1. Morgan Kaufmann Publishers
April 25, 2017
Technology Trends
Electronics technology continues to evolve
Increased capacity and performance
Reduced cost
DRAM capacity
Year
Technology
Relative performance/cost
1951
Vacuum tube 1
1965
Transistor 35
1975
Integrated circuit (IC)
900
1995
Very large scale IC (VLSI)
2,400,000
2013
Ultra large scale IC
250,000,000,000
Chapter 1 — Computer Abstractions and Technology — 1
Chapter 1 — Computer Abstractions and Technology

2. Microprocessor Complexity
Gordon Moore Intel Cofounder
# of transistors on an IC
2X Transistors / Chip
Every 1.5 years
Called
“Moore’s Law”
Year

3. Memory Capacity (Single-Chip DRAM)
year size (Mbit)
1986 1
1989 4
(1Gbit)
(2Gbit)
Bits
Now 1.4X/yr, or 2X every 2 years
8000X since 1980!
Year

4. Computer Technology – Dramatic Change!
Memory
DRAM capacity: 2x / 2 years (since ‘96); 64x size improvement in last decade
Processor
Speed 2x / 1.5 years (since ‘85); [slowing!] 100X performance in last decade
Disk
Capacity: 2x / 1 year (since ‘97) 250X size in last decade

5. Performance Metrics Purchasing perspective
given a collection of machines, which has the
best performance ?
least cost ?
best cost/performance?
Design perspective
faced with design options, which has the
best performance improvement ?
Both require
basis for comparison
metric for evaluation
Our goal is to understand what factors in the architecture contribute to overall system performance and the relative importance (and cost) of these factors
Or smallest/lightest
Longest battery life
Most reliable/durable (in space)

6. Morgan Kaufmann Publishers
April 25, 2017
Defining Performance
Which airplane has the best performance?
Chapter 1 — Computer Abstractions and Technology — 6
Chapter 1 — Computer Abstractions and Technology

7. Response Time and Throughput
Morgan Kaufmann Publishers
April 25, 2017
Response Time and Throughput
Response time
How long it takes to do a task
Throughput
Total work done per unit time
e.g., tasks/transactions/… per hour
How are response time and throughput affected by
Replacing the processor with a faster version?
Adding more processors?
We’ll focus on response time for now…
Chapter 1 — Computer Abstractions and Technology — 7
Chapter 1 — Computer Abstractions and Technology

8. Morgan Kaufmann Publishers
April 25, 2017
Relative Performance
Define Performance = 1/Execution Time
“X is n time faster than Y”
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
Chapter 1 — Computer Abstractions and Technology — 8
Chapter 1 — Computer Abstractions and Technology

9. Measuring Execution Time
Morgan Kaufmann Publishers
April 25, 2017
Measuring Execution Time
Elapsed time
Total response time, including all aspects
Processing, I/O, OS overhead, idle time
Determines system performance
CPU time
Time spent processing a given job
Discounts I/O time, other jobs’ shares
Comprises user CPU time and system CPU time
Different programs are affected differently by CPU and system performance
Chapter 1 — Computer Abstractions and Technology — 9
Chapter 1 — Computer Abstractions and Technology

10. Morgan Kaufmann Publishers
April 25, 2017
CPU Clocking
Operation of digital hardware governed by a constant-rate clock
Clock period
Clock (cycles)
Data transfer and computation
Update state
Clock period: duration of a clock cycle
e.g., 250ps = 0.25ns = 250×10–12s
Clock frequency (rate): cycles per second
e.g., 4.0GHz = 4000MHz = 4.0×109Hz
Chapter 1 — Computer Abstractions and Technology — 10
Chapter 1 — Computer Abstractions and Technology

11. Review: Machine Clock Rate
Clock rate (clock cycles per second in MHz or GHz) is inverse of clock cycle time (clock period)
CC = 1 / CR
one clock period
10 nsec clock cycle => 100 MHz clock rate
5 nsec clock cycle => 200 MHz clock rate
2 nsec clock cycle => 500 MHz clock rate
1 nsec (10-9) clock cycle => 1 GHz (109) clock rate
500 psec clock cycle => 2 GHz clock rate
250 psec clock cycle => 4 GHz clock rate
200 psec clock cycle => 5 GHz clock rate
A clock cycle is the basic unit of time to execute one operation/pipeline stage/etc.

