Transcript ppt - CS Course Webpages

Computer Organization
Lecture 1
Bryant’s Book Chapter 1
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Course Overview
• Topics:
– Theme
– Five Great Realities of Computer Systems
– Computer System Overview
– Summary
– NOTE: Most slides are from the textbook and the co-author
Randal E Bryant of Carnegie Mellon University
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Course Theme
• Abstraction is good, but don’t forget reality!
• Courses to date emphasize abstraction
– Abstract data types
– Asymptotic analysis
• These abstractions have limits
– Especially in the presence of bugs
– Need to understand underlying implementations
• Useful outcomes
– Become more effective programmers
• Able to find and eliminate bugs efficiently
• Able to tune program performance
– Prepare for later “systems” classes in CSE & ECE
• Compilers, Operating Systems, Networks, Computer Architecture,
Embedded Systems
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Great Reality #1
• Int’s are not Integers, Float’s are not Reals
• Examples
– Is x2 ≥ 0?
• Float’s:
Yes!
• Int’s:
– 40000 * 40000 --> 1600000000
– 50000 * 50000 --> ??
– Is (x + y) + z = x + (y + z)?
• Unsigned & Signed Int’s:
Yes!
• Float’s:
– (1e20 + -1e20) + 3.14 --> 3.14
– 1e20 + (-1e20 + 3.14) --> ??
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Computer Arithmetic
• Does not generate random values
– Arithmetic operations have important mathematical properties.
• Cannot assume “usual” properties
– Due to finiteness of representations
– Integer operations satisfy “ring” properties
• Commutativity, associativity, distributivity
– Floating point operations satisfy “ordering” properties
• Monotonicity, values of signs
• Observation
– Need to understand which abstractions apply in which contexts
– Important issues for compiler writers and serious application
programmers
– Entire courses offered on computer arithmetic (ECEN 653 Computer
Arithmetic Unit Design)
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Great Reality #2
• You’ve got to know assembly
• Chances are, you’ll never write a program in assembly
– Compilers are much better & more patient than humans
• Understanding assembly is the key to understanding machinelevel execution model
– Behavior of programs in presence of bugs
• High-level language model breaks down
– Tuning program performance
• Understanding sources of program inefficiency
– Implementing system software
• Compiler has machine code as target
• Operating systems must manage process state
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Great Reality #3
• Memory Matters
• Memory is not unbounded
– It must be allocated and managed
– Many applications are memory dominated
• Memory referencing bugs especially pernicious
– Effects are distant in both time and space
• Memory performance is not uniform
– Cache and virtual memory effects can greatly affect program
performance
– Adapting program to characteristics of memory system can lead to
major speed improvements
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Memory Referencing Errors
• C and C++ do not provide any memory protection
– Out of bounds array references
– Invalid pointer values
– Abuses of malloc/free
• Can lead to nasty bugs
– Whether or not bug has any effect depends on system and compiler
– Action at a distance
• Corrupted object logically unrelated to one being accessed
• Effect of bug may be first observed long after it is generated
• How can I deal with this?
–
–
–
–
Program in Java, Lisp, or ML
Understand what possible interactions may occur
Use or develop tools to detect referencing errors
Use debugged library routines
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Great Reality #4
• There’s more to performance than asymptotic complexity
• Constant factors matter too!
– Easily see 10:1 performance range depending on how code written
– Must optimize at multiple levels: algorithm, data representations,
procedures, and loops
• Must understand system to optimize performance
– How programs compiled and executed
– How to measure program performance and identify bottlenecks
– How to improve performance without destroying code modularity and
generality
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Great Reality #5
• Computers do more than execute programs
• They need to get data in and out
– I/O system critical to program reliability and performance
• They communicate with each other over networks
– Many system-level issues arise in presence of network
•
•
•
•
Concurrent operations by autonomous processes
Coping with unreliable media
Cross platform compatibility
Complex performance issues
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Course Overview
• Topics:
– Theme
– Five great realities of computer systems
– Computer System Overview
– Summary
– NOTE: Most slides are from the textbook and the co-author
Randal E Bryant of Carnegie Mellon University
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Terms
• bit, byte, word
• KB, MB, GB, TB, PB, EB, ZB, YB…
– https://en.wikipedia.org/wiki/File_size
• sec, ms, us, ns, ps
Hardware Organization
CPU
Register file
ALU
PC
System bus Memory bus
Main
memory
I/O
bridge
Bus interface
I/O bus
USB
controller
Mouse
Keyboard
Graphics
adapter
Disk
controller
Display
Disk
Expansion slots for
other devices such
as network adapters
hello executable
stored on disk
Inside Pentium 4
Intel Core i7 950 Processor
Hardware Terms
•
•
•
•
CPU: Central Processing Unit
ALU: Arithmetic/Logic Unit
PC: Program Counter
USB: Universal Serial Bus
Main Memory
• A temporary storage device that holds both a
program and the data it manipulates.
