1 CS 501 Spring 2002 CS 501: Software Engineering Lecture 23 Reliability III.
1 CS 501 Spring 2002 CS 501: Software Engineering Lecture 19 Performance of Computer Systems.
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Transcript of 1 CS 501 Spring 2002 CS 501: Software Engineering Lecture 19 Performance of Computer Systems.
2 CS 501 Spring 2002
Administration
Quiz 3
Collect after class.
Final presentations
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Available time slots are on the web site.
3 CS 501 Spring 2002
Performance of Computer Systems
In most computer systems
The cost of people is much greater than the cost of hardware
Yet performance is important
Future loads may be much greater than predicted
A single bottleneck can slow down an entire system
4 CS 501 Spring 2002
Moore's Law
Original version:
The density of transistors in an integrated circuit will double every year. (Gordon Moore, Intel, 1965)
Current version:
Cost/performance of silicon chips doubles every 18 months.
5 CS 501 Spring 2002
Moore's Law and System Design
Design system: 2002
Production use: 2005
Withdrawn from production: 2015
Processor speeds: 1 1.9 28
Memory sizes: 1 1.9 28
Disk capacity: 1 2.2 51
System cost: 1 0.4 0.01
6 CS 501 Spring 2002
Moore's Law: Rules of Thumb
Planning assumptions:
Every year: cost/performance of silicon chips improves 25% cost/performance of magnetic media improves 30%
10 years = 100:120 years = 10,000:1
7 CS 501 Spring 2002
Parkinson's Law
Original: Work expands to fill the time available. (C. Northcote Parkinson)
Planning assumptions:
(a) Demand will expand to use all the hardware available.
(b) Low prices will create new demands.
(c) Your software will be used on equipment that you have not envisioned.
8 CS 501 Spring 2002
False Assumptions
Unix file system will never exceed 2 Gbytes (232 bytes).
AppleTalk networks will never have more than 256 hosts (28 bits).
GPS software will not last 1024 weeks.
Nobody at Dartmouth will ever earn more than $10,000 per month.
etc., etc., .....
9 CS 501 Spring 2002
Moore's Law and the Long Term
1965 When?
What level?
2000?
Within your working life?
10 CS 501 Spring 2002
Predicting System Performance
• Mathematical models
• Simulation
• Direct measurement
• Rules of thumb
All require detailed understanding of the interaction between software and systems.
11 CS 501 Spring 2002
Understand the Interactions between Hardware and Software
:Thread :Toolkit :ComponentPeer target:HelloWorld
runrun callbackLoop
handleExpose
paint
12 CS 501 Spring 2002
Decompress
Stream audioStream video
fork
join
start state
stop state
Understand Interactions between Hardware and Software
13 CS 501 Spring 2002
Look for Bottlenecks
Possible areas of congestion
Network load
Database access
how many joins to build a record?
Locks and sequential processing
CPU performance is rarely a factor, except in mathematical algorithms. More likely bottlenecks are:
Reading data from disk
Moving data from memory to CPU.
14 CS 501 Spring 2002
Look for Bottlenecks: Utilization
utilization =
mean service timemean inter-arrival time
When the utilization of any hardware component exceeds 30%, be prepared for congestion.
15 CS 501 Spring 2002
Techniques for Eliminating Bottlenecks
Serial and Parallel Processing
Single thread v. multi-thread
e.g., Unix fork
Granularity of locks on data
e.g., record locking
Network congestion
e.g., back-off algorithms
16 CS 501 Spring 2002
Mathematical Models: Queues
arrive wait in line service depart
Single server queue
18 CS 501 Spring 2002
Mathematical Models
Queueing theory
Good estimates of congestion can be made for single-server queues with:
• arrivals that are independent, random events (Poisson process)
• service times that follow families of distributions (e.g., negative exponential, gamma)
Many of the results can be extended to multi-server queues.
20 CS 501 Spring 2002
Simulation
Model the system as set of states and events
advance simulated time determine which events occurred update state and event listrepeat
Discrete time simulation: Time is advanced in fixed steps (e.g., 1 millisecond)
Next event simulation: Time is advanced to next event
Events can be simulated by random variables (e.g., arrival of next customer, completion of disk latency)
21 CS 501 Spring 2002
Timescale
Operations per second
CPU instruction: 400,000,000
Disk latency: 60 read: 25,000,000 bytes
Network LAN: 10,000,000 bytesdial-up modem: 6,000 bytes
22 CS 501 Spring 2002
Measurements on Operational Systems
• Benchmarks: Run system on standard problem sets, sample inputs, or a simulated load on the system.
• Instrumentation: Clock specific events.
If you have any doubt about the performance of part
of a system, experiment with a simulated load.
23 CS 501 Spring 2002
Example: Performance of Disk Array
Each transaction must:
wait for specific disk platter
wait for I/O channel
signal to move heads on disk platter
wait for I/O channel
pause for disk rotation
read data
Close agreement between: results from queuing theory, simulation, and direct measurement (within 15%).
24 CS 501 Spring 2002
Discussion of Pfleeger, Chapter 7
Format:
State a question.
Ask a member of the class to answer.
(Sorry if I pronounce your name wrongly.)
Provide opportunity for others to comment.
When answering:
Stand up.Give your name or NetID. Make sure the TA hears it.
Speak clearly so that all the class can hear.
25 CS 501 Spring 2002
Question 1: Documentation Standards
(a) What is the purpose of standard documentation?
(b) Who should create the documentation standards?
(c) What documentation standards are you following in your project? (Be honest!)
26 CS 501 Spring 2002
Question 2: Good Programming
You are judging a Good Programmer Competition.
(a) Give four criteria for what constitutes good programming style.
(b) Give four additional criteria for what constitutes a good programmer.
(c) What is the most flattering thing that you can say about a programmer?
27 CS 501 Spring 2002
Question 3: Maintenance
Most production programs are maintained by people other than the programmers who originally wrote them.
(a) What factors make a program easy for somebody else to maintain?
(b) What factors make a program hard for somebody else to maintain?
28 CS 501 Spring 2002
Question 4: Documentation Standards
(a) What is the purpose of standard documentation?
(b) Who should create the documentation standards?
(c) What documentation standards are you following in your project? (Be honest!)
29 CS 501 Spring 2002
Question 5: Data Structures
(a) How should you decide what data structures to use in a program?
(b) What can you do in designing data structures to help the people who will maintain your programs?
(c) What should you do about the performance of algorithms that create and work on your data structures?