CS457 - System Performance Evaluation - Winter 2008

1. Metrics PDF

Lecture 8 - Baby Modelling

In Lecture 5, I asked you to read this system description. Today we will complete analysing it as an example of how to make a system abstraction.

We do this as methodically and mechanically as possible.

1. Study goal and System description

1a. Simplifications

2. Services Provided

3. Metrics

Load Balancing

  1. Reponse to client requests
  2. Delay in redirection
  3. Utilization


  1. Time data is stale


  1. System availability for login
  2. Time to complete redirections

4. List Parameters

System Parameters

These describe system propertis that might be expected to affect the performance of the system in dimensions described by the goals of the study

  1. network delay
  2. frequency of transmission of shopping cart content
  3. frequency of server failure
  4. downtime on server failure
  5. load balancing algorithm
  6. service capacity: how many services a server can provide per second

Workload Parameters

  1. arrival rate of requests of each type

Hybrid Parameters

  1. service (response) time, which depends on service capacity and arrival rate of requests

5. Choose Factors

Falls under the heading of experiment design which, for now, remains outside the scope of system abstraction.

Choosing a factor to examine requires you to choose levels at which you want to know how the system performs.


An extreme example of abstraction.

Two methods for modelling a system

  1. Stochastic
  2. Operational

1a. Study Goals

Get analytic results under as general a set of assumptions as possible. Results will be in the following areas:

  1. Response Time
  2. Throughput
  3. Utilization

1b. System Description

  1. A server that processes requests
  2. A queue that holds requests currently being processed
  3. A client that makes requests.

Note. `Request', these notes, and `job', textbook, are exactly the same thing for the purposes of this course. `Request' better characterizes what the client does and is both a noun and a verb, which is handy when writing and speaking; `job' better describes the actual thing that is requested and has a narrower range of meanings in English, which can reduce misunderstanding.

1c. Simplifications


2. Services Provided

Handling requests.

3. Metrics

  1. Statistics of response times, such as
  2. Statistics of throughput
  3. Statistics of utilization

4. Parameters

System Parameters

  1. Service statistics

Workload Parameters

  1. Arrival statistics
  2. Think time, which is really part of arrival statistics
  3. Number of users, which is really part of arrival statistics.

5. Factors

Same as parameters

Little's Law (or Formula)

  1. Picture of arrival and departure generating system load diagram
  2. Area under system load is total response time: nR
  3. Average load, Q, times time interval, L is total response time: QL = nR
  4. n/L = X = throughput = \lambda = interarrival time.


Little's formula holds only if the system is stable, which is defined by U < 0

6. Workload Models

  1. Infinite population of clients
  2. Finite population of clients - think-time model

Applying Little's Formula in the Think-time Model

1a. Goal

1b. System Description

1c. Simplifications

2. Services Provided

3. Metrics

  1. X - Throughput
  2. R - average response time

4/5. Parameters/Factors

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