AOK-Lecture03.pdf

Arsitektur dan Organisasi Komputer Computers Performance

Lecture 03 (27 Feb 2014)

Henry Novianus Palit [email protected]

Designing for Performance (1)

Computer trends: The cost of computer systems continue to drop

The performance and capacity of computer systems continue to rise; e.g., todays laptops have the computing power of an IBM mainframe from 10-15 years ago

The speed of a computer in executing a program is affected by

design of its instruction set

design of its hardware

design of its software (including the OS and the compiler)

technology in which the hardware is implemented

Arsitektur & Organisasi Komputer 2


The speed of switching between 0 and 1 states in logic circuits is largely determined by the size of the transistors that implement the circuits; i.e., smaller transistors switch faster

Reducing transistor sizes has two advantages: Instructions can be executed faster

More transistors can be placed on a chip, leading to more logic functionality and more memory storage capacity

Gordon Moore (Intel co-founder): The number of transistors incorporated in a chip will approximately

double every 24 months

The raw speed of a microprocessor will not achieve its potential unless it is fed by a constant stream of work (i.e., computer instructions)



Techniques to exploit the raw speed of a processor: Pipelining (a kind of instruction-level parallelism) the processor works on multiple instructions by moving data or instructions into a conceptual pipe with all stages of the pipe processing simultaneously; e.g., while an instruction is being executed, the next instruction is being fetched and decoded Branch prediction the processor looks ahead in the instruction code fetched from the memory, predicts which branches or groups of instructions are likely to be processed next, and prefetches those instructions (could be multiple branches ahead); thus, it increases the amount of work available for the processor to execute Data flow analysis the processor analyzes which instructions are dependent on each others results to create an optimized schedule of instructions; thus, it prevents unnecessary delay Speculative execution using branch prediction and data flow analysis, some processors speculatively execute instructions ahead of their actual appearance in the program execution, holding the results in temporary locations; thus, it keeps the execution engines busy


Five-Stage Pipeline


Parallelism (1)

Instruction-level parallelism Pipelining also see the previous slides

Pipelining allows a trade-off between latency (how long it takes to execute an instruction) and processor bandwidth (how many instructions/sec the CPU can complete)

Superscalar architectures A dual pipeline or a single pipeline with multiple functional units

The two instructions must neither conflict over resource usage (e.g., registers) nor depend on the result of the other either guaranteed by the compiler or detected & eliminated during execution by extra hardware

Most of the functional units in stage 4 take appreciably longer than one clock cycle to execute, certainly the ones that access memory or do floating-point arithmetic


Superscalar Architectures


Parallelism (2)

Processor-level parallelism Multicore processors

Fabricating multiple processing units on a single chip, e.g., dual-core, quad-core, hex-core, etc.

Data parallel processors SIMD (Single Instruction-stream Multiple Data-stream) procrs consists of a large number of identical processors that perform the same sequence of instructions on different sets of data (e.g., GPUs / Graphics Processing Units)

Vector processors very efficient at executing a sequence of operations on pairs of data elements, but all of the operations are performed in a single, heavily pipelined functional unit (e.g., SSE / Streaming SIMD Extension from Intel)


SIMD Processor

Processing steps per cycle:

The scheduler selects two threads to execute on the processor

The next instruction from each thread then executes on up to 16 SIMD cores

If each thread is able to use all of 16 SIMD cores, a fully loaded GPU with 32 SMs (stream multiprocrs) can perform 512 ops / cycle a similar-sized general purpose quad-core CPU would struggle to achieve 1/32 as much processing


Parallelism (3)

Processor-level parallelism (contd) Multiprocessors (SMP / symmetric multiprocessing)

Computer systems that contain many processors, each possibly containing multiple cores

Used for either executing a number of different application tasks concurrently or executing subtasks of a single large task in parallel

All processors usually have access to all of the memory shared-memory multiprocessor

Multicomputers (distributed or cluster computing) Using an interconnected group of computers to achieve high total computational power

Computers normally have access only to their own memory units

Sharing data is done by exchanging messages over a communication network message-passing multicomputer


Multiprocessor


A single-bus multiprocessor A multiprocessor with local memories (NUMA / Non-Uniform Memory Access)

Tianhe-2

Worlds Fastest Computer (by November 2013)

source: http://www.top500.org

Processors : Intel Xeon E5-2692 (12C) + Intel Xeon Phi 31S1P Total cores : 3,120,000 Memory : 1 PB Interconnect : TH Express-2 Linpack performance : 33,862.7 TFlop/s (peak = 54,902.4 TFlop/s) Power : 17,8 MW OS : Kylin Linux MPI : MPICH2


Performance Assessment

Performance is a key parameter in evaluating a computer system, along with cost, size, security, reliability, and power consumption

Raw speed is far less important than how a processor performs when executing a given application

Some measures of computers performance: Clock speed

Instruction execution rate

Benchmarks

Amdahls Law

Littles Law


Clock Speed

Clock speed or clock rate is measured in cycles/second or Hertz (Hz)

Clock signals typically are generated by a quartz crystal, which generate a constant signal wave while power is applied; the wave is in turn converted into a digital voltage pulse stream

Since the execution of an instruction involves a number of steps such as fetching the instruction from memory, decoding the instruction, loading & storing data, and performing arithmetic & logical operations most instructions require multiple clock cycles to complete

