Computer Architecture (“MAMAS”, 234267) Spring 2011 Lecturer: Dan Tsafrir
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Transcript Computer Architecture (“MAMAS”, 234267) Spring 2011 Lecturer: Dan Tsafrir
Computer Architecture
(“MAMAS”, 234267)
Spring 2011
Lecturer: Dan Tsafrir
Reception: Mon 18:30, Taub 611
28/2/2011
Presentation based on slides by David Patterson, Avi Mendelson, Lihu Rappoport, and Adi Yoaz
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Computer Architecture 2011 – Introduction (lec1)
General Info
Grade
20% Exercise (mandatory) תקף
80% Final exam
Textbook
“Computer Architecture:
A Quantitative Approach” (4th Edition)
by: Patterson & Hennessy
Other course information
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Course web site:
http://webcourse.cs.technion.ac.il/234267/Spring2011
Lectures will be upload to the web a day before the class
Computer Architecture 2011 – Introduction (lec1)
Computer System Structure
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Computer Architecture 2011 – Introduction (lec1)
Classical Motherboard Diagram
Cache
More to the “north”
= closer to the CPU
= faster
CPU
CPU BUS
North Bridge
External
Graphics
Card
DDR2 or DDR3
Channel 1
PCI express 2.0
IOMMU
On-board Memory
Graphics controller
Serial Port
Parallel Port
IO Controller
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DDR2 or DDR3
Channel 2
PCI express ×1
South Bridge
Floppy
Drive
Mem BUS
keybrd
USB
controller
mouse
SATA
controller
DVD
Drive
Hard
Disk
PCI
Sound
Card
speakers
Lan
Adap
LAN
Computer Architecture 2011 – Introduction (lec1)
Intel Core2 Duo
Northbridge = MCH =
mem controller hub
Notice bandwidths
Southbridge = ICH =
I/O controller hub
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Computer Architecture 2011 – Introduction (lec1)
Intel Core i Series (i3 i5 i7)
For high-end i-Series chips,
Northbridge functionality
moved onto processor
(=> made faster)
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Computer Architecture 2011 – Introduction (lec1)
Intel - Sandy Bridge
The trend
continues
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Computer Architecture 2011 – Introduction (lec1)
Course Focus
Start from CPU (=processor)
Move on to Memory Hierarchy
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Caching
Main memory
Virtual Memory
Move on to PC Architecture
Instruction set, performance
Pipeline, hazards
Branch prediction
Out-of-order execution
Motherboard & chipset, DRAM, I/O, Disk, peripherals
End with some Advanced Topics
Computer Architecture 2011 – Introduction (lec1)
The Processor
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Computer Architecture 2011 – Introduction (lec1)
Architecture vs. Microarchitecture
Architecture:
= The processor features as seen by its user
= Interface
Microarchitecture:
= Manner by which the processor is implemented
= Implementation details
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Caches size and structure, number of execution units, …
Note: different processors with different u-archs
can support the same arch
Instruction set, number of registers, addressing modes,…
Example: Intel Pentium-IV vs. Intel Core2 Duo
We will address both
Computer Architecture 2011 – Introduction (lec1)
Why Should We Care?
Abstractions enhance productivity, so:
Same goes for arch
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If we know the arch (=interface),
Why should we care about the u-arch (=internals)?
Just details for a programmer of a high-level language
Abstractions only work so long as what’s below
works
Computer Architecture 2011 – Introduction (lec1)
Recent Processor Trends
Source: http://www.scidacreview.org/0904/html/multicore.html
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Computer Architecture 2011 – Introduction (lec1)
Well-Known Moore’s Law
Graph taken from: http://www.intel.com/technology/mooreslaw/index.htm
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Computer Architecture 2011 – Introduction (lec1)
The Story in a Nutshell
Transistors
(1000s)
clock speed
(MHz)
power (W)
Instructions/cycle
(ILP)
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Computer Architecture 2011 – Introduction (lec1)
Took the Industry by Surprise
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Computer Architecture 2011 – Introduction (lec1)
Dire Implications: Performance
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Computer Architecture 2011 – Introduction (lec1)
Dire Implications: Sales
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Computer Architecture 2011 – Introduction (lec1)
Dire Implications: Sales
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Computer Architecture 2011 – Introduction (lec1)
Dire Implications: Programmers
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Computer Architecture 2011 – Introduction (lec1)
Supercomputing: “Top 500 list”
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Computer Architecture 2011 – Introduction (lec1)
Dire Implications: Supercomputing
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Computer Architecture 2011 – Introduction (lec1)
Processor Performance
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Computer Architecture 2011 – Introduction (lec1)
Metrics: IC, CPI, IPC
CPUs work according to a clock signal
Instruction Count (IC)
Clock cycle: measured in nanoseconds (10-9 of a second)
Clock frequency = 1/|clock cycle|: in GHz (109 cycles/sec)
Total number of instructions executed in the program
Cycles Per Instruction (CPI)
Average #cycles per Instruction (in a given program)
CPI =
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#cycles required to execute the program
IC
IPC (= 1/CPI) : Instructions per cycles.
