computing systems
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Computing Systems. Assessing and Understanding Performance. Performance. Measure, Report, and Summarize performance Make intelligent choices See through the marketing hype - PowerPoint PPT PresentationTRANSCRIPT
Performance
Measure, Report, and Summarize performance Make intelligent choices See through the marketing hype Key to understanding underlying organizational motivation
Why is some hardware better than others for different programs?
What factors of system performance are hardware related?(e.g., Do we need a new machine, or a new operating system?)
How does the machine's instruction set affect performance?
We are looking for metrics for measuring performance from the viewpoint of both a computer user and a designer
Defining performance
Airplane Pass. capacity
Range
(miles)
Speed
(m.p.h.)
Pass. throughput
(pass. x m.p.h.)
Boeing 777 375 4630 610 228750
Boeing 747 470 4150 610 286700
Concorde 132 4000 1350 178200
Douglas DC 8-50
146 8720 544 79424
• How much faster is the Concorde compared to the 747 ?• Is the Concorde faster compared to the 747 ? • How much bigger is the 747 than the Douglas DC-8?• Which of these airplanes has the best performance ?
Understanding performance
The performance of a program depends on:
- the algorithm,- the language, - the compiler, - the architecture- the actual hardware
Computer performance: Time, Time, Time !!!
Response Time = Execution Time = Latency
- The time between the start and completion of a task
Throughput- Total amount of work completed in a given time
If we upgrade a machine with a new faster
processor what do we increase?
If we add a new processor to a system that uses
multiple processors what do we increase?
Execution Time Execution Time (response time or elapsed time)
total time to complete a program, it counts everything (disk accesses, memory accesses, input/output activities)
a useful number, but often not good for comparison purposes
CPU (execution) time doesn't count time spent waiting for I/O or time spent running
other programs can be broken up into system time (CPU time spent in the
OS), and user time (CPU time spent in the program)
Our focus: user CPU time time spent executing the lines of code that are "in" our
program
Book’s definition of performance
- For some program running on machine X,
- "X is n times faster than Y"
xX TimeExecution
1ePerformanc
nTimeExecution
TimeExecution
ePerformanc
ePerformanc
X
Y
Y
X
Problem (Relative Performance) :– machine A runs a program in 10 seconds– machine B runs the same program in 25 seconds– How much faster is machine A compared to B ?
Measuring Time
Instead of reporting execution time in seconds, we often use cycles (= clock cycles = ticks = clock ticks = clocks = clock periods)
Clock “ticks” indicate when to start activities:
cycle time = time between ticks = seconds per cycle clock rate (frequency) = cycles per second (1 Hz. = 1 cycle/sec)
Example: a 4 GHz. clock has a 250 ps. cycle time
time
cycle
seconds
program
cycles
program
seconds
How to improve performance
So, to improve performance (everything else being equal) you can either (increase or decrease?)
________ the # of required cycles for a program, or________ the clock cycle time or, said another way, ________ the clock rate.
e_timeclock_cycl_a_programcycles_forCPU_clock_
programa_for__time_execution_CPU
cycle
seconds
program
cycles
program
secondsTime
Example - improving performance
Our favorite program runs in 10 seconds on computer A, which has a 4 GHz. clock. We are trying to help a computer designer build a new machine B, that will run this program in 6 seconds. The designer can use new (or perhaps more expensive) technology to substantially increase the clock rate, but has informed us that this increase will affect the rest of the CPU design, causing machine B to require 1.2 times as many clock cycles as machine A for the same program. What clock rate should we tell the designer to target?"
Cycles required for a program Can we assume that # cycles = # instructions ?
This assumption is incorrect. Different instructions take different amounts of time. Why? remember that these are machine instructions, not lines of C code Multiplication takes more time than addition Floating point operations take longer than integer ones Accessing memory takes more time than accessing registers
Important point: changing the cycle time often changes the number of cycles required for various instructions (more later)
Clock cycles per instruction
It is clear that the execution time of a program must depends on the number of machine instructions generated by the compiler:
the average number of clock cycles each instruction takes to execute is often abbreviated CPI
CPI provides a way of comparing two different implementations of the same ISA (since the IC required for a program will be the same)
uction_per_instrock_cyclesaverage_cl
rogramns_for_a_pinstructiocyclesCPU_clock_
The “performance equation”
A given program will require:
some number of instructions (machine instructions)
some number of cycles per each instruction
some number of seconds per cycle
This useful formula separate the 3 key factors that affect performance.
timecycle_clock_CPIcount_ninstructioimeCPU_t
Performance - the BIG picture The only complete and reliable measure of performance is
determined by execution time
Do any of the other variables equal performance? NO! # of cycles to execute program? # of instructions in program? # of cycles per second? average # of cycles per instruction (CPI)? average # of instructions per second (MIPS)?
Common pitfall: thinking one of the variables is indicative of performance when it really isn’t.
cycle_Clock
Seconds
nInstructio
cyclesClock_
Program
nsInstructio
Program
SecondsTime
Example - CPI Suppose we have two implementations of the same
instruction set architecture (ISA). For some program: Machine A has a clock cycle time of 250 ps and a CPI of 2.0 Machine B has a clock cycle time of 500 ps and a CPI of 1.2
What machine is faster for this program, and by how much?
If two machines have the same ISA which of the following quantities will always be identical?
- clock rate, - CPI, - execution time, - # of instructions, - # of cycles, - MIPS
Example - Number of InstructionsA compiler designer is trying to decide between two code sequencesfor a particular machine. Based on the hardware implementation, there are three different classes of instructions: Class A, Class B, and Class C, and they require one, two, and three cycles (respectively).