A straight comparison of clock speeds on different processors does not tell the whole story about performance (e.g., when pipelining is used)


Instruction Execution Rate (1)

For a given processor, the number of clock cycles required varies for different types of instructions Average Cycles Per Instruction (CPI) for a given program is where CPIi = number of cycles required for instruction type i Ii = number of executed instructions of type i Ic = instruction count (number of instructions) n = number of instruction types The processor time (T) needed to execute a given program is where = the constant cycle time = 1/f (f = the constant frequency)


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To differentiate memory and processor cycle times, the preceding equation can be rewritten as where p = number of processor cycles needed to decode & execute the instruction m = number of memory references needed (on avg) k = ratio between memory & processor cycle times System attributes that influence the performance factors (Ic, p, m, k, )


kmpIT c


A common measure of performance for a processor is the rate at which instructions are executed, expressed as millions of instructions per second (MIPS)

Another common performance measure deals only with floating-point instructions (which are common in many scientific and game applications) is expressed as millions of floating-point operations per second (MFLOPS)


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Benchmarks (1)

MIPS and MFLOPS often are inadequate to evaluate the processors performance (e.g., CISC vs. RISC machines may have different MIPS rates although both take about the same amount of time)

In early 1990s, measuring the performance of systems is shifted to using a set of benchmark programs

Desirable characteristics of a benchmark program: Written in a high-level language, making it portable across machines

Representative of a particular kind of programming style such as systems programming, numerical programming, or commercial programming

Can be measured easily

Has wide distribution


Benchmarks (2)

SPEC (System Performance Evaluation Corporation) benchmarks defined and maintained by an industry consortium (e.g., SPEC CPU2006, SPECjvm98, SPECjbb2000, SPECweb99, SPECmail2001)

Averaging results run a number of different benchmark programs on each machine and then average the results where Ri = high-level language instruction execution rate for benchmark i


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Benchmarks (3)

SPEC benchmarks concern with speed metric and rate metric Speed metric measures the ability of a machine to complete a task

Results are reported as the ratio of the reference run time to the system (under test) run time The overall performance measure for the system under test is calculated by averaging the ratios values by a geometric mean

Rate metric measures the throughput or rate of a machine carrying out a number of tasks

Multiple copies (i.e., as many as the number of processors) of the benchmarks are run simultaneously and a ratio is reported

Here, Tsuti is the elapsed time from the start of the execution of the program on all N processors until the completion of all copies A geometric mean is used to determine the overall performance measure


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Amdahls Law (1)

Proposed by Gene Amdahl and deals with the potential speedup of a program using multiple processors compared to a single processor

Consider a program running on a single processor such that a fraction (1f) of the execution time involves code that is inherently serial and a fraction f involves code that is inherently parallelizable with no scheduling overhead Let T be the total execution time of the program using a single processor The speedup using a parallel processor with N processors that fully exploits the parallel portion of the program is


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Amdahls Law (2)

Two drawn conclusions: When f is small, the use of parallel processors has little effect As N approaches infinity, speedup is bound by 1/(1f), so there are diminishing returns for using more processors

The conclusions are too pessimistic, as a server can execute multiple threads or multiple tasks in parallel and exploit data parallelism Speedup in general can be expressed as If a targeted enhancement is applied to fraction f, and the fractions speedup after enhancement is SUf, the overall speedup is


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Littles Law

Based on a queuing theory, Littles Law can be applied to any system that is statistically in steady state and in which there is no leakage

General setup: Suppose there is a steady state system where items arrive at an average rate of items per unit time

The item stay in the system an average of W units of time

There is an average of L units in the system at any one time

Littles Law relates these three variables as L = W

Under steady state conditions, the average number of items in a queuing system equals the average rate at which items arrive multiplied by the average time that an item spends in the system


Example: MIPS rate

Consider a program that is executed on a 400-MHz processor. The instruction mix and the CPI for each instruction type are given on the table below Calculate the MIPS rate!

Solution: Average CPI = (1 x 0.6)+(2 x 0.18)+(4 x 0.12)+(8 x 0.1) = 2.24 MIPS rate = (400 x 106) / (2.24 x 106) 178.6


Example: Speed Metric

The table below shows the SPEC integer speed ratios for twelve benchmark programs on the Sun Blade 6250 Calculate the speed metric! Solution: Speed metric = (17.5 x 14.0 x 13.7 x 17.6 x 14.7 x 18.6 x 17.0 x 31.3 x 23.7 x 9.23 x 10.9 x 14.7)1/12 = 16.09


Example: Speedup (Amdahls Law)

Problem: Suppose that a task makes extensive use of floating-point operations, with 40% of the time is consumed by floating-point operations

With a new hardware design, the floating-point module is speeded up by a factor of K

Calculate the maximum overall speedup!

Solution: Thus, independent of K, the maximum speedup is 1.67


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Homework

Four benchmark programs are executed on three computers with the following results:

The table shows the execution time in seconds, with 100,000,000 instructions executed in each program.

Calculate the MIPS values for each computer for each program!

Calculate the arithmetic and harmonic means assuming equal weights for the four programs, and rank the computers based on arithmetic mean and harmonic mean!


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