Computer Architecture 2014 – Introduction1
Computer Architecture Computer Architecture (“MAMAS”, 234267)(“MAMAS”, 234267)
Spring 2014Spring 2014
Lecturer: Yoav EtsionReception: Mon 15:00, Fishbach 306-8
TAs: Nadav Amit, Gil Einziger, Franck Sala
Presentation based on slides by David Patterson, Avi Mendelson, Lihu Rappoport, Adi Yoaz and Dan Tsafrir
Computer Architecture 2014 – Introduction33
COMPUTER SYSTEM COMPONENTSCOMPUTER SYSTEM COMPONENTSArchaicArchaic
CPU - Memory BUS
MAINMEMORY
CACHEBUSADAPTER
I/O BUSCPU
I/O CONTROLLERS
Disk
Network +WLAN
printer scannerkeyboard mouse...
046267 Computer Architecture 1 U Weiser
Computer Architecture 2014 – Introduction4
4
COMPUTER SYSTEM COMPONENTSCOMPUTER SYSTEM COMPONENTSYesterdayYesterday
I/O CONTROLLERSprinter scannerkeyboard mouse...
MAINMEMORYNorth Bridge
South Bridge
CPUcache
Disk
Network +WLAN
046267 Computer Architecture 1 U Weiser
Computer Architecture 2014 – Introduction5
COMPUTER SYSTEM COMPONENTSCOMPUTER SYSTEM COMPONENTSnownow
Printer, scannerKeyboard, mouse...
MAINMEMORY
South Bridge
CPUMC+cache+G
Disk/SSD
Network +WLAN
5046267 Computer Architecture 1 U Weiser
Computer Architecture 2014 – Introduction6
Classical Motherboard Classical Motherboard DiagramDiagram
CPU
PCI
North Bridge DDR2 or DDR3Channel 1
mouse
LAN
LanAdap
External Graphics
CardMem BUS
CPU BUS
Cache
SoundCard
speakers
South Bridge
PCI express 2.0
IO Controller
HardDisk
Pa
rall
el
Po
rt
Se
ria
l P
ort Floppy
Drivekeybrd
DDR2 or DDR3Channel 2
USBcontroller
SATAcontroller
PCI express ×1
Memory controller
On-board Graphics
DVDDrive
IOMMU
More to the “north” = closer to the CPU = faster
Computer Architecture 2014 – Introduction7
Course FocusCourse Focus Start from CPU (=processor)
Instruction set, performance Pipeline, hazards Branch prediction Out-of-order execution
Move on to Memory Hierarchy Caching Main memory Virtual Memory
Move on to PC Architecture System & chipset, DRAM, I/O, Disk, peripherals
End with some Advanced Topics
Computer Architecture 2014 – Introduction9
Architecture vs. Architecture vs. MicroarchitectureMicroarchitecture
Architecture:= The processor features as seen by its user= Interface Instruction set, number of registers, addressing modes,
…
Microarchitecture:= Manner by which the processor is implements the Architecture= Implementation details Caches size and structure, number of execution units, …
Note: different processors with different u-archs can support the same arch Example: ARM V8, ARM V9
We will address both
Computer Architecture 2014 – Introduction10
Why Should We Care?Why Should We Care?
Abstractions enhance productivity, so:If we know the arch (=interface),Why should we care about the u-arch (=internals)?