The first code sequence has 5 instructions: 2 of A, 1 of B, and 2 of C. The second sequence has 6 instructions: 4 of A, 1 of B, and 1 of C.
Which sequence will be faster? How much?What is the average CPI for each sequence?
Hint:
N = number of instruction classes,
Ci = count of the # of instructions of class i executed
N
1iii )C(CPIcyclesCPU_clock_
MIPS
Million instructions per second
Problems using MIPS for comparing computers MIPS specifies the instruction execution rate but does not
take into account that instructions may have different capabilities
MIPS varies between programs on the same computer; thus a computer cannot have a single MIPS rating for all programs
MIPS can vary inversely with performance !!!
610timeExecution
countnInstructioMIPS
Example – MIPS Two different compilers are being tested for a 4 GHz. machine with
three different classes of instructions: Class A, Class B, and Class C, which require one, two, and three cycles (respectively). Both compilers are used to produce code for a large piece of software.
The first compiler's code uses:
- 5 million Class A instructions, - 1 million Class B instructions,- 1 million Class C instructions.
The second compiler's code uses:
- 10 million Class A instructions, - 1 million Class B instructions, - 1 million Class C instructions.
Which sequence will be faster according to MIPS? Which sequence will be faster according to execution time?
Benchmarks
Performance best determined by running a real application Use programs typical of expected workload or typical of expected class of applications
(e.g., compilers/editors, scientific applications, graphics, etc.)
Small benchmarks nice for architects and designers easy to standardize can be abused
SPEC (System Performance Evaluation Cooperative) companies have agreed on a set of real program and inputs valuable indicator of performance (and compiler technology) can still be abused (see Intel’s benchmark )
Benchmark “games” An embarrassed Intel Corp. acknowledged Friday that a bug in a
software program known as a compiler had led the company to overstate the speed of its microprocessor chips on an industry benchmark by 10 percent. However, industry analysts said the coding error…was a sad commentary on a common industry practice of “cheating” on standardized performance tests…The error was pointed out to Intel two days ago by a competitor, Motorola …came in a test known as SPECint92…Intel acknowledged that it had “optimized” its compiler to improve its test scores. The company had also said that it did not like the practice but felt to compelled to make the optimizations because its competitors were doing the same thing…At the heart of Intel’s problem is the practice of “tuning” compiler programs to recognize certain computing problems in the test and then substituting special handwritten pieces of code…
Saturday, January 6, 1996 New York Times
Benchmarks
Different classes and applications of computers requires different types of benchmark suites SPEC CPU2000 SPECweb99 EEMBC
The execution time measurements are normalized by dividing the execution time on a Sun Ultra 5_10 with a 300 MHz processor by the execution time on a measured computer (this measure is called SPEC ratio)
The guiding principle in reporting performance measurements should be reproducibility (an important aspect of reproducibility is the choice of input)
Spec’89 – Compiler enhancement and performance
0
100
200
300
400
500
600
700
800
tomcatvfppppmatrix300eqntottlinasa7doducspiceespressogcc
BenchmarkCompiler
Enhanced compiler
SP
EC
pe
rfo
rman
ce r
atio
SPEC2000
Does doubling the clock rate double the performance?
Clock rate in MHz
500 1000 1500 30002000 2500 35000
200
400
600
800
1000
1200
1400
Pentium III CINT2000
Pentium 4 CINT2000
Pentium III CFP2000
Pentium 4 CFP2000
Ratio PIII P4CINT2000/clock rate in MHz 0.47 0.36CFP2000/clock rate in MHz 0.34 0.39
SPEC2000Can a machine with a slower clock rate have better performance?
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
SPECINT2000 SPECFP2000 SPECINT2000 SPECFP2000 SPECINT2000 SPECFP2000
Always on/maximum clock Laptop mode/adaptiveclock
Minimum power/minimumclock
Benchmark and power mode
Pentium M @ 1.6/0.6 GHz
Pentium 4-M @ 2.4/1.2 GHz
Pentium III-M @ 1.2/0.8 GHz
Performance, power, and energy efficiency
Power is increasingly becoming a key limitation in processor performance (especially for embedded processors)
In CMOS technology the primary source of power dissipation is:
For power limited application, the most important metric is energy efficiency, which is computed by taking performance and dividing by average power consumption when running the benchmark
_frequencyswitchingVoltage_loadcapacitivepowerswitching_ 2
Summarizing performance Although summarizing measurements result in less
information, marketers and even users often prefer to have a single number to compare performance
Arithmetic mean of the execution times (underlying assumption that the programs in the workload are each run an equal number of times)
Weighted arithmetic mean (wi frequency of the program in the workload)
n
1iiTime
n
1AM
n
1iii TimewWAM
Amdahl’s law
The performance enhancement possible with a given improvement is limited by the amount that the improved feature is used
Principle: make the common case fast
Execution time after improvement =
Execution time affected + Execution time unaffectedAmount of improvement
Example – Amdahl’s law
Suppose a program runs in 100 seconds on a
machine, with multiply responsible for 80 seconds of
this time.
How much do we have to improve the speed of
multiplication if we want the program to run 4 times
faster?
How about making it 5 times faster?
SummaryPerformance is specific to a particular program
Execution time is the only valid and unimpeachable measure of performance
For a given ISA, increases in CPU performance come from threesources
Increases in clock rate Improvement in processor organization that lower the CPI Compiler enhancements that lower the instruction count and/or the
average CPI (e.g. by using simpler instructions)
Pitfalls Using a subset of the “performance equation” as performance
metric Expecting the improvement of one aspect of a machine’s
performance to increase total performance by an amount proportional to the size of the partial improvement
Designing only for performance without considering cost, functionality and other requirements is unrealistic