Transcript Interconnection networks 1 - Thayer School of Engineering

ENGS 116 Lecture 19
1
Interconnection Networks
Vincent H. Berk
November 24, 2008
Reading for today: Sections 6.1 – 6.4
Reading for Monday: Sections 6.5 – 6.9
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Project Reports
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Due by Beginning of class on Monday, December 1st
Content
– Introduction and description of the topic
– Coverage of topic: breadth/depth, appropriate background information
– Analysis and discussion
– References: correct citations, proper form
Writing
– Spelling
– Grammar
– Style and presentation
Assume that the reader is familiar with basic architecture concepts
Must use appropriate citations.
Argument all your decisions.
Email all source code to <[email protected]>
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Project Presentations
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16 minutes – absolutely no more – Practice your timing!
All group members talk
1st (3) of December, 3rd (3) of December
Present:
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research question
approach
results
conclusions
• EVERYONE ATTENDS these presentations
General Announcement
• Hardware Security Project starting this Winter term
• Design and implementation of Hardware Based Profiler
• Need: 1 or 2 Masters or PhD students
• Contact: Steve Taylor or Vincent Berk
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Networks
• Common topics of conversation:
– direct (point-to-point) vs. indirect (multi-hop)
– topology (e.g., bus, ring, directed acyclic graph, star)
– routing algorithms
– switching (aka multiplexing)
– wiring (e.g., choice of media, copper, coax, fiber)
• What really matters:
– latency
– bandwidth
– cost
– reliability
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ABCs of Networks
• Starting point: Send bits between 2 computers
• Queue on each end
• Can send both ways (“Full Duplex”)
• Rules for communication? “protocol”
– Inside a computer:
• Loads/Stores: Request (Address) & Response (Data)
• Need request & response signaling
– Name for standard group of bits sent: packet
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A Simple Example
• What is the format of a packet? (Protocol)
– Fixed? Number of bytes?
Request/
Response
Address/Data
1 bit
32 bits
0: Please send data from address
1: Packet contains data corresponding to request
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Questions About Simple Example
• What if more than 2 computers want to communicate?
– Need computer address field (destination) in packet
• What if packet is garbled in transit?
– Add error detection field in packet (e.g., CRC)
• What if packet is lost?
– More elaborate protocols to detect loss (e.g., NAK, ARQ, time
outs)
• What if multiple processes per machine?
– Queue per process
• Questions such as these lead to more complex protocols and packet
formats
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A Simple Example Revisited
• What is the format of a packet?
– Fixed? Number of bytes?
Request/
Response
Address/Data
CRC
2 bits
32 bits
4 bits
00:
01:
10:
11:
Request—Please send data from address
Reply—Packet contains data corresponding to request
Acknowledge request
Acknowledge reply
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Additional Background
• Connection of 2 or more networks: Internetworking
• 3 cultures for 3 classes of networks
– SAN: server (storage) networks, performance
– LAN: workstations, cost
– WAN: telecommunications, long range
• Cost
• Performance (BW, latency)
• Reliability
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Interconnections (Networks)
• Examples:
– SAN networks (infiniband): 100s nodes; ≤ 10 meters per link
– Local Area Networks (Ethernet): 100s nodes; ≤ 100 meters
– Wide Area Network (ATM): 1000s nodes; ≤ 5,000,000 meters
Node
Node
Node
SW Interface
SW Interface
SW Interface
HW Interface
HW Interface
HW Interface
Link
Link
Link
Interconnect
Node
...
SW Interface
HW Interface
...
Link
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Software to Send and Receive
• SW Send steps
1: Application copies data to OS buffer
2: OS calculates checksum, starts timer
3: OS sends data to network interface HW and says start
• SW Receive steps
3: OS copies data from network interface HW to OS buffer
2: OS calculates checksum, if matches send ACK; if not, deletes
message (sender resends when timer expires)
1: If OK, OS copies data to user address space and signals
application to continue
• Sequence of steps for SW: protocol
– Example similar to UDP/IP protocol in UNIX
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Network Performance Measures
Node
Node
...
SW Interface
SW Interface
HW Interface
HW Interface
Overhead
Overhead
Link
Link
Bandwidth
...
Link
Interconnect
Bisection Bandwidth
Latency
Link
Bandwidth
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Universal Performance Metrics
Sender
Sender
Overhead
Transmission time
(size ÷ bandwidth)
(processor
busy)
Time of
Flight
Transmission time
(size ÷ bandwidth)
Receiver
Overhead
Receiver
Transport Latency
(processor
busy)
Total Latency
Total Latency = Sender Overhead + Time of Flight + Message Size ÷ BW + Receiver Overhead
Includes header/trailer in BW calculation?
