# COMPUTER ARCHITECTURE &amp; OPERATIONS I Instructor: Yaohang Li

```COMPUTER ARCHITECTURE &amp;
OPERATIONS I
Instructor: Yaohang Li
Review
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Last Class
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Program and Computer
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Components of a Computer
This Class
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Definition of Computer Performance
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Measure of Computer Performance
Next Class
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Quiz 1
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Power Wall
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Assignment 1
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Which airplane has the best performance?
Boeing 777
Boeing 777
Boeing 747
Boeing 747
BAC/Sud
Concorde
BAC/Sud
Concorde
Douglas
DC-8-50
Douglas DC8-50
0
100
200
300
400
0
500
Boeing 777
Boeing 777
Boeing 747
Boeing 747
BAC/Sud
Concorde
BAC/Sud
Concorde
Douglas
DC-8-50
Douglas DC8-50
500
1000
Cruising Speed (mph)
4000
6000
8000 10000
Cruising Range (miles)
Passenger Capacity
0
2000
1500
0
100000 200000 300000 400000
Passengers x mph
&sect;1.4 Performance
An Analogy
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That depends on …
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If performance means
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“the least time of transferring 1 passenger from one
place to another”
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“the least time of transferring 450 passenger from
one place to another”
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Concorde
Boeing 747
Performance can be defined in different
ways
Response Time and Throughput
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Response time (AKA Execution Time)
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Total time required for a computer to complete a task
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Measured by time
Throughput (AKA Bandwidth)
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Number of tasks done work done per unit time
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Response Time and Throughput
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Assuming each task in a computer is a serial
task. How are response time and throughput
affected by
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Replacing with a faster processor?
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Reduce response time
Increase throughput
Increase throughput
Same response time
We’ll focus on response time for now…
Performance and Execution Time
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Performance
Performanc e X  1 Execution time X
Relative Performance
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“X is n time faster than Y”
Performanc e X Performanc e Y
 Execution time Y Execution time X  n
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Example: time taken to run a program
10s on A, 15s on B
Execution TimeB / Execution TimeA
= 15s / 10s = 1.5
 So A is 1.5 times faster than B
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Measuring Execution Time
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Elapsed (Wallclock) time
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Total response time, including all aspects
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Processing, I/O, OS overhead, idle time
Determines system performance
CPU time
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Time spent processing a given job
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Comprises user CPU time and system CPU time
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Discounts I/O time, other jobs’ shares
User CPU time: CPU time spent in a program itself
System CPU time: CPU time spent in the OS performing task
on behalf of the program
Different programs are affected differently by CPU
and system performance
CPU Clocking
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Operation of digital hardware governed by a
constant-rate clock
Clock period
Clock (cycles)
Data transfer
and computation
Update state
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Clock period: duration of a clock cycle
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e.g., 250ps = 0.25ns = 250&times;10–12s
Clock frequency (rate): cycles per second
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e.g., 4.0GHz = 4000MHz = 4.0&times;109Hz
CPU Time
CPU Time  CPU Clock Cycles  Clock Cycle Time
CPU Clock Cycles

Clock Rate
Performance Improvement
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Performance improved by either
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Increasing clock rate
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Reducing number of clock cycles
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=&gt; Shorter clock period
=&gt; More but shorter instructions
=&gt; More clock cycles
=&gt; Longer clock period
=&gt; Less but Longer Instructions
=&gt; Reducing clock rate
Hardware designer must often trade off
clock rate against cycle count
CPU Time Example
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A Program on Computer A: 2GHz clock, 10s CPU time
Designing Computer B
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Aim for 6s CPU time
Can do faster clock, but causes 1.2 &times; clock cycles
How fast must Computer B clock be?
Clock Cycles B 1.2  Clock Cycles A
Clock Rate B 

CPU Time B
6s
Clock Cycles A  CPU Time A  Clock Rate A
 10s  2GHz  20  109
1.2  20  109 24  109
Clock Rate B 

 4GHz
6s
6s
Instruction Set Architecture
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Instruction Set Architecture (ISA)
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An abstract interface between the hardware
and the lowest-level software that
encompasses all the information necessary to
write a machine language program that will
run correctly
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Repertoire of instructions
Registers
Memory access
I/O
Clock Cycles per Instruction (CPI)
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Clock Cycles per Instruction (CPI)
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Average number of clock cycles per
instruction for a program
Instruction Count and CPI
Clock Cycles  Instructio n Count  Cycles per Instructio n
CPU Time  Instructio n Count  CPI  Clock Cycle Time
Instructio n Count  CPI

Clock Rate
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Instruction Count (IC) for a program
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Determined by program, ISA and compiler
Average cycles per instruction
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Determined by CPU hardware
If different instructions have different CPI
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Average CPI affected by instruction mix
CPI Example
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Computer A: Cycle Time = 250ps, CPI = 2.0
Computer B: Cycle Time = 500ps, CPI = 1.2
Same ISA
Which is faster, and by how much?
CPU Time
CPU Time
A
 Instructio n Count  CPI  Cycle Time
A
A
A is faster…
 I  2.0  250ps  I  500ps
B
 Instructio n Count  CPI  Cycle Time
B
B
 I  1.2  500ps  I  600ps
B  I  600ps  1.2
CPU Time
I  500ps
A
CPU Time
…by this much
CPI in More Detail
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If different instruction classes take different
numbers of cycles
n
Clock Cycles   (CPIi  Instructio n Count i )
i1
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Weighted average CPI
n
Clock Cycles
Instructio n Count i 

CPI 
   CPIi 

Instructio n Count i1 
Instructio n Count 
Relative frequency
CPI Example
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Alternative compiled code sequences using
instructions in classes A, B, C
Class
A
B
C
CPI for class
1
2
3
IC in sequence 1
2
1
2
IC in sequence 2
4
1
1
Sequence 1: IC = 5
Clock Cycles
= 2&times;1 + 1&times;2 + 2&times;3
= 10
Avg. CPI = 10/5 = 2.0
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Sequence 2: IC = 6
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Clock Cycles
= 4&times;1 + 1&times;2 + 1&times;3
=9
Avg. CPI = 9/6 = 1.5
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Summary
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Response Time and Throughput
Performance Measure
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CPI (Cycles per Instruction)
IC (Instructions Count)
Performance Definition
What I want you to do
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Review Chapter 1
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