Can be > 1; see the “story in a nutshell slide”
Computer Architecture 2011 – Introduction (lec1)
Minimizing 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 + u-arch (e.g.: SSETM)
SSE = streaming SIMD extension (Intel)
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Computer Architecture 2011 – Introduction (lec1)
Alternative Way to Calculate CPI
ICi = #times instruction of type-i is executed in program
n
IC = #instruction executed in program =
IC ICi
i 1
Fi = relative frequency of type-i instruction = ICi/IC
CPIi = #cycles to execute type-i instruction
e.g.: CPIadd = 1, CPImul = 3
n
#cycles required to execute the program:
# cyc CPI i ICi
i 1
CPI:
n
# cyc
CPI
IC
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CPI IC
i 1
i
IC
i
n
ICi n
CPIi
CPIi Fi
IC i 1
i 1
Computer Architecture 2011 – Introduction (lec1)
Measure & Compare Performance
Use benchmarks & measure how long it takes
Mathematical analysis typically impossible
Use real applications (=> no absolute answers)
Preferably standardized benchmarks (+input), e.g.,
SPEC INT: integer apps
• Compression, C complier, Perl, text-processing, …
Sometimes you see FLOPS (“pick” or “sustained”)
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SPEC FP: floating point apps (mostly scientific)
TPC benchmarks: measure transaction throughput (DB)
SPEC JBB: models wholesale company (Java server, DB)
Supercomputers (top500 list), against LINPACK
Computer Architecture 2011 – Introduction (lec1)
Evaluating 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
3%
Bad S-curve
2%
6%
Positive
outliers
Good S-curve
Positive
outliers
4%
1%
0%
2%
-1%
-2%
Negative
outliers
-3%
-4%
0%
Small negative
outliers
-2%
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Computer Architecture 2011 – Introduction (lec1)
Amdahl’s Law
Suppose we accelerate the computation such that
P = proportion of computation we make faster
S = speedup experienced by the proportion we improved
For example
If an improvement can speedup 40% of the computation
=> P = 0.4
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If the improvement makes the portion run twice as fast
=> S = 2
Then overall speedup
=
1
(1 P) P
S
Computer Architecture 2011 – Introduction (lec1)
Amdahl’s Law - Example
FP operations improved to run 2x faster
S = 2, but…
P = only affects 10% of the program
Speedup:
1
1
1.053
0.1
0.95
(1 P) P
(1
0.1)
S
2
Conclusion
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1
Better to make common case fast…
Computer Architecture 2011 – Introduction (lec1)
Amdahl’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:
1
(1 P) P
N
Serial component imposes a hard limit
1
1
lim
N
(1 P)
(1 P) P
N
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Computer Architecture 2011 – Introduction (lec1)
Instruction Set Design
software
The ISA is what the user /
compiler see
instruction set
hardware
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The HW implements the
ISA
Computer Architecture 2011 – Introduction (lec1)
Considerations 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)
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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 2011 – Introduction (lec1)
Basing Design Decisions on Workload
Immediate argument’s size in bits (histogram)
30%
Int. Avg.
FP Avg.
20%
10%
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14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0%
Immediate data bits
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1% of data values > 16-bits
Having 16 bits is likely good enough
Computer Architecture 2011 – Introduction (lec1)
CISC 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
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Computer Architecture 2011 – Introduction (lec1)
But it Turns Out…
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
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Computer Architecture 2011 – Introduction (lec1)
CISC 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
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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 over more than a single cache line
An instruction may be over more than a single page
Computer Architecture 2011 – Introduction (lec1)
RISC 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
ALU operations on registers only
• Memory is accessed using Load and Store instructions only
• Many orthogonal registers
• Three address machine:
Add dst, src1, src2
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Fixed length instructions
Examples: MIPSTM, SparcTM, AlphaTM, PowerTM
Computer Architecture 2011 – Introduction (lec1)
RISC Processors (Cont.)
Simple arch => simple u-arch
Compiler can be smarter
Better pipeline usage
Better register allocation
Existing RISC processor are not “pure” RISC
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Room for larger on die caches
Smaller => faster
Easier to design & validation (=> cheaper to manufacture)
Shorten time-to-market
More general-purpose registers (=> less memory refs)
e.g., support division which takes many cycles
Computer Architecture 2011 – Introduction (lec1)
Compilers 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
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Easier if lots of registers
Computer Architecture 2011 – Introduction (lec1)
Still, CISC Is Dominant
x86 (CISC) dominates the processor market
Legacy
CISC internally arch emulates RISC
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A vast amount of existing software
Intel, AMD, Microsoft benefit
But put lot of money to compensate for disadvantage
Starting at Pentium II and K6, x86 processors translate
CISC instructions into RISC-like operations internally
Inside core looks much like that of a RISC processor
Computer Architecture 2011 – Introduction (lec1)
Software 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:
128-bits
x3
x2
x1
128-bits
x0
x3
x2
+
y3
y2
x0
+
y1
y0
x3+y3 x2+y2 x1+y1 x0+y0
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x1
y3
y2
y1
y0
y3
y2
y1
x0+y0
Computer Architecture 2011 – Introduction (lec1)
BACKUP
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Computer Architecture 2011 – Introduction (lec1)
Compatibility
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)
Architecture-independent SW
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For example: MS Word 2003 can open MS Word 2010 doc
Commonly supports one or two generations behind
Run SW on top of VM that does JIT (just in time compiler):
JVM for Java and CLR for .NET
Interpreted languages: Perl, Python
Computer Architecture 2011 – Introduction (lec1)