Same goes for archJust details for a programmer of a high-level language
Computer Architecture 2014 – Introduction11
Recent Processor TrendsRecent Processor Trends
Source: http://www.scidacreview.org/0904/html/multicore.html
Computer Architecture 2014 – Introduction12
Well-Known Moore’s LawWell-Known Moore’s Law
Graph taken from: http://www.intel.com/technology/mooreslaw/index.htm
Computer Architecture 2014 – Introduction14
The Story in a NutshellThe Story in a NutshellTransistors(1000s)
clock speed(MHz)
power (W)
Instructions/cycle(ILP)
Computer Architecture 2014 – Introduction15
Took the Industry by SurpriseTook the Industry by Surprise
Computer Architecture 2014 – Introduction16
Dire Implications: PerformanceDire Implications: Performance
Computer Architecture 2014 – Introduction19
Dire Implications: ProgrammersDire Implications: Programmers
Computer Architecture 2014 – Introduction20
Supercomputing: “Top 500 list”Supercomputing: “Top 500 list”
Computer Architecture 2014 – Introduction21
Dire Implications: Dire Implications: SupercomputingSupercomputing
Computer Architecture 2014 – Introduction23
Metrics: IC, CPI, IPCMetrics: IC, CPI, IPC CPUs work according to a clock signal
Clock cycle: measured in nanoseconds (10-9 of a second) Clock frequency = 1/|clock cycle|: in GHz (109 cycles/sec)
Instruction Count (IC) Total number of instructions executed in the program
Cycles Per Instruction (CPI) Average #cycles per Instruction (in a given program)
IPC (= 1/CPI) : Instructions per cycles.Can be > 1; see the “story in a nutshell slide”
CPI =#cycles required to execute the program
IC
Computer Architecture 2014 – Introduction24
Minimizing Execution TimeMinimizing Execution Time CPU Time - time required to execute a program
CPU Time = IC CPI clock cycle
Our goal:
minimize CPU Time (any of above components) Minimize clock cycle: increase GHz (processor design)
Minimize CPI: u-arch (e.g.: more execution
units)
Minimize IC: arch (e.g. SSE instruction)
SSE = streaming SIMD extension (Intel)
Computer Architecture 2014 – Introduction25
Alternative Way to Calculate Alternative Way to Calculate CPICPI
ICi = #times instruction of type-i is executed in program
IC = #instruction executed in program =
Fi = relative frequency of type-i instruction = ICi/IC
CPIi = #cycles to execute type-i instruction e.g.: CPIadd = 1, CPImul = 3
#cycles required to execute the program:
CPI: CPIcyc
IC
CPI IC
ICCPI
IC
ICCPI F
i ii
n
ii
i
n
i ii
n
# 1
1 1
1
#n
i ii
cyc CPI IC
IC ICii
n
1
Computer Architecture 2014 – Introduction26
Performance Evaluation: Performance Evaluation: How?How?
Performance depends on Application Input
Mathematical analysis
Computer Architecture 2014 – Introduction27
BenchmarksBenchmarks
Use benchmarks & measure how long it takes Use real applications (=> no absolute answers)
Preferably standardized benchmarks (+input), e.g., SPEC INT: integer apps
• Compression, C complier, Perl, text-processing, … SPEC FP: floating point apps (mostly scientific) TPC benchmarks: measure transaction throughput (DB) SPEC JBB: models wholesale company (Java server, DB)
Sometimes you see FLOPS (“pick” or “sustained”) Supercomputers (top500 list), against LINPACK
Computer Architecture 2014 – Introduction28
-2%
0%
2%
4%
6%
Evaluating PerformanceEvaluating Performance Use a performance simulator to evaluate
the performance of a new feature / algorithm Models the uarch to a great detail Run 100’s of representative applications
Produce the performance s-curve Sort the applications according to the IPC increase Baseline (0%) is the processor without the new
feature
-4%
-3%
-2%
-1%
0%
1%
2%
3%
Negativeoutliers
Positiveoutliers
Bad S-curve
Small negativeoutliers
Positiveoutliers
Good S-curve
Computer Architecture 2014 – Introduction29
Amdahl’s LawAmdahl’s Law
Suppose we accelerate the computation such thatP = portion of computation we make fasterS = speedup experienced by the portion we improved
For exampleIf an improvement can speedup 40% of the computation
=> P = 0.4If the improvement makes the portion run twice as fast
=> S = 2
Then overall speedup = 1
(1 ) PP S
Computer Architecture 2014 – Introduction30
Amdahl’s Law - ExampleAmdahl’s Law - Example
FP operations improved to run 2x fasterS = 2, but…P = only affects 10% of the programSpeedup:
ConclusionBetter to make common case fast…
1 1 11.053
0.1 0.95(1 ) (1 0.1) 2PP S
Computer Architecture 2014 – Introduction31
Amdahl’s Law – ParallelismAmdahl’s Law – Parallelism
When parallelizing a program P = proportion of program that can be made parallel 1 - P = inherently serial N = number of processing elements (say, cores)Speedup:
Serial component imposes a hard limit
1
(1 ) PP N
1 1lim
(1 )(1 )N P PP N
Computer Architecture 2014 – Introduction32
The ISA is what the user & compiler see
The HW implements the ISA
instruction set
software
hardware
Instruction Set DesignInstruction Set Design
Computer Architecture 2014 – Introduction33
Considerations in ISA DesignConsiderations in ISA Design Instruction size
Long instructions take more time to fetch from memory Longer instructions require a larger memory
• Important for small (embedded) devices, e.g., cell phones
Number of instructions (IC) Reduce IC => reduce runtime (at a given CPI &
frequency)
Virtues of instructions simplicity Simpler HW allows for: higher frequency & lower power Optimization can be applied better to simpler code Cheaper HW
Computer Architecture 2014 – Introduction34
Basing Design Decisions on Basing Design Decisions on WorkloadWorkload
Immediate argument’s size in bits (histogram)
1% of data values > 16-bits Having 16 bits is likely good enough
0%
10%
20%
30%
0 1 2 3 4 5 6 7 8 9
10
11 12
13
14
15
Immediate data bits
Int. Avg.