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Simplified Latency Model
• Total Latency ≈ Overhead + Message Size / BW
• Overhead = Sender Overhead + Time of Flight + Receiver Overhead
• Example: show what happens as we vary the following
– Overhead: 1, 25, 500 µsec
– BW: 10, 100, 1000 Mbit/sec (factors of 10)
– Message Size: 16 Bytes to 4 MB (factors of 4)
• If overhead is 500 µsec,
how big a message is needed to get > 10 Mb/s of bandwidth?
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Overhead, Bandwidth, Size
1000
bw1000
o1, bw1000
o500, bw1000
Effective Bandwidth (Mbits/sec)
o25, bw1000
100
bw100
o1, bw100
o500, bw100
10
bw10
o500, bw10
o1, bw10
o1, bw100
1
o1, bw1000
o25, bw10
o25, bw100
o25, bw1000
0.1
o500, bw10
o500, bw100
o500, bw1000
0.01
Message Size (bytes)
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Measurement: Sizes of Message for NFS
100%
90%
Msgs
Cumulative %
80%
70%
Why?
Bytes
60%
50%
40%
30%
20%
10%
0%
0
1024
2048
3072
4096
Packet size
5120
• 95% messages, 30% bytes for packets ≤ 200 bytes
• > 50% data transferred in packets = 8KB
6144
7168
8192
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HW Interface Issues
• Where to connect network to computer?
– Cache consistent to avoid flushes? ( memory bus)
– Latency and bandwidth? ( memory bus)
– Standard interface card? ( I/O bus)
– MPP  memory bus; LAN, WAN  I/O bus
CPU
Network
Network
$
I/O
Controller
L2 $
Memory Bus
Memory
Bus Adaptor
I/O
Controller
I/O bus
ideal: high bandwidth,
low latency, standard
interface
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SW Interface Issues
• How to connect network to software?
– Programmed I/O? (low latency)
– DMA? (best for large messages)
– Receiver interrupted or received polls?
• Things to avoid
– Invoking operating system in common case
– Operating at uncached memory speed
(e.g., check status of network interface)
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CM-5 Software Interface
– Time per poll 1.6 secs; time
per interrupt 19 secs
– Minimum time to handle
message: 0.5 secs
– Enable/disable 4.9/3.8 secs
• As rate of messages arriving
changes, use polling or
interrupt?
– Solution: Always enable
interrupts, have interrupt
routine poll until no messages
pending
– Low arrival rate  interrupt
– High arrival rate  polling
Overhead
100
90
80
message overhead (µsecs)
• CM-5 example (MPP)
70
60
Polling
50
40
30
Interrupts
20
10
0
0
10
20
30
40
50
60
70
80
message interarrival (µsecs)
Time between messages
90
100
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Network Media
Twisted Pair:
Coaxial Cable:
Fiber Optics
Transmitter
– L.E.D
– Laser Diode
light
source
Copper, 1mm thick, twisted to avoid
antenna effect (telephone)
Plastic Covering
Braided outer conductor
Insulator
Copper core
Air
Total internal
reflection
Silica
Used by cable companies: high
BW, good noise immunity
Receiver
– Photodiode
3 parts are
cable, light
source, light
detector.
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Connecting Multiple Computers
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•
Shared Media vs. Switched:
pairs communicate at same time,
“point-to-point” connections
Aggregate BW in switched network
is many times that of shared
– point-to-point faster since no
arbitration, simpler interface
•
Shared Media (Ethernet)
Node
Node
Node
Switched Media (CM-5, Fast-Ethernet)
Node
Node
Arbitration in shared network?
– Central arbiter for LAN?
– Listen to check if being used
(“Carrier Sensing”)
– Listen to check if collision
(“Collision Detection”)
– Random resend to avoid repeated
collisions; not fair arbitration;
– OK if low utilization
Switch
Node
Node
(a.k.a. data switching
interchanges, multistage
interconnection networks,
interface message processors)
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Switch Topology
• Structure of the interconnect
• Determines
– Degree: number of links from a node
– Diameter: max number of links crossed between nodes
– Average distance: number of hops to random destination
– Bisection: minimum number of links that separate the network into
two halves (worst case)
• Warning: these three-dimensional drawings must be mapped onto
chips and boards which are essentially two-dimensional media
– Elegant when sketched on the blackboard may look awkward when
constructed from chips, cables, boards, and boxes (largely 2D)
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A Simple Example
Figure 8.15 A ring network topology.
