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Chapter 1 — Computer Abstractions and Technology — 1
Technology Trends• Electronics
technology continues to evolve– Increased capacity and
• Now 1.4X/yr, or 2X every 2 years• 8000X since 1980!
Bit
s
Year
• 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
Computer Technology – Dramatic Change!
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 ?• least cost ?• best cost/performance?
• 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
Chapter 1 — Computer Abstractions and Technology — 6
Defining Performance• Which airplane has the best performance?
0 100 200 300 400 500
DouglasDC-8-50
BAC/ SudConcorde
Boeing 747
Boeing 777
Passenger Capacity
0 2000 4000 6000 8000 10000
Douglas DC-8-50
BAC/ SudConcorde
Boeing 747
Boeing 777
Cruising Range (miles)
0 500 1000 1500
DouglasDC-8-50
BAC/ SudConcorde
Boeing 747
Boeing 777
Cruising Speed (mph)
0 100000 200000 300000 400000
Douglas DC-8-50
BAC/ SudConcorde
Boeing 747
Boeing 777
Passengers x mph
Chapter 1 — Computer Abstractions and Technology — 7
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 — 8
Relative Performance• Define Performance = 1/Execution Time• “X is n time faster than Y”
n XY
YX
time Executiontime Execution
ePerformancePerformanc
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 — 9
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 — 10
CPU Clocking• Operation of digital hardware governed by a
constant-rate clock
Clock (cycles)
Data transferand computation
Update state
Clock period
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
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