Lamport's Mutual Exclusion Distributed Systems Synchronization

Summary

In this informative video, Professor Rutu Shakatam introduces Lampson's mutual exclusion algorithm, a crucial concept in distributed systems where multiple processes must access a shared resource without conflict. The video explains that mutual exclusion ensures only one process can enter a critical section at a time. Since distributed systems lack shared memory and synchronized clocks, message passing is the solution. Two types of algorithms are discussed: non-token based, which uses timestamps for synchronization, and token-based, which involves a 'token' that grants access to the critical section. The video outlines the three essential messages for mutual exclusion: request, reply, and release. It also details the algorithm's steps, including sending a request with a timestamp, receiving replies from other processes, and the conditions required to enter the critical section. The performance of Lampson's algorithm is highlighted as 3n-1 messages per critical section invocation, where n is the number of processes. The video concludes with an invitation for viewers to engage with the content and seek clarification on any doubts.

Takeaways

• 📘 Mutual exclusion is a concept where only one process can access a critical section or shared resource at a time.
• 🌐 In distributed systems, mutual exclusion is more complex due to the lack of shared memory and synchronized clocks.
• 🚫 Traditional synchronization techniques like semaphores are not suitable for distributed systems.
• 💌 Message passing is the basis for achieving mutual exclusion in distributed systems.
• 🔄 There are two types of algorithms for mutual exclusion in distributed systems: non-token based and token-based algorithms.
• 🕒 Non-token based algorithms like Lampson's and Ricart-Agrawala use timestamps to manage message passing and synchronization.
• 🎟️ Token-based algorithms, such as Raymen's tree and Suzuki-Kasami, use a 'token' to indicate which process has access to the critical section.
• 🔢 Lampson's logical clock assigns timestamps to requests to determine the priority for entering the critical section.
• 📝 Each process maintains a queue to store timestamps, assuming messages are delivered in FIFO order.
• 🤝 Three types of messages are involved in the process: request, reply, and release.
• ⏳ A process enters the critical section only after its timestamp is at the top of the queue and it has received replies from all other processes.
• 📉 Lampson's mutual exclusion algorithm has a performance of 3n-1 messages per critical section invocation, where n is the number of processes.

Q & A

What is mutual exclusion in the context of computer science?

Mutual exclusion is a concept where only one process can execute in a critical section or utilize a shared resource at any given point in time, preventing simultaneous access by multiple processes to prevent conflicts and ensure data consistency.

Why does the concept of mutual exclusion become more complex in distributed systems?

In distributed systems, the lack of shared memory and synchronized clocks among different nodes makes traditional mutual exclusion techniques like semaphores infeasible. Thus, message passing becomes the basis for synchronization in such environments.

What are the two types of algorithms used to achieve mutual exclusion in distributed systems?

The two types of algorithms are non-token based algorithms, which use message passing and timestamps for synchronization, and token-based algorithms, which involve a token that is passed among processes to grant access to the critical section.

How does Lampson's mutual exclusion algorithm work?

Lamport's mutual exclusion algorithm uses message passing and timestamps to determine which process has the highest priority to enter the critical section. Processes send request messages with timestamps to all other processes, receive reply messages, and only enter the critical section if their timestamp is at the top of their queue and they have received replies from all other processes.

What are the three types of messages involved in Lampson's mutual exclusion algorithm?

The three types of messages are request, reply, and release. Request is sent by a process wanting to enter the critical section, reply is sent by other processes to grant permission, and release is sent by a process after it has finished using the critical section to allow other processes to enter.

What is the significance of timestamps in Lampson's algorithm?

Timestamps are used to determine the priority of critical section requests. The smaller the timestamp, the higher the priority. This ensures a consistent and fair order of process execution within the critical section.

How does the logical clock concept by Lamport contribute to the mutual exclusion problem?

Lamport's logical clock provides a way to assign timestamps to events in a distributed system. This helps in ordering the requests to the critical section and determining which process should be given access based on the earliest timestamp, thus preventing conflicts.

What is the performance of Lampson's mutual exclusion algorithm in terms of message complexity?

The performance of Lampson's mutual exclusion algorithm is 3n-1 messages per critical section invocation, where n is the number of processes. This includes n-1 request, n-1 reply, and n-1 release messages.

How does the queue maintained by each process in Lampson's algorithm function?

Each process maintains a queue that stores timestamps and process IDs (PIDs) of requests to enter the critical section. This queue is used to keep track of pending requests and to determine which process has the highest priority to enter the critical section based on the timestamp.

What are the two requirements for a process to enter the critical section in Lampson's algorithm?

The two requirements are: (1) The process's timestamp must be at the top of its queue, indicating it has the highest priority, and (2) The process must have received reply messages from all other processes, indicating they are not currently using or have granted permission to use the critical section.

How does the release message function in Lampson's mutual exclusion algorithm?

After a process has completed its execution in the critical section, it sends a release message to inform all other processes that the critical section is now free. This allows other processes that have requested access to the critical section to proceed.

What is the role of the 'first in, first out' (FIFO) order in the context of message delivery in distributed systems?

