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06013214 (Distributed Systems)

Synchronization

Akharin Khunkitti

Faculty of Information Technology

KMITL

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Topics

Introduction

Clock Synchronization

Logical Clocks

Election Algorithms

Mutual Exclusion

Summary

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Introduction

How processes cooperate and synchronize withone another

Cooperate in granting each other temporaryexclusive access

Need to agree on the ordering of events

Discussion of issues Actual time

Relative ordering, rather than absolute time

Election algorithms for a coordinator

Distributed mutual exclusion to protect sharedresources against simultaneous access by multipleprocesses

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Clock Synchronization When a process wants to know the time, it makes a system

call and the kernel tells it

In a centralized system, its easy to know the time value andorder of the times

In a distributed system, achieving agreement on time is nottrivial Example: UNIX make program on different machines

When each machine has its own clock, an event that occurred

after another event may nevertheless be assigned an earlier time.

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Clock Synchronization Algorithms The goal is to keep time of all the machines together as well

as possible

Each machine is assumed to have a timer that causes asinterrupt H times a second The interrupt handler adds 1 to a software clock

Let Cp is the value of the clock on machine p

The UTC time is t

The perfect clock would be Cp(t) = t ;or dC/dt = 1

The relation between clock time and UTC when clocks tick at different rates.

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Cristians Algorithm

Get CUTC from a time server

The correct time is CUTC + Message propagation time from server to client+ Servers internal processing time

The best estimate of the message propagation time is (T1-T0)/2 Correct time is CUTC + (T1-T0)/2

The best estimate of the one-way propagation time is (T1-T0-I)/2 Correct time is CUTC + (T1-T0-I)/2

Can be averaged to get a better value

Getting the current time from a time server.

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The Berkeley Algorithm

a) The time daemon asks all the other machines for their clock valuesb) The machines answer

c) The time daemon tells everyone how to adjust their clock

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Logical Clocks It is not essential that all machines time

agree with the real time as announced on theradio every hour

Logical clocks certain class of algorithms, it

is the internal consistency of the clocks, notwhether they are close to the real time

The agreement on the order in which eventsoccur

Need not be real time, or physical clocks

Lamport (1978) showed that although clocksynchronization is possible, it need not beabsolute

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Lamport Timestamps (1) To synchronize logical clocks, Lamport

defined a relation called happens-before If a and b are events in the same process, and a

occurs before b, then a -> b is true If a is the event of a message being sent by one

process, and b is the event of the message beingreceived by another process, then a -> b is alsotrue

Happen-before is a transitive relation

If a -> b and b -> c, then a -> c

Time can be measured for every event

If a -> b, then C(a) < C(b)

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Lamport Timestamps (2)

a) Three processes, each with its own clock.

The clocks run at different rates

b) Lamports algorithm corrects the clocks

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Election Algorithms Many Distributed algorithms require one process to

act as coordinator, initiator, or otherwise performsome special roles

Each process has a unique number E.g. network address

In general, election algorithms attempt to Locate the process with the highest process number, and

Designate it as coordinator

Assume that every process knows the processnumber of every other process Do not know which ones are currently up or down

The goal of an election algorithm is to ensure that When an election starts, it concludes with all processesagreeing on who the new coordinator is to be

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The Bully Election Algorithm (1) Bully algorithm has 3 types of messages

ELECTION send to all higher-number processes

OK or Answer response of ELECTION message

COORDINATOR elected coordinator send to all runninglower-number processes

Any process can initiates an election By sending ELECTION message to all processes with

higher numbers

Try to find the running highest number process

A process, P, holds an election as follow; P sends an ELECTION message to all processes with

higher numbers

If no one responds, P wins the election and becomescoordinator

If one of the higher-ups answers, it takes over. Ps job isdone

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The Bully Election Algorithm (2)

The bully election algorithma) Process 4 holds an election

b) Process 5 and 6 respond, telling 4 to stop

c) Now 5 and 6 each hold an election

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The Bully Election Algorithm (3)

d) Process 6 tells 5 to stop

e) Process 6 wins and tells everyone

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A Ring Election Algorithm (1) Based on the use of a ring

Processes are physically or logically ordered Each process knows its successor

When any process notices that the coordinator is notfunctioning, it Builds an ELECTION message containing its own process number

Sends the message to its successor

It the successor is down, the sender skips over the successor, andgoes to the next member along the ring, or one after that until arunning process is located

At each step, the sender adds its own process number to the list in themessage => making itself a candidate to be elected as coordinator

The message gets back to the process that started it Recognizes the received message containing its own process number

Find who is the coordinator by looking in the list member with thehighest number

Send COORDINATOR message with coordinator (highest) number

When COORDINATOR message has circulated once, it is removed

Everyone knows who is the new coordinator, and goes back to work

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A Ring Election Algorithm (2)

Election algorithm using a ring.

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Mutual Exclusion:

A Distributed Algorithm (2)

a) Two processes want to enter the same critical region at thesame moment.

b) Process 0 has the lowest timestamp, so it wins.

c) When process 0 is done, it sends an OK also, so 2 can nowenter the critical region.

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Mutual Exclusion:

A Token Ring Algorithm (1)

A logical ring is constructed in which each process isassigned a position in the ring Each process knows who is the next in line after itself

When the ring is initialized, a process is given a token The token circulates around the ring

When a process acquire the token, it checks to see if it isattempting to enter a critical region If so, the process enters the region, does al the work it needs to,

and leaves the region After it has exited, it passes the token along the ring

If not interested in entering a critical region, it just passes the tokenalong

If the token is lost, it must be regenerated Detecting lost token is difficult, since amount of time between

successive appearances of the token is unbounded => long usedby a process

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Mutual Exclusion:A Token Ring Algorithm (2)

a) An unordered group of processes on a network.

b) A logical ring constructed in software.

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Mutual Exclusion:A Comparison of the Three Algorithms

A comparison of three mutual exclusion algorithms.

Lost token,

process crash0 to n 11 to Token ring

Crash of any

process2 ( n 1 )2 ( n 1 )Distributed

Coordinator crash23Centralized

ProblemsDelay before entry

(in message times)

Messages per

entry/exitAlgori thm

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Summary Synchronization is all about doing the right things at the

right time

Processes on different machines have their own clocks

There are various ways to synchronize clock, but allmethods are essentially based on exchanging clock values Variation in communication delays and the way those variations are

dealt with, largely determine the accuracy of clock synchronizationalgorithms

In many cases, knowing the absolute time is not necessary Only want to know the correct order of related events at different

processes

Lamport showed that by introducing a notion of logical clocks When event a happened before b, C(a) < C(b)

Synchronization between processes often requires that oneprocess acts as a coordinator The decision is taken by means of election algorithms

Distributed mutual exclusion algorithms ensure that in adistributed collection of processes At most one process at a time has access to a shared resource

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END

Questions?