• Consists of a collection of dynamic random
access memory (DRAM) chips.
• Logically a linear array of bytes.
Processor
• The engine that interprets instructions stored in main
memory.
• At any point in time, the PC points at some machine
language instruction in main memory.
• A processor repeatedly executes the instruction
pointed at by the PC.
• A processor operates according to a very simple
instruction execution model, defined by its
instruction set architecture (ISA).
Abstraction
• Both hardware and software consists of
hierarchical layers (abstraction).
– To cope with the complexity of computer systems.
• The interface between the hardware and lowlevel software: Instruction Set Architecture
(ISA)
ISA
• Includes anything programmers need to know
to make a binary machine language program
work correctly.
• Includes instructions, I/O devices, and so on.
• Modern ISAs:
– 80x86/Pentium, PowerPC, DEC Alpha, MIPS,
SPARC, HP, …
What is Computer Architecture?
Computer Architecture =
Instruction Set Architecture +
Machine Organization
What Is Computer Architecture?
Application
Operating
System
Compiler
Instr. Set Proc.
Firmware
I/O system
Datapath & Control
Digital Design
Circuit Design
Layout
Instruction Set
Architecture
Register File, ALU
• The register file is a small storage device that
consists of a collection of word-sized registers,
each with its own unique name.
• The ALU computes new data and address
values (add, subtract, multiply, divide, or, and,
xor, etc.)
Memory Hierarchy
L0:
Smaller,
faster,
and
costlier
(per byte)
storage
devices
Registers
L1:
L2:
L3:
Larger,
slower,
and
cheaper
(per byte)
storage
devices
L5:
L4:
CPU registers hold words
retrieved from cache memory.
On-chip L1
cache (SRAM
)
Off-chip L2
cache (SRAM)
L1 cache holds cache lines
retrieved from the L2 cache.
L2 cache holds cache lines
retrieved from memory.
Main memory
(DRAM)
Local secondary storage
(local disks)
Remote secondary storage
(distributed file systems, Web servers)
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Main memory holds disk
blocks retrieved from local
disks.
Local disks hold files
retrieved from disks
on remote network
servers.
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Memory Hierarchy
Faster
REGISTERS
Smaller
CACHE
MEMORY
Slower
DISK
Larger
Cache Memory
CPU chip
Register file
L1
cache
(SRAM)
ALU
Cache bus
L2 cache
(SRAM)
System bus
Memory
bridge
Bus interface
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Memory bus
Main
memory
(DRAM)
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Memory Hierarchy
• Storage at one level serves as a cache for
storage at the next lower level.
• The register file is a cache for the L1 cache, L1
is a cache for L2, and so forth.
DRAM vs. SRAM
• DRAM stands for Dynamic Random Access Memory. It is a
type of semiconductor memory in which the memory is
stored in the form of a charge. Each memory cell in a DRAM is
made of a transistor and a capacitor. The data is stored in the
capacitor. Capacitors loose charge due to leakage and hence
DRAM's are volatile devices. To keep the data in the memory,
the device must be regularly refreshed whereas SRAM is
static, so it will retain a value as long as power is supplied.
SRAM is typically faster than DRAM since it doesn't have
refresh cycles. Since each SRAM memory cell is comprised of
6 Transistors unlike a DRAM memory cell, which is comprised
of 1 Transistor and 1 Capacitor, the cost per memory cell is far
greater in an SRAM compared to a DRAM. With similar
reasoning, it can be asserted that DRAMs come in larger
densities than SRAMs given a fixed area. Example : SRAMs are
used in Caches because of higher speed and DRAMs are used
for main memory in a PC because of higher densities.
Smartphone RAM vs. Internal
Memory
• “My cellphone has 4GB RAM and 16GB memory (or
internal storage).”
• RAM is …
• The internal memory (or storage) is flash memory.
• Flash memory is an electronic non-volatile computer
storage medium that can electrically erased and
reprogrammed.
