Lecture 2b:Lecture 2b:

Performance Metrics Performance Metrics

Performance Metrics

Measurable characteristics of a computer system:

• Count of an event

• Duration of a time interval

• Size of a parameter

Rate:

• Operations executed per second

Performance Mertrics

Clock Speed

Clock speed/frequency (f): the rate of clock pulses (ex: 1GHz)

Cycle time (Tc): time between two clock pulses (Tc = 1/f)

Tc

Performance MertricsInstruction Execution Rate

Cycles per Instruction (CPI): is an average depends on the design of micro-architecture

(hardwired/microprogrammed, pipelined)

Number of instructions: is the number of instructions executed at runtime Depends on

• instruction set architecture (ISA)

• compiler

CPI =

n

i 1ii )ICPI(

n

i 1iI

CPIi: number of cycles required for instruction i

Ii: number of executed instructions of type i

Performance Metrics

CPU Performance

CPU time of a program (T) = instructions x cycles x time program instruction cycle

CPI (cycles per instruction)

T = instruction count x CPI x 1 f

Performance Metrics

CPU Performance

Drawbacks:

In modern computers, no program runs without some operating system running on the hardware

Comparing performance between machines with different operating systems will be unfair

Performance Metrics

Execution time

• Elapsed time / wall-clock time / response time• Latency to complete a task, including disk access,

memory access, I/O, operating system overhead, and everything (includes time consumed by other programs in a time-sharing system)

• CPU time• The time CPU is computing, not including I/O time or

waiting time• User time / user CPU time

• CPU time spent in the program• System time / system CPU time

• CPU time spent in the operating system performing tasks requested by the program

Performance Metrics

Performance Comparison

Relative performance

Performancex = 1 . Execution timeX

Performance Ratio = PerformanceX = Execution timeY

PerformanceY Execution timeX

Performance Metrics

Relative Performance

• If workload consists of more than one program, total execution time may be used.

• If there are more than one machine to be compared, one of them must be selected as a reference.

Performance Metrics

Throughput

• Total amount of work done in a given time

• Measured in tasks per time unit

• Can be used for

• Operating system performance

• Pipeline performance

• Multiprocessor performance

Performance Metrics

MIPS (Million instructions per second)

• Includes both integer and floating point performance

• Number of instructions in a program varies between different computers

• Number of instructions varies between different programs on the same computer

MIPS = Instruction count = Clock rate Execution time x 106 CPI x 106

Performance Metrics

MFLOPS

(Million floating point operations per second)

• Give performance of only floating-point operations

• Different mixes of integer and floating-point operations may have different execution times:

• Integer and floating-point units work independently

• Instruction and data caches provide instruction and data concurrently

Performance Metrics Utilization

Speciality ratio

• 1 general purpose

Utilization = Busy time . Total time

Speciality ratio = Maximum performance . Minimum performance

Performance Metrics

Asymptotic and Half performance

• r – asymptotic performance

• n1/2 – half performance

T = r (n + n1/2)

r = 1/tn1/2 = t0/t

Slope = r-1

t0

-n1/2n1/2

2t0

Performance Metrics

Speedup

• Express how much faster is system 2 than system 1

• Calculated directly from execution time

Performancex = 1 = 1 Execution timeX TX

Speedup2,1 = Performance2 = T1

Performance1 T2

Performance Metrics

Relative Change

• It expresses the performance of system 2 relative to system 1

Performancex = 1 = 1 Execution timeX TX

Relative change2,1 = Performance2 - Performance1 = T1 - T2 = Speedup2,1 - 1 Performance1 T2

Performance Metrics

Statistical Analysis

• Used to compare performance

• Workload consists of many programs

• Depends on the nature of the data as well as distribution of the test results

Performance Metrics

Indices of Central Tendency

Used to summarize multiple measurements

• Mean

• Median

• Mode

Performance Metrics

• Mean (average)

Gives equal weight to all measurements

Arithmetic mean = xi , 1 ≤ i ≤ n n

Measurement Execution time

X1 10

X2 20

X3 15

X4 18

X5 16

Mean 15.8

Performance Metrics

• Median1. Order all n measurements

2. The middle value is the median. If n is even, median is the mean of the middle 2 values

Using Median instead of Mean reduces the skewing effect of the outliers.

