1. Chapter 2: Performance
CS 447
Jason Bakos
Fall 2001
CS 447

2. Introduction Performance measures are important
Factors that contribute to performance
Instruction utilization in software
How well the hardware implements the instructions
How well the memory and I/O system perform
What are the best metrics to use for determining performance?
Fall 2001
CS 447

3. Defining Performance How do we define performance?
response (or execution) time
the time between the start and the finish of a task
throughput
total amount of work done in a given time
Almost just an inverse relationship, but throughput can be increased with parallelism
Fall 2001
CS 447

4. Execution Time For now, we are only concerned with execution Time
For two machines, X and Y…
Fall 2001
CS 447

5. Execution Time
If X is n times faster than Y…
Fall 2001
CS 447

6. Execution Time Execution time is measured in seconds
Program execution time – “wall-clock time” to execute a program
CPU execution time – amount of time the CPU spends executing a program (on a time-shared system)
Extra time passes while other programs are being executed
Extra time passes while a program waits for I/O
User CPU time – CPU time spent executing a program
System CPU time – amount of CPU time the operating system spends on behalf of a program
Ex. system calls, such as I/O, synchronization, etc.
Fall 2001
CS 447

7. More Terminology
System performance – elapsed time on an unloaded system
CPU performance – CPU time
Fall 2001
CS 447

8. Clock Cycles
CPUs use a clock to synchronize events that occur in hardware
Events occur on the rising or falling edge of the clock signal
The clock is a global signal
Has a discrete time intervals called clock cycles
Has cycle time or clock rate (inverses)
Ex. 500MHz or 2 ns
This is the PERIOD of the clock (compare to duty cycle)
Fall 2001
CS 447

9. Relating the Metrics
This is why a CPU clock rate doesn’t give a direct indication of CPU performance…
Fall 2001
CS 447

10. Example
Our favorite program runs in 10 seconds on computer A, which has a 400 MHz clock. We are trying to help a computer designer build a machine, B, that will run this program in 6 seconds. The designer has determined that a substantial increase in the clock rate is possible, but this increase will affect the rest of the CPU design, causing machine B to require 1.2 times as many clock cycles as machine A for this program. What clock rate should we tell the designer to target?
Fall 2001
CS 447

11. Cycles per Instruction
CPI = cycles per instruction
Average number of clock cycles to execute an instruction
For each architecture, different instructions take different amounts of clock cycles to execute
CPI provides one way of comparing two different implementations of the same instruction set architecture
This is because in this case, the instruction count for any given program will be the same
Fall 2001
CS 447

12. Basic Peformance Equation (Revisited)
Three key factors that affect performance
Instruction count
CPI
Clock cycle time
Fall 2001
CS 447

13. Considerations We get CPU execution time from running a program
The clock cycle time is published
Instruction count and CPI are more difficult to get
Why is it difficult to get the instruction count?
Some processors actually keep track of this
CPI depends on I/O and memory system
We only need one of these
Fall 2001
CS 447

14. More Considerations
Use this equation when there are different classes of instructions that require different numbers of clock cycles
Fall 2001
CS 447

15. Benchmarks
Benchmarks are a set of programs that are used to evaluate a machine’s performance
Ex. SPEC95 int/fp benchmarks
Processor companies may target the hot code segments in the benchmarks for optimization in order to make their product look better
Fall 2001
CS 447

16. Amdahl’s Law for Improving Performance
Suppose a program runs in 100 seconds on a machine, with multiply operations responsible for 80 seconds of this time
How much do I have to improve the speed of multiplication if I want my program to run five times faster?
Answer? Cannot be done!
Amdahl’s Law dictates that we should make the common case fast…
The opportunity for improvement is affected by how much time the event occurs
Fall 2001
CS 447

17. MIPS
There are 3 problems with using MIPS as a measure for comparing machines
MIPS specifies the instruction execution rate but does not take into account the capabilities of the instructions
Affects instruction count
MIPS varies between programs on the same computer!
Depends on average CPI of the instruction mix
MIPS can vary inversely with performance!
This occurs when CPI is higher on a machine with a higher MIPS
Fall 2001
CS 447