Computer ArchitectureComputer Architecture

Part I-C: Performance

What does faster mean?What does faster mean?

Response time The time spent to complete an event Also referred to as execution time or

latency Throughput

Amount of work done in a given time Also referred to as bandwidth In general, faster response time means

an improvement in throughput

Execution Time and PerformanceExecution Time and Performance

Quantitatively, execution time is inversely proportional to performance. improve performance = increase

performance improve execution time = decrease

execution time X is n times faster than Y means

X

Y

Y

X

t

t

P

Pn

Make the Common Case FastMake the Common Case Fast

A rule of thumb in computer design is to make the event that occurs more frequently, faster In making a design trade-off, favor the

frequent case over the infrequent case In general, this move should increase

overall performance

Amdahl’s LawAmdahl’s Law

The performance improvement to be gained from using some faster mode of operations is limited by the fraction of time that faster mode can be used.

Speedup due to enhancement E

ExTime w/o E Performance w/ ESpeedup(E) = ------------- = -------------------

ExTime w/ E Performance w/o E

(for an entire task)

Factors Affecting the SpeedupFactors Affecting the Speedup

The fraction of computation time in the original machine that can be converted to take advantage of the enhancement

The improvement gained by the enhanced execution mode, i.e. how much faster the task would run if the enhanced mode were used for the entire program.

1enhancedSpeedup

1enhancedFraction

Applying Amdahl’s LawApplying Amdahl’s Law

ExTimenew = ExTimeold x (1 - Fractionenhanced) + Fractionenhanced

Speedupoverall =ExTimeold

ExTimenew

Speedupenhanced

=1

(1 - Fractionenhanced) + Fractionenhanced

Speedupenhanced

Using Amdahl’s Law: An ExampleUsing Amdahl’s Law: An Example

Suppose that we are considering an enhancement that runs 10 times faster than the original machine but is only usable 40% of the time. What is the overall speedup gained by incorporating the enhancement?

Answer:

Fractionenhanced = 0.4

Speedupenhanced = 10

Speedupoverall = 1 / [0.6+(0.4/10)] = 1/0.64 = 1.56

Measuring CPU Processing Measuring CPU Processing Speed: The ClockSpeed: The Clock

A circuit which generates a signal that defines regular time intervals or cycles during which basic CPU steps are performed

Provides control as to when each step of the instruction cycle takes place

Clock CyclesClock Cycles

One clock pulse is the burst of current when the clock output is equal to 1

A clock cycle is the interval between the beginning of a pulse to the beginning of the next

Measured in Hertz, a unit of measurement of electrical vibrations.

I Hz = 1 cycle/second Basic unit of CPU speed = 1

million Hz or 1 MHz

pulse cycle

Locality of ReferenceLocality of Reference

Programs tend to reuse data and instructions they have used recently.

A program may spend 90% of its execution time in only 10% of the code. Based on a program’s recent past, one

can predict with reasonable accuracy what instructions and data will use in the near future.

Two Types of LocalityTwo Types of Locality

Temporal Locality recently accessed items are likely to be

accessed in the near future

Spatial Locality items whose addresses (or location) are

near one another tend to be referenced close together in time

Metrics of PerformanceMetrics of Performance

Compiler

Programming Language

Application

DatapathControl

Transistors Wires Pins

ISA

Function Units

(millions) of Instructions per second: MIPS(millions) of (FP) operations per second: MFLOP/s

Cycles per second (clock rate)

Megabytes per second

Answers per monthOperations per second

MIPS BenchmarkMIPS Benchmark

Millions of Instructions Per Second Easy to understand and

straightforward Dependent on instruction set Varies between programs on the same

computer MIPS can vary inversely with

performance!

MFLOPS BenchmarkMFLOPS Benchmark

Millions of Floating-point Operations Per Second (MegaFLOPS)

Intended to measure floating-point operations but some programs don’t use any

Floating-point operations are not consistent across machines

MFLOPS ratings for the same machine may differ depending on instruction mix

Programs as EvaluatorsPrograms as Evaluators

Four types (in decreasing order of accuracy): Real programs Kernels Toy Benchmarks Synthetic Benchmarks

Synthetic BenchmarksSynthetic Benchmarks

Programs which try to match the average number and frequency of operations of a typical workload, e.g. dhrystone, whetstone, etc.

Not real programs, may not reflect program behavior for factors not measured

Compilers and hardware optimizations can artificially inflate results

Toy BenchmarksToy Benchmarks

Small, simple programs Produce a result the user already

knows Example: quicksort, Sieve of

Erastosthenes, etc.

Kernel BenchmarksKernel Benchmarks

Small, key pieces from real programs put together to evaluate machine performance Examples: Linpack, Livermore Loops, etc.

No user would run kernel programs because they exist solely for performance evaluation

Best used to isolate performance of individual features of machines to explain the reasons for differences in real programs

Real ProgramsReal Programs

Common programs like compilers (e.g. C), word processors (e.g. TeX, MS Word), computer-aided design tools (e.g. Spice), etc.

Real programs have the input, output, and options that a user can select.

When Benchmarks DisagreeWhen Benchmarks Disagree

0.000.200.400.600.801.001.201.401.601.80

Imp

rove

men

tWhat isMMX’sreal speed?

source: adapted from Byte April 1998

Popular BenchmarksPopular Benchmarks Bapco SYSmark - application, tests system BYTEmark - synthetic, tests processor Intel Media - synthetic, tests processor

(multimedia, uses MMX instructions) CaffeineMark - synthetic, tests JVM SPEC CPU95 - synthetic, tests processor

(two suites: integer and floating-point) SPEC Glperf - synthetic, tests 3-D graphics SPEC Viewperf - application, 3-D graphics Norton Multimedia - synthetic, tests system

(multimedia, uses MMX instructions)

Popular BenchmarksPopular Benchmarks

TPC-C (Transaction Processing Council) - database application, tests transaction-processing performance

TPC-D - database application, tests decision support and data-warehousing performance

ZDBOp (Ziff-Davis Benchmark Operation): BrowserComp - application, tests browsers CPUmark32 - synthetic, tests processor NetBench - application, tests network performance ServerBench - application, tests server performance WebBench - application, tests web server WinBench - application, tests component subsystems Winstone - application, tests system

Programs as EvaluatorsPrograms as Evaluators

Companies may design features that would make their machines run faster on the benchmarks than on real programs

A standard set of programs is hard to obtain because each program run differently for each machine and companies would want to use programs that run fast on their machines