Homework 1: Discrete-Event Simulation

Hints, Partial solution, Suggested format of Log File, Directions for mailing homework to Lab Instructor,

Write in C a program to do the discrete-event simulation of a computer system. We assume that there is a CPU and two disks as represented in the following queueing network:

                    ----+       +--+        SWITCH
 INPUT ---->x------->||||------>|  |--------->x-----> OUTPUT
            ^       ----+       +--+         /|
            |       QUEUE1     SERVER1      / |
            |                   (CPU)      |  |
            |                              |  |
            |   +--+     +----             v  |
            x---|  |<----||||<-------------+  |
            ^   +--+     +----                | 
            |  SERVER2   QUEUE2               |
            |  (Disk1)                        |
            |                                 |
            |   +--+     +----                v
            x---|  |<----||||<----------------+
                +--+     +----
               SERVER3   QUEUE3
               (Disk2)
                

• The system runs from INITTIME to FINTIME.
• Jobs arrive to the INPUT randomly, say, with interarrival time uniformly distributed in the time interval AMIN..AMAX.
• At SWITCH the jobs go to the output with probability POUT, to QUEUE2 with probability P1, and to QUEUE3 with probability P2 (clearly POUT+P1+P2=1).
• When jobs get to a server, if the server is free the jobs move directly to the server; otherwise they wait in the queue of that server until their turn to move to the server arrives.
• Jobs are serviced at a server for a random amount of time, uniformly distributed, say, in the interval S1MIN..S1MAX, S2MIN..S2MAX, and S3MIN..S3MAX respectively.
• At FINTIME the simulation terminates, that is, we do not consider further the jobs that are being serviced or that are waiting in the queues.

The user is asked at run time to specify:

• a seed SEED for the random number generator
• the values of INITTIME, FINTIME, AMIN, AMAX, POUT, P1, S1MIN, S1MAX, S2MIN, S2MAX, S3MIN, S3MAX.
• the name of a log file where will be logged each significant event [the significant events are the arrival of a new job into the system, the completion of a job at a server, the termination of the simulation]. [Here is the suggested format for the log file.]

Print out the values of SEED, INITTIME, ..

Determine and print out:

• The average and the maximum size of each Queue.
• The Utilization of each Server [= time-the-server-is-busy / total-time; where total-time = time of termination of the simulation( = FINTIME) - start time of simulation(= INITTIME)].
• The Average and Maximum Response Time at each Server [or Turn-Around Time = Time a job completes service at the Server - Time that job arrived at that server and queue].
• The Average and Maximum Response Time for the whole system

Run the program a number of times with different values for the parameters and random seed. Examine in correspondence the utilizations and queue sizes. If for a given choice of parameters by changing the random seed we obtain utilization and size values that are stable [i.e. do not change much, say, change at most 10%], then we have a good simulation.

Mail the homework to the TA [instructions] Be sure to receive within 24 hours a message back from the TA confirming that he has received the homework. If you do not receive that reply contact both the instructor and the TA.