12. Morgan Kaufmann Publishers
April 25, 2017
CPU Time
Performance improved by
Reducing number of clock cycles
Increasing clock rate
Hardware designer must often trade off clock rate against cycle count
Chapter 1 — Computer Abstractions and Technology — 12
Chapter 1 — Computer Abstractions and Technology

13. Morgan Kaufmann Publishers
April 25, 2017
CPU Time Example
Computer A: 2GHz clock, 10s CPU time
Designing Computer B
Aim for 6s CPU time
Can do faster clock, but causes 1.2 × clock cycles
How fast must Computer B clock be?
Chapter 1 — Computer Abstractions and Technology — 13
Chapter 1 — Computer Abstractions and Technology

14. Instruction Count and CPI
Morgan Kaufmann Publishers
April 25, 2017
Instruction Count and CPI
Instruction Count for a program
Determined by program, ISA and compiler
Average cycles per instruction
Determined by CPU hardware
If different instructions have different CPI
Average CPI affected by instruction mix
Chapter 1 — Computer Abstractions and Technology — 14
Chapter 1 — Computer Abstractions and Technology

15. Morgan Kaufmann Publishers
April 25, 2017
CPI Example
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?
A is faster…
…by this much
Chapter 1 — Computer Abstractions and Technology — 15
Chapter 1 — Computer Abstractions and Technology

16. Morgan Kaufmann Publishers
April 25, 2017
CPI in More Detail
If different instruction classes take different numbers of cycles
Weighted average CPI
Relative frequency
Chapter 1 — Computer Abstractions and Technology — 16
Chapter 1 — Computer Abstractions and Technology

17. Morgan Kaufmann Publishers
April 25, 2017
CPI Example
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
IC in sequence 2 4
Sequence 1: IC = 5
Clock Cycles = 2×1 + 1×2 + 2×3 = 10
Avg. CPI = 10/5 = 2.0
Sequence 2: IC = 6
Clock Cycles = 4×1 + 1×2 + 1×3 = 9
Avg. CPI = 9/6 = 1.5
Chapter 1 — Computer Abstractions and Technology — 17
Chapter 1 — Computer Abstractions and Technology

18. A Simple Example  = Op Freq CPIi Freq x CPIi ALU 50% 1 . Load 20% 5
Store
10% 3
Branch 2
 =
How much faster would the machine be if a better data cache reduced the average load time to 2 cycles?
How does this compare with using branch prediction to shave a cycle off the branch time?
What if two ALU instructions could be executed at once?

19. A Simple Example  = Op Freq CPIi Freq x CPIi ALU 50% 1 Load 20% 5
Store
10% 3
Branch 2
 = .5
1.0 .3 .4 .5 .4 .3 .5
1.0 .3 .2
.25
1.0 .3 .4
2.2
1.6
2.0
1.95
How much faster would the machine be if a better data cache reduced the average load time to 2 cycles?
How does this compare with using branch prediction to shave a cycle off the branch time?
What if two ALU instructions could be executed at once?
For lecture
CPU time new = 1.6 x IC x CC so 2.2/1.6 means 37.5% faster
CPU time new = 2.0 x IC x CC so 2.2/2.0 means 10% faster
CPU time new = 1.95 x IC x CC so 2.2/1.95 means 12.8% faster

20. Determinates of CPU Performance
CPU time = Instruction_count x CPI x clock_cycle
Instruction_ count
CPI
clock_cycle
Algorithm
Programming language
Compiler
ISA
Core organization
Technology
For class handout

21. Determinates of CPU Performance
CPU time = Instruction_count x CPI x clock_cycle
Instruction_ count
CPI
clock_cycle
Algorithm
Programming language
Compiler
ISA
Core organization
Technology X X X X X X
For lecture X X X X X X

22. Morgan Kaufmann Publishers
April 25, 2017
Performance Summary
The BIG Picture
Performance depends on
Algorithm: affects IC, possibly CPI
Programming language: affects IC, CPI
Compiler: affects IC, CPI
Instruction set architecture: affects IC, CPI, Tc
Chapter 1 — Computer Abstractions and Technology — 22
Chapter 1 — Computer Abstractions and Technology

23. Morgan Kaufmann Publishers
April 25, 2017
Power Trends
In CMOS IC technology
×40
5V → 1V
×1000
Chapter 1 — Computer Abstractions and Technology — 23
Chapter 1 — Computer Abstractions and Technology

24. Morgan Kaufmann Publishers
April 25, 2017
Reducing Power
Suppose a new CPU has
85% of capacitive load of old CPU
15% voltage and 15% frequency reduction
The power wall
We can’t reduce voltage further
We can’t remove more heat
How else can we improve performance?
Chapter 1 — Computer Abstractions and Technology — 24
Chapter 1 — Computer Abstractions and Technology