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b) Linear array
a) Bus
c) Star
e) Tree
d) Ring
f) Near-neighbor mesh
h) 3–cube (hypercube)
g) Completely connected
Examples of Static Interconnection Network Topologies
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Figure 8.16: Network topologies that have appeared in commercial
MPPs.
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Important Topologies
Type
Degree
Diameter
Avg Dist
Bisection
N = 1024
Diam
Avg D
1D mesh
≤2
N-1
N/3
1
2D mesh
≤4
2(N1/2 - 1)
2N1/2 / 3
N1/2
63
21
3D mesh
≤6
3(N1/3 - 1)
3N1/3 / 3
N2/3
~ 30
~ 10
Ring
2
N/2
N/4
2
2D torus
4
N1/2
N1/2 / 2
2N1/2
32
16
Hypercube
n
n = LogN
n/2
N/2
10
5
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0
0
0
0
0
1
0
0
0
3
2
1
1
0
1
1
2
0
2
1
3
0
1
2
3
4
5
6
7
8
9
10
11
12
Figure 8.14 A fat-tree topology for 16 nodes.
0
13
3
14
1
15
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Figure 8.13 Popular switch topologies for eight nodes.
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b) 8  8 Baseline
a) Crossbar switch
= processor
= switch
Examples of dynamic interconnection network topologies
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Connection-Based vs. Connectionless
• Telephone: operator sets up connection between the caller and the
receiver
– Once the connection is established, conversation can continue for
hours
• Share transmission lines over long distances by using switches to
multiplex several conversations on the same lines
– “Time division multiplexing” divide B/W transmission line into a
fixed number of slots, with each slot assigned to a conversation
• Problem: lines busy based on number of conversations, not amount of
information sent
• Advantage: reserved bandwidth
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Connection-Based vs. Connectionless
• Connectionless: every package of information must have an
address => packets
– Each package is routed to its destination by looking at its
address
– Analogy, the postal system (sending a letter)
– Also called “Statistical multiplexing”
• Each packet requires a new/separate routing decision
• Depending on implementation the switching stations may
also be called routers.
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Routing Messages
• Within a network:
– Shared media:
• Broadcast to everyone
• Internetwork routing. Options:
– Source-based routing: message specifies path to the destination
(changes of direction)
– Virtual circuit: circuit established from source to destination,
message picks the circuit to follow
– Destination-based routing: message specifies destination, switch
must pick the path: deterministic vs. non-deterministic
• deterministic: always follow same path
• adaptive: pick different paths to avoid congestion, failures
• randomized routing: pick between several good paths to balance
network load
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Deterministic Routing Examples
• mesh: dimension-order routing
– (x1, y1)  (x2, y2)
– first x = x2 – x1,
– then y = y2 – y1,
• hypercube: edge-cube routing
– X = xox1x2 . . .xn  Y = yoy1y2 . .
.yn
– R = X xor Y
– Traverse dimensions of differing
address in order
• tree: common ancestor
• Deadlock free?
110
010
111
011
100
000
001
101
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Store and Forward vs. Cut-Through
• Store-and-forward policy: each switch waits for the full packet to
arrive in switch before sending to the next switch (good for WAN)
• Cut-through routing or wormhole routing: switch examines the
header, decides where to send the message, and then starts
forwarding it immediately
– In wormhole routing, when head of message is blocked, message
stays strung out over the network, potentially blocking other
messages (needs only buffer the piece of the packet that is sent
between switches). CM-5 uses it, with each switch buffer being
4 bits per port.
– Cut-through routing lets the tail continue when head is blocked,
“accordioning” the whole message into a single switch.
(Requires a buffer large enough to hold the largest packet).
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Congestion Control
• Packet switched networks do not reserve bandwidth; this leads to
contention (connection-based limits input)
• Solution: prevent packets from entering until contention is reduced
(e.g., freeway on-ramp metering lights)
• Options:
– Packet discarding: If packet arrives at switch and no room in buffer,
packet is discarded (e.g., UDP)
– Flow control: between pairs of receivers and senders;
use feedback to tell sender when allowed to send next packet
• Back-pressure: separate wires to tell to stop
• Window: give original sender right to send N packets before getting
permission to send more; overlaps latency of interconnection with overhead
to send & receive packet (e.g., TCP), adjustable window
– Choke packets: aka “rate-based”; each packet received by busy
switch in warning state sent back to the source via choke packet.
Source reduces traffic to that destination by a fixed % (e.g., ATM,
ICMP source quench)
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Practical Issues for Interconnection Networks
• Standardization advantages:
– low cost (components used repeatedly)
– stability (many suppliers to chose from)
• Standardization disadvantages:
– Time for committees to agree
– When to standardize?
