# Estimating Optimal Checkpoint Intervals Using GPSS Simulation

Queues, or queueing systems, are very often used in computer systems. In this project we illustrate how queueing simulation may be used to find the optimal interval for checkpointing problems and compare results with theoretical computations for simple systems that may be treated analytically.We consider a relatively simple model for an internet banking facility. From time to time, the application server breaks down. The information at the time of the breakdown has to be passed onto the back up server before service may be resumed. To make the change over as efficient as possible, information of the state of user’s accounts is saved at regular intervals. This is known as checkpointing. Firstly, we use GPSS (a queueing simulation tool) to find, by simulation…

Contents

1 INTRODUCTION
2 OBJECTIVE
3 METHOD
3.1. Using GPSS
3.1.1. Maximizing availability of the server
3.1.2. Minimizing the number of customers that are blocked by the system
3.1.3. Minimizing the expected time of customers transactions
3.2. Model for the optimal checkpoint
3.3. Optimum checkpoint interval by Gelenbe
4 RESULTS
4.1. Maximizing availability
4.2. Minimizing the number of blocked customers
Results from our theoretical model – parameter set 1
Results of Gelenbe’s model – parameter set 1
4.3. Minimizing the expected transaction time
Minimizing expected transaction time – parameter set 1
Minimizing expected transaction time – parameter set 2
Maximizing availability – parameter set 2
Results of Gelenbe’s model –parameter set 2
Results from our model –parameter set 2
5 CONCLUSIONS AND DISCUSSION
REFERENCES
A SIMULATION CODE AND SOME RESULTS
A.1 Simulation code for the availability problem
A.2 An example of Micro-GPSS output
A.3 The problem of minimizing number of customers
A.3.1 The case with checkpointing as D/D/1 queueing system
A.3.2 The case with checkpointing as M/M/1 queueing system
A.4 Code for minimizing average time case
A.4.1 The ‘optimal’ choice of checkpointing interval
A.4.2 A worse case scenario

Author: Savatovic, Anita,Cakic, Mejra