Measurement Execution time

X1 10

X2 20

X3 15

X4 18

X5 16

X6 200

Mean 46.5

Measurement Execution time

X1 10

X3 15

X5 16

X4 18

X2 20

X6 200

2

54 XX Median =

= 17

Performance Metrics

• ModeMode is the value that occurs most frequently

• If all values occur once, there is no mode

• If there are several samples that all have the same value, there would be several modes

Measurement Execution time

X1 10

X2 20

X3 36

X4 20

X5 20

X6 20

Mode = 20

Mean, Median, Mode

Mean Incorporates information from the entire measured values Sensitive to outliers

Median and Mode Less sensitive to outliers Do not effectively use all information

ex

Performance Metrics

Arithmetic mean (average)

• May be misleading if the data are skewed or scattered

Arithmetic mean = xi , 1 ≤ i ≤ n n

MA MB MC

Prog1 50 100 500

Prog2 400 800 800

Prog3 5550 5100 4700

Average 2000 2000 2000

Performance Metrics

Weighted average

• weight is the frequency of each program in daily processing

• Results may change with a different set of execution frequencies

Weighted average = ∑ wi . xi 1 ≤ i ≤ n

weight MA MB MC

Prog1 60% 50 100 500

Prog2 30% 400 800 800

Prog3 10% 5550 5100 4700

Average 705 810 1010

Performance Metrics

Geometric mean

• Results are stated in relation to the performance of a reference machine

Geometric mean = ( xi )1/n , 1 ≤ i ≤ n

MA Normalized to MA

MB

(reference)

Normalized to MB

MC Normalized to MC

Prog1 50 2 100 1 500 0.2

Prog2 400 2 800 1 800 1

Prog3 5550 0.92 5100 1 4700 1.085

Average 1.54 1 0.60

• Results are consistent no matter which system is chosen as reference

Performance Metrics

Harmonic mean

• Used to compare performance results that are expressed as a rate (e.g. operations per second, throughput, etc.)

• Slowest rates have the greatest influence on the result

It identifies areas where performance can be improved

Harmonic mean = n , 1 ≤ i ≤ n

∑ 1/xi

Performance Metrics

Characteristics of a good performance metric

• If the time values are averaged, then the resulting mean value must be directly proportional to the total time.

• If the rate values are averaged, then the resulting mean value must be inversely proportional to the total time.

Performance Metrics

Ex• n benchmark programs

• Ti is the execution time of program i

• F floating-point operations in each program

• Mi = F / Ti is the execution rate of program i (MFLOP/s)

Arithmetic mean

• Inappropriate for summarizing rates

TA = TA is directly proportional to the total execution time

n

1iiT

n

1

MA = =

n

1iiM

n

1

n

1i iT

1

n

F MA is inversely proportional to the total execution time

Performance Metrics

Harmonic mean

• Inappropriate for summarizing execution times

• Appropriate for summarizing rates

TH = TH is not directly proportional to the total execution time

n

1ii1/T

n

MH = =MH is inversely proportional to the total execution time

n

1ii1/M

n

n

1iiT

n F

Performance Metrics

Ex

Performance Metrics

Geometric mean

• Inappropriate for summarizing execution times

• Inappropriate for summarizing rates

TG = TG is not directly proportional to the total execution time

MH = =MH is not inversely proportional to the total execution time

n

n

1iiT

n

n

1iiM

n

n

1iiF/T

Performance MetricsGeometric mean

• Produces a consistent ordering of the systems but it is the wrong ordering

M1 M2 M3

Prog1 417 244 134

Prog2 83 70 70

Prog3 66 153 135

Prog4 39499 33527 66000

Prog5 772 368 369

Geometric mean (Normalized wrt S1) 1.0 0.86 0.84

Geometric mean (Normalized wrt S2) 1.17 1.0 0.99

Rank 3 2 1

Total time 40787 34362 66798

Arithmetic mean 8157 6872 13342

Rank 2 1 3

Performance Metrics

Histogram• Used to display the distribution of a set of measured

values (variability)

• First find the minimum and maximum values. Then divide the range into b subranges, called cells.

Histogram

Message size (kbytes)

Network A Network B

0 < xi ≤ 5 11 39

5 < xi ≤ 10 27 25

10 < xi ≤ 15 41 18

15 < xi ≤ 20 32 5

20 < xi ≤ 25 21 19

25 < xi ≤ 30 12 42

30 < xi ≤ 35 4 0

Performance Metrics

Index of Dispersion

Index of dispersion is used to compare the spread of measurements around the mean value

• Range is the simplest metric for an index of dispersion

• Range is sensitive to a few extreme values

)(min)(maxmax iiii xxR

Performance Metrics

Index of Dispersion

• Maximum of the absolute values of the difference of each measurement from the mean

• It is also sensitive to extreme values

|)(|maxmax xxii

Performance Metrics

Index of Dispersion

• Sample variance is the simplest metric for an index of dispersion

)1(

)(2

1 1

2

2

nn

xxns

n

i

n

ii

Requires 2 passes through the data to calculate first x and then s2

Requires 1 pass

1

)(2

12

n

xxs

n

i i

Performance Metrics

Index of Dispersion

• Standard deviation

• Coefficient of variance (COV): normalizes standard deviation wrt the mean

1

)(2

12

n

xxss

n

i i

xsCOV /