that are spaced T minutes apart. Pumping gas and paying takes P minutes. If both pumps in a lane are free when a customer arrives, the customer will use the "down stream" pump. A car at pump 1 cannot pass a car at the pump in front of it even if it has finished.

  1. What are the resident entities in this system? What are their attributes? What are the transient entities and their attributes? Describe, in words, the resident entity cycles and transient entity path(s). List any additional assumptions you are making about this system? Give an event graph model for the resident entities in this system. At a minimum, your graph could be used to measure the utilization of each pump.
  2. Create a SIGMA model of the turnpike gas station. Modify the problem so that if there are four or more cars in line when a customer arrives, the customer will go on to another station. Run you model until 20 cars have been served and analyze the queue size and utilization of each pump (charts, numbers, intuition, etc.). Start the system empty. Assume that car interarrival times are uniformly distributed between 1 and 5 minutes and service times are between 1 and 7 minutes.

A Turnpike Gas Station (continued) (Mini Simulation Project)

Enrich the preceding turnpike gas station model to include two access lanes, one on either side of the pumps. Potential customers arrive from the right at intervals that are randomly spaced. Assume that customer interarrival times are uniformly distributed between 2 and 6 minutes when coming from the left and uniformly spaced between 2 and 4 minutes when coming from the right. Service times are between 3 and 5 minutes at each pump.

  1. Customers cannot pass cars in front of them even if they have finished. Each pump can pump to only one car at a time in either lane and cars cannot drive around to change lanes. If there are more than four cars waiting in a lane, customers arriving from that direction will not stop. Draw an event graph of this system that can be used to study the resident entities (pumps and queues). Carefully justify your event graph model; do not just tell what you did and how, include why!
  2. Create a SIGMA model of your graph and print the English translation (*.ENG) and the event graph for your model. Run your model and discuss the output (what kinds of questions can you answer with your model?).
  3. Using the PUT and GET functions, collect waiting time data for customers in each lane. Present and discuss histograms and time plots of the waiting time data.
  4. Using event parameters, tell us how you would enrich your model to include several service islands.
  5. Consider two service options: "full" service where service times are between 3.5 and 4.5 minutes at each pump and "self" service where service times are uniformly between 1 and 7 minutes at each pump. If both pumps are free when a customer arrives, that customer will use the "down stream" pump only half the time unless there is a "full" service attendant on duty. Create a SIGMA model of this system that can be used to study these two alternatives. Run your model under each alternative for 1 simulated hour. Write up your experiment and recommendations in less than five pages. How much can you afford to pay the extra attendant needed for the full service option (in terms of profit from lost sales)?

Parts Assembly

In an assembly operation, machines A, B, and C make parts that are joined together by machine D. It takes 3 parts from machine A, 2 parts from B, and 1 part from C for each assembly operation at machine D. All processing times have exponential (Erlang{1}) distributions. The mean processing times are input to the simulation as parameters of the first vertex. Build a simulation model of this simple assembly operation. Run your model for 10 complete assembly operations where the mean processing times for machines A, B, C, and D are 0.1, 0.2, 0.3, and 0.4. Which machine appears to be the bottleneck machine in this operation? What if the machine D's processing time were reduced to 0.05 minutes? Translate your model to C or Pascal and run it until 1000 assemblies are finished. Does your bottleneck machine change once the system has warmed up?

The Texas Ferry Service (Mini Simulation Project)

Along the Texas Coast, an east-west four-lane highway was built to promote the tourist trade to the Padre Islands. Unfortunately, this highway must cross a wide channel used by large oil tankers. Providing free automobile ferry service is considered to be an alternative to building a bridge. There is room for up to 6 loading/unloading berths on each side of the channel. Whenever a tanker comes through the channel, the ferries have to wait until it passes. Evaluate the ferry option. Do a sensitivity analysis to various levels of demand. (How much can we spend on a bridge? How bad can it be?) Hint: Consider modeling aggregate arrival processes; e.g, ferry-loads/hour for the different size ferries.

Relevant data is as follows: Each berth costs $2,500,000 to build. There are three sizes of ferries that can use the berths:

Type 0 holds 25 cars and costs $1,500,000,
Type 1 holds 50 cars and costs $3,250,000,
Type 2 holds 100 cars and costs $5,500,000. 

On Friday afternoons during a two-hour peak period, it is estimated that cars will arrive roughly according to a Poisson process with a rate of 20 per minute from the east (Corpus Christi toward the Padre Islands). The time between arrivals is thus 20*ERL{1}. On Sunday afternoon over a four-hour period, roughly the same number of cars will return to Corpus Christi. At other times (during the tourist season) traffic in both directions is expected to arrive at a rate of about 6 cars per minute.

The ferries take between 12 and 18 minutes to cross the channel (depending on the tides). Tankers come by according to a Poisson process at a rate of 1 every half-hour, and it takes 8 minutes for a tanker to pass the ferry lanes. Cars can be loaded onto a ferry at a rate of 3 per minute and unloaded at a rate of 5 per minute.

Note: They decided to use ferries instead of build the bridge. Five berths on each side were built and 6 medium-sized ferries operate during the peak periods. Do you think this was a good decision? What factors, in addition to cost, might have guided their decision?

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