Ch1 - Lecture2

Chapter 1 — Computer Abstractions and Technology —
Defining Performance
Which airplane has the best performance?

Chapter 1 — Computer Abstractions and Technology —
Response Time and Throughput
Response time (PC user)
How long it takes to do a task
Throughput (Datacenter manager)
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 —
Understanding Performance
Algorithm
Determines number of operations executed
Programming language, compiler, architecture
Determine number of machine instructions executed per operation
Processor and memory system
Determine how fast instructions are executed
I/O system (including OS)
Determines how fast I/O operations are executed

Chapter 1 — Computer Abstractions and Technology —
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 —
Measuring Execution Time
Elapsed time (wall clock time, response 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 —
CPU Clocking
Operation of digital hardware governed by a constant-rate clock
Clock (cycles)
Data transfer
and 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

Chapter 1 — Computer Abstractions and Technology —
CPU Time
Performance improved by
Reducing number of clock cycles
Increasing clock rate
Hardware designer must often trade off clock rate against cycle count
A program takes 2500 clock cycles
to run on a computer with 2.5 GHz
processor. What is CPU time?

Chapter 1 — Computer Abstractions and Technology —
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 —
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 —
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 —
CPI in More Detail
If different instruction classes take different numbers of cycles
Weighted average CPI
Relative frequency

Chapter 1 — Computer Abstractions and Technology —
CPI Example
Alternative compiled code sequences using instructions in classes A, B, C
Which code sequence executes the most instructions?
Which will be faster?
What is the CPI for each sequence?

Chapter 1 — Computer Abstractions and Technology —
CPI Example
Alternative compiled code sequences using instructions in classes A, B, C
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 —
Performance Summary
Performance depends on
Algorithm: affects IC, possibly CPI (float)
Programming language: affects IC, CPI
Compiler: affects IC, CPI
Instruction set architecture: affects IC, CPI, Tc

Chapter 1 — Computer Abstractions and Technology —
Power Trends
In CMOS IC technology
×1000
×30
5V → 1V

Chapter 1 — Computer Abstractions and Technology —
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 —
Uniprocessor Performance
Constrained by power, instruction-level parallelism, memory latency

Chapter 1 — Computer Abstractions and Technology —
Multiprocessors
Multicore microprocessors
More than one processor per chip
Requires explicitly parallel programming
Compare with instruction level parallelism
Hardware executes multiple instructions at once
Hidden from the programmer
Hard to do
Programming for performance
Load balancing
Optimizing communication and synchronization

Chapter 1 — Computer Abstractions and Technology —
Manufacturing ICs
Yield: proportion of working dies per wafer

Chapter 1 — Computer Abstractions and Technology —
AMD Opteron X2 Wafer
X2: 300mm wafer, 117 chips, 90nm technology
X4: 45nm technology