FP Avg.
Computer Architecture 2014 – Introduction35
CISC ProcessorsCISC Processors CISC - Complex Instruction Set Computer
Example: x86 The idea: a high level machine language
• Once people programmed in assembly, CISC supposedly easier
Characteristic Many instruction types, with a many addressing modes Some of the instructions are complex
• Execute complex tasks• Require many cycles
ALU operations directly on memory (e.g., arr[j] = arr[i]+n)• Registers not used (and, accordingly, only a few registers exist)
Variable length instructions• common instructions get short codes save code length
Computer Architecture 2014 – Introduction36
Rank instruction % of total executed
1 load 22%
2 conditional branch 20%
3 compare 16%
4 store 12%
5 add 8%
6 and 6%
7 sub 5%
8 move register-register 4%
9 call 1%
10 return 1%
Total 96%
Simple instructions dominate instruction frequency
But it Turns Out…But it Turns Out…
Computer Architecture 2014 – Introduction37
CISC DrawbacksCISC Drawbacks Complex instructions and complex addressing modes
complicates the processor slows down the simple, common instructions contradicts Make The Common Case Fast
Compilers don’t use complex instructions / indexing
methods
Variable length instructions are real pain in the neck Difficult to decode few instructions in parallel
• As long as instruction is not decoded, its length is unknown It is unknown where the instruction ends It is unknown where the next instruction starts
An instruction may be longer than a cache line• Or even longer longer than a page (in theory)
Computer Architecture 2014 – Introduction38
RISC ProcessorsRISC Processors RISC - Reduced Instruction Set Computer
The idea: simple instructions enable fast hardware Characteristic
A small instruction set, with only a few instructions formats
Simple instructions• execute simple tasks• Most of them require a single cycle (with pipeline)
A few indexing methods Load/Store machine: ALU operations on registers only
• Memory is accessed using Load and Store instructions only
• Many orthogonal registers • Three address machine: Add dst, src1, src2
Fixed length instructions
Examples: MIPSTM, SparcTM, AlphaTM, PowerTM
Computer Architecture 2014 – Introduction39
RISC Processors (Cont.)RISC Processors (Cont.) Simple arch => simple u-arch
Room for larger on die caches Smaller => faster Easier to design & validate (=> cheaper to
manufacture) Shorten time-to-market More general-purpose registers (=> less memory refs)
Compiler can be smarter Better pipeline usage Better register allocation
Existing RISC processor are not “pure” RISC Various complex operations added along the way
Computer Architecture 2014 – Introduction40
Compilers and ISACompilers and ISA Ease of compilation
Orthogonality: • no special registers• few special cases • all operand modes available with any data type or
instruction type Regularity:
• no overloading for the meanings of instruction fields streamlined
• resource needs easily determined
Register assignment is critical too Easier if lots of registers
Computer Architecture 2014 – Introduction41
Still, CISC Is DominantStill, CISC Is Dominant x86 (CISC) dominates the processor market
Not necessarily because it is CISC…
Legacy A vast amount of existing software Intel, AMD, Microsoft benefit But put lot of money to compensate for
disadvantage
CISC internally arch emulates RISC Starting at Pentium II and K6, x86 processors
translate CISC instructions into RISC-like operations internally
Inside core is a RISC machine
Computer Architecture 2014 – Introduction42
Software Specific ExtensionsSoftware Specific Extensions Extend arch to accelerate exec of specific
apps
Example: SSETM – Streaming SIMD Extensions 128-bit packed (vector) / scalar single precision FP
(4×32) Introduced on Pentium® III on ’99 8 new 128 bit registers (XMM0 – XMM7) Accelerates graphics, video, scientific calculations,
…
Packed: Scalar:
x0x1x2x3
y0y1y2y3
x0+y0x1+y1x2+y2x3+y3
+
128-bits
x0x1x2x3
y0y1y2y3
x0+y0y1y2y3
+
128-bits
Computer Architecture 2014 – Introduction44
CompatibilityCompatibility Backward compatibility (HW responsibility)
When buying new hardware, it can run existing software:• i5 can run SW written for Core2 Duo, Pentium4,
PentiumM, Pentium III, Pentium II, Pentium, 486, 386, 268
BTW:
Forward compatibility (SW responsibility) For example: MS Word 2003 can open MS Word 2010 doc Commonly supports one or two generations behind
Architecture-independent SW Run SW on top of VM that does JIT (just in time compiler):
JVM for Java and CLR for .NET Interpreted languages: Perl, Python