Transcript Interconnection networks 1

ENGS 116 Lecture 19
1
Interconnection Networks
Vincent H. Berk
November 14, 2005
Reading for Friday: 8.1-8.7
Reading for Monday: 8.8-8.13
ENGS 116 Lecture 19
2
Project Reports
•
•
•
•
•
•
•
Due by Beginning of class next Monday, November 24
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]>
ENGS 116 Lecture 19
3
Project Presentations
•
•
•
•
15 minutes – absolutely no more – Practice your timing!
All group members talk
24th (4) of November, 1st (4) of December
Present:
–
–
–
–
research question
approach
results
conclusions
• EVERYONE ATTENDS these presentations
ENGS 116 Lecture 19
4
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
ENGS 116 Lecture 19
5
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
ENGS 116 Lecture 19
6
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
ENGS 116 Lecture 19
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
7
ENGS 116 Lecture 19
8
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
ENGS 116 Lecture 19
9
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
ENGS 116 Lecture 19
10
Interconnections (Networks)
• Examples:
– SAN networks (infiniband): 100s nodes; ≤ 100 meters per link
– Local Area Networks (Ethernet): 100s nodes; ≤ 1000 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
ENGS 116 Lecture 19
11
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
ENGS 116 Lecture 19
12
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
ENGS 116 Lecture 19
13
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?
ENGS 116 Lecture 19
14
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?
ENGS 116 Lecture 19
15
Overhead, Bandwidth, Size
Effective Bandwidth (Mbits/sec)
1000
bw1000
o1, bw1000
o500, bw1000
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)
ENGS 116 Lecture 19
16
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
ENGS 116 Lecture 19
17
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
ENGS 116 Lecture 19
18
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)
ENGS 116 Lecture 19
19
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
ENGS 116 Lecture 19
20
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.
ENGS 116 Lecture 19
21
Connecting Multiple Computers
•
•
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, ATM)
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)
ENGS 116 Lecture 19
22
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)
ENGS 116 Lecture 19
23
A Simple Example
Figure 8.15 A ring network topology.
ENGS 116 Lecture 19
24
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
ENGS 116 Lecture 19
Figure 8.16: Network topologies that have appeared in commercial
MPPs.
25
ENGS 116 Lecture 19
26
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
ENGS 116 Lecture 19
27
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
ENGS 116 Lecture 19
Figure 8.13 Popular switch topologies for eight nodes.
28
ENGS 116 Lecture 19
29
b) 8  8 Baseline
a) Crossbar switch
= processor
= switch
Examples of dynamic interconnection network topologies