• Before anything built? => Committee does design?
• Too early suppresses innovation
• Perfect interconnect vs. Fault Tolerant?
– Will SW crash on single node prevent communication?
(MPP typically assumes perfect)
• Reliability (vs. availability) of interconnect
• Most successful system is not always the best design.
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Practical Issues
Interconnection
Example
Standard
Fault Tolerance?
Hot Insert?
MPP
CM-5
No
No
No
LAN
Ethernet
Yes
Yes
Yes
WAN
ATM
Yes
Yes
Yes
• Standards: required for WAN, LAN!
• Fault Tolerance: Can nodes fail and still deliver messages to other
nodes? Required for WAN, LAN!
• Hot Insert: If the interconnection can survive a failure, can it also
continue operation while a new node is added to the interconnection?
Required for WAN, LAN!
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Inter-Network-Routing
• Connecting >2 networks together.
• Requires:
– Addressing Hierarchy
– Common Protocols
– Courtesy and Security
• Each step in a route (hop) decides:
– What first?
– Where next?
• Transparent or explicit
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Bridging (transparent routing)
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OSI model
• This one has to be in every network presentation
7. Application
Web browser
6. Presentation
Network library interface
5. Session
TCP
4. Transport
IP
3. Network
Packet
2. Data Link
Ethernet Frame
1. Physical
Electrical signals
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Networking Protocols: HW/SW Interface
• Internetworking: allows computers on independent and incompatible
networks to communicate reliably and efficiently;
– Enabling technologies: SW standards that allow reliable
communications without reliable networks
– Hierarchy of SW layers, giving each layer responsibility for
portion of overall communications task, called
protocol families or protocol suites
• Transmission Control Protocol/Internet Protocol (TCP/IP)
– This protocol family is the basis of the Internet
– IP makes best effort to deliver; TCP “guarantees” delivery
– TCP/IP used even when communicating locally: NFS uses IP
even though communicating across homogeneous LAN
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Protocol
Message
Logical
Message
Actual
H
T
H
T
Actual
H
T
Logical
H
T
H
Actual
T T
H H
T T
T
H
T
Actual
H H
T T
H H
T T
H H
T T
H H
T T
Actual
• Key to protocol families is that communication occurs logically at the
same level of the protocol, called peer-to-peer, but is implemented via
services at the lower level
• Danger is each level increases latency if implemented as hierarchy
(e.g., multiple check sums)
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IP, TCP, and UDP
• IP = internet protocol, used at network layer
– IP routes datagrams to destination machine, makes best effort to deliver
packets but does not guarantee delivery or order of datagrams
– For IP, every host and router must have unique IP address
• IPv4 uses 32-bit addresses
• IPv6 uses 16-byte addresses (not that straight forward, though!!!)
• TCP = transmission control protocol, used at transport layer
– TCP is connection-oriented, makes guarantee of reliable, in-order delivery
– Up to 4 retries on failure to deliver (or acknowledge!)
• UDP = user data protocol, used at transport layer
– Connectionless protocol, makes no guarantees of delivery
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Packet Formats
CM-5
Route
T L
Data (4 - 20)
C
32 bits
Ethernet
ATM
Preamble
Preamble
Destination
Destination
Source
Source
Length
Destination
C
Data (48)
Data (0 - 1500)
Pad (0 -46)
Checksum
32 bits
32 bits
• Fields: Destination, Checksum (C), Length (L), Type (T)
• Data/Header Sizes in bytes: (4 to 20)/4, (0 to 1500)/26, 48/5
T
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Networking Summary
• Protocols allow heterogeneous networking
• Protocols allow operation in the presence of failures
• Routing issues: store and forward vs. cut-through, congestion, ...
• Standardization key for LAN, WAN
• Internetworking protocols used as LAN protocols  large overhead
for LAN
• Integrated circuit revolutionizing networks as well as processors
• Switch is a specialized computer
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Cluster (Multicomputer)
• A collection of low-cost nodes connected by a fast network.
• Applications:
–
–
–
–
Less synchronization required than for MP applications
Less need for communication
No need for one large homogeneous memory
Many copies of one application run in parallel
• Each node:
– cheap
– redundant
• Easily expandable
– Scales if the software application scales
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Applications
• Distributed Database
– Each node works as the query engine for data on local disk(s)
– All nodes together implement redundancy:
• Failure of 1 or more nodes doesn’t damage the database
• Scientific applications:
– Nuclear or Oceanographic simulations
– Diskless nodes. Each node uses NFS (of similar SAN-based
system) to access central data repository.
– Applications are started over the network.
– Think SETI@home (BOINC)