Advanced Computer Architecture - Lecture 41: Networks and clusters

CS 704
Advanced Computer Architecture

Lecture 41
Networks and Clusters
(Networks: Interconnection and Topology )

Prof. Dr. M. Ashraf Chughtai


Today’s Topics
Recap:
A Simple Network
Network Topology
Internetworking
Summary

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Recap: I/O Systems and Storages
Last time we concluded our discussion on the
storage I/Os and communication I/Os
Here, we noticed that the dependability,
reliability, availability of the storage I/Os
mostly influence the overall performance of

computer systems
Dependability is the quality of delivered
service such that confidence can be placed on
this service; and measured by quantifying the
transitions between service accomplishment
and service interruption
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Recap: Dependability, reliability and Availability
The dependability is measured in terms of the
reliability and availability of a module
The reliability of a module is the measure of
the continuous service accomplishment or the
measure of the time to failure, from a reference
initial instant
The availability of a module is the measure of
the service accomplishment with respect to
the swinging between the accomplishment and
interruption states
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Recap: I/O and Storage Systems
The storages are interfaced with the processor
using channel and backplane and network
interconnects
The networks are capable of sustaining high
bandwidth transfer and their file-server
operating system supports remote file access
Hence, the network attached storages have
very high dependability, but are more
vulnerable to the reliability, so to improve the
availability and performance of network ……
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Recap: Network Attached Storages
….. attached storage system, disk arrays are
introduced
Here, the data is stripped across a set of disks
which makes the collection appears to the
software as a single large disk
The throughput of disk arrays is improved due
many small disk drives having high bandwidth

The drawback to an array with more devices is
that dependability of the device increases,
hence, the reliability decreases; as generally N
devices have 1/N reliability
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Recap: Redundant Arrays of Disks
The dependability of disk array is improved by
adding redundant disks to the array to tolerate
faults

Such a disk array is called Redundant Array of
Inexpensive Disk – RAID
There exist several different approaches to
include redundant disks in the disk array
These approaches are usually classified by
numerical value which identifies the RAID level
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Recap: Redundant Arrays of Disks
RAID 0 is the disk array without any redundant
disk, but employs the stripping of data across
a set of disks
RAID 1 or disk Mirror array is one where each
disk is fully duplicated onto its "shadow“
RAID 3 or Bit-Interleaved Parity Disk employs
a parity disk for each group of data; the parity
computed across recovery group to protect
against hard disk failures
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Recap: Redundant Arrays of Disks
RAID 4 or Block Interleaved Parity and the
RAID 5 or Block Interleaved Distributed Parity,
both use the same ratio of data disk to parity
disk as RAID 3, but they access data
differently
In RAID 4 level, the parity disk is associated to
each data block, identical to it is associated to
each data group in RAID 3, so it supports a
mixture of both the small and large reads and
writes

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Recap: Redundant Arrays of Disks
In RAID 5 level, the parity disk is associated to
each data block
The data blocks are distributed among
different disks in each row;
i.e., the stripped data units are not located in
the same disk
This allows simultaneous read and write of
more than one block
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Interconnection Networks
Till now the focus of our studies has been the
architecture of the components of a single
computer and their performance
Now today and in the following a few lectures

we will talk about how to connect computers
together forming network of computers
The formation of a generic interconnection
network is depicted here
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Interconnection Networks
Node

Node

Hardware Interface

Hardware Interface

Software Interface

Software Interface

Link

Link

Interconnection Network

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Interconnection Networks
The standard components of a computer
network are:
– Computer nodes
(also called host or end system)
– H/W and S/W interface
– Links to the interconnection network
– Interconnection networks
(also called network or communication
subnet)
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Interconnection Networks
The coordinated use of interconnected
computers in a machine room is referred to as
the cluster

The connection of two or more interconnection networks is called Internetworking
The typical example of Internetworking is the
Internet
Internetworking relies on the communication
standards to convert information from one
kind of network to another
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Interconnection Networks
Depending on the number of nodes and their
proximity or nearness the interconnections are
designated as:
Local Area Network-LAN: Hundreds of
computer distributed in a building within a
distance of up to a few kilometers
Wide Area Network-WAN: Interconnection of
thousands of computers distributed
throughout the world at a maximum distance
of thousands of kilometer – Automatic Teller
Machine (ATM) is a typical example
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Interconnection Networks
System Area Network-SAN: Interconnection
network of hundreds of nodes within the
machine room; so the distance of the link is
less than 100 meters
SAN is basically the cluster
However, the Moor’s Law have contracted the
definition of network to an extent that it
defines the interconnection of components
within a single computer
In order to discuss the complexities and ….
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Networks Communication Model
… performance of networks, let us consider a
simple interconnection model of two
computers and understand the implications of
network parameters
The communication model depicted here
shows that two machines are connected via

two unidirectional wires with a FIFO (queue) at
the end to hold the data
Here, each machine wants receive a word or
message from the other
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Networks Communication Model
A

B

The machine A to get data from B, it sends a
request to B, which responds by sending a
reply along with the data
In order to send a request and reply a
message contains extra information beyond
data, as shown in the example message format
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Networks Message Format
Header [1 bit]

0
Header [1 bit]

1

Payload [32 bits]

Address
Payload [32 bits]

Data

Here, a 1 bit header specifies the message as a
request (header=0) or reply (header=1)
The request carries the address of the data
word and the reply the data word
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Networks Interconnection Software
Interconnection networks involve software to

establish communication
For the simple network considered here, the
software is invoked to translate the request
and reply messages
The network software:
– cooperate with the operating system to
distinguish between the processes on the
other networks
– protect the processes running on networks
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Networks Interconnection Software
– Ensures reliable delivery of message, i.e., to
ensure that the message is neither distorted
nor lost in transit
It is worth mentioning here that reliability
the message format is modified by adding
an error detection code (checksum or CRC)
and using 2-bit header as shown here

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Networks Interconnection Software
This information is calculated at the sendingend and is added to the message;
then at the receiving-end this message is
checked; and the receiver sends an acknowledgment if the message passes the test
Header [2 bits]
bits]

Payload [32 bits]

Trailer [4
(Checksum)

Data
00: Request
10: Acknowledge Request
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01: Reply
11: Acknowledge Reply

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Networks Interconnection Software
Furthermore, to ensure reliable deliver of
message, the sender activates a timer each time a
message is sent;
The sender copies the data into an operating
system buffer to resend the message if
acknowledgement doesn’t arrive by time the timer
expires, as it is presumed to be lost
At the receiving end the message is copied into
the operating system buffer
The checksum is checked, if it passes the test
acknowledgment is sent other wise the message
is deleted from the buffer
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Networks Interconnection Protocol
So far we have been talking about the acknowledgment - protocol for reliable communication on a simple network
However, there are many more issues of
reliable communication; e.g.
 Two machines from two different
manufacturers might be using different byteorder within a word (Big Endean or Little
Endean) – so the software must have to
reverse the order accordingly
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Networks Interconnection Protocol
 The duplicate delivery of message should be
guarded against the late delivery of the
original message, if it was stuck in the
network
 The order or the sequence of the message
should not change; so sequence number
should be included in the message
 It must also work when the receiver’s FIFO is
full; so some feedback mechanism be
incorporated
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