– Cellphone storage
– USB drive
– Solid-state drive (SSD) as a replacement for hard
drives
Operating System
• A layer of software interposed between the
application program and the hardware.
• All attempts by an application program to manipulate
the hardware must go through the OS.
• Two primary purposes of OS
– Protect HW from misuse by runaway applications.
– Provide applications with simple and uniform mechanisms
for manipulating complicated and often wildly different lowlevel hardware devices.
OS abstracts HW
Application programs
Software
Operating system
Processor
Main memory
Hardware
I/O devices
Processes
Virtual memory
Files
Processor
Main memory
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I/O devices
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Process
• The operating system’s abstraction for a
running program.
• Multiple processes can run concurrently on
the same machine.
– Traditional systems (uniprocessor) can execute
only one program at a time.
– Multicore processors can execute several
programs simultaneously.
Process
• In either case, a single CPU can execute
multiple processes concurrently by having the
processor switch among them.
=> Context switch
• The OS keeps track of all the state information
(context) that the process needs in order to
run.
• Context: PC value, register file values, …
Process
• At any point in time, a uniprocessor system
can only execute the code for a single process.
• When the OS decides to transfer control from
the current process to another process, it
performs a context switch by saving the
context of the current process, restoring the
context of the new process, and passing
control to the new process.
Thread
• A process can actually consists of multiple
execution units, called threads, each running
in the context of the process and sharing the
same code and global data.
Virtual Memory
• An abstraction that provides each process
with the illusion that it has exclusive use of
the main memory.
• Each process has the same uniform view of
memory, called virtual address space.
Memory Performance Example
• Implementations of Matrix Multiplication
– Multiple ways to nest loops
/* ijk */
for (i=0; i<n; i++) {
for (j=0; j<n; j++) {
sum = 0.0;
for (k=0; k<n; k++)
sum += a[i][k] * b[k][j];
c[i][j] = sum;
}
}
/* jik */
for (j=0; j<n; j++) {
for (i=0; i<n; i++) {
sum = 0.0;
for (k=0; k<n; k++)
sum += a[i][k] * b[k][j];
c[i][j] = sum
}
}
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Matmult Performance
(Alpha 21164)
Too big for L1 Cache
Too big for L2 Cache
160
140
120
ijk
100
ikj
jik
80
jki
kij
60
kji
40
20
0
matrix size (n)
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Amdahl’s Law
• When we speed up one part of a system, the effect
on the overall system performance depends on both
how significant this part was and how much it sped
up.
Told: original execution time
Tnew: execution time after improvement
α: fraction of time that will be improved
k: improvement factor
Tnew = (1 - α) Told + (α Told )/k = Told [ ( 1 - α) + α /k]
Speedup S = Told / Tnew = 1 / [ (1 - α) + α /k]
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Example
• A new CPU is 10 times faster in the Web
serving application than the original processor.
Assuming that the original CPU is busy with
computation 40% of the time and is waiting
for I/O 60% of the time, what is the overall
speedup gained by the enhancement?
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Example
• Suppose you work as a truck driver, and you
have been hired to carry a load of potatoes
from Boise, Idaho to Minneapolis, Minnesota,
a total distance of 2,500 kilometers. You
estimate you can average 100 Km/hr driving
within the speed limits, requiring a total of 25
hours for the trip.
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Example
• You hear on the news that Montana has just
abolished its speed limit, which constitutes
1,500 Km of the trip. Your truck can travel at
150 Km/hr. What will be your speedup for the
trip?
• How fast must you travel through Montana
toget an overall speedup for your trip of
1.67x?
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Course Perspective
• Most Systems Courses are Builder-Centric
– Computer Architecture
• Design pipelined processor in Verilog
– Operating Systems
• Implement large portions of operating system
– Compilers
• Write compiler for simple language
– Networking
• Implement and simulate network protocols
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Course Perspective (Cont.)
• Our Course is Programmer-Centric
– Purpose is to show how by knowing more about the underlying
system, one can be more effective as a programmer
– Enable you to
• Write programs that are more reliable and efficient
• Incorporate features that require hooks into OS
– E.g., concurrency, signal handlers
– Not just a course for dedicated hackers
• We bring out the hidden hacker in everyone
– Cover material in this course that you won’t see
elsewhere
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Summary
• The computer system is more than just hardware!
• We need to understand both the hardware and
system interfaces to properly use a computer
• We shall focus on more details to such concepts
through out this course.
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