The FIFO order ensures that messages are delivered in the same sequence they were sent. This is crucial in distributed systems for maintaining the correct order of operations, especially when dealing with timestamps and determining the priority of processes wanting to enter the critical section.

Outlines

00:00

🤖 Introduction to Mutual Exclusion in Distributed Systems

This paragraph introduces the concept of mutual exclusion in the context of distributed systems. It explains that mutual exclusion ensures that only one process can access a shared resource or critical section at a time. The paragraph also discusses the challenges of implementing mutual exclusion in distributed systems due to the lack of shared memory and synchronized clocks. It outlines two types of algorithms to achieve mutual exclusion: non-token-based algorithms, which use message passing and timestamps, and token-based algorithms, which involve a token being passed among processes. The paragraph concludes with an introduction to the general system model and the three types of messages involved in mutual exclusion: request, reply, and release.

05:00

📨 Lampard's Mutual Exclusion Algorithm and Process Synchronization

This paragraph delves into the specifics of Lampard's mutual exclusion algorithm, which is a non-token-based algorithm that uses message passing and timestamps to achieve synchronization among processes. It describes the process of a process sending a request to enter the critical section along with its timestamp, other processes replying to the request, and the use of a queue to maintain timestamps. The algorithm uses the smallest timestamp to determine which process gets priority access to the critical section. The paragraph also illustrates how the algorithm works with three processes, showing how they communicate and synchronize to ensure mutual exclusion. It concludes by explaining the requirements for a process to enter the critical section and the importance of the release message after the critical section is used.

10:02

🔄 Execution and Release of the Critical Section in Lampard's Algorithm

This paragraph continues the discussion on Lampard's mutual exclusion algorithm, focusing on the execution and release of the critical section. It explains the steps a process must take to enter the critical section, including making a request with a timestamp, receiving replies from other processes, and ensuring its timestamp is at the top of the queue. The paragraph also details the process of releasing the critical section, which involves a process removing its timestamp from the queue and sending a release message to all other processes. The performance of Lampard's algorithm is quantified as 3n-1 messages per critical section invocation, where n is the number of processes. The paragraph concludes with a summary of the algorithm's steps and a reminder of its efficiency and process synchronization capabilities.

Keywords

💡Mutual Exclusion

Mutual exclusion is a fundamental concept in concurrent and distributed systems that ensures only one process can access a shared resource or critical section at a time. It is crucial for preventing data corruption and race conditions. In the video, mutual exclusion is the central theme, as it discusses how to achieve this in distributed systems where shared memory and synchronized clocks are not available.

💡Distributed Systems

Distributed systems are a collection of autonomous computers that work together to achieve a common goal. They are connected through a network and operate independently. The video emphasizes the challenges of implementing mutual exclusion in such systems due to the lack of shared memory and synchronized clocks.

💡Message Passing

Message passing is a method of communication between processes in distributed systems. It is the basis for inter-process communication and is essential for implementing mutual exclusion algorithms. In the context of the video, message passing is the mechanism used to coordinate processes' access to critical sections.

💡Lamport's Logical Clock

Lamport's logical clock is a concept used to provide a total ordering of events in a distributed system without a centralized clock. It assigns timestamps to events that can be used to determine the order in which they occurred. In the video, it is mentioned as a method to timestamp critical section requests, which helps in determining the priority of processes.

💡Timestamps

Timestamps are used to record the time at which events occur. In the context of the video, timestamps are attached to messages to ensure that the order of requests to enter the critical section is maintained. They are crucial for the Lampard's mutual exclusion algorithm to function correctly.

💡Critical Section

A critical section is a part of a program where shared resources are accessed and is protected by mutual exclusion mechanisms to prevent simultaneous access by multiple processes. The video discusses how mutual exclusion algorithms, like Lampard's, ensure that only one process enters the critical section at any given time.

💡Non-Token Based Algorithms

Non-token based algorithms are a class of mutual exclusion algorithms that do not use a token to control access to the critical section. Instead, they rely on message passing and timestamps. Lampard's mutual exclusion algorithm is an example of such an algorithm, as discussed in the video.

💡Token Based Algorithms

Token-based algorithms use a 'token', a special message or signal, to control access to the critical section. A process must possess the token to enter the critical section. The video contrasts these with non-token based algorithms, mentioning Raymen's tree algorithm and Suzuki-Kasami algorithm as examples.

💡Request, Reply, and Release Messages

These are the three types of messages used in Lampard's mutual exclusion algorithm. A process sends a 'request' message when it wants to enter the critical section. Other processes respond with 'reply' messages to grant or deny access. Once done, a 'release' message is sent to indicate the process is done with the critical section. The video explains how these messages facilitate mutual exclusion.

💡Process ID (PID)

Process ID is a unique identifier assigned to each process in a system. In the context of the video, PID is used in conjunction with timestamps to identify the process requesting access to the critical section. It helps in maintaining the order and priority of requests.

💡First-In-First-Out (FIFO)

FIFO is a principle used in queuing systems where the first item to be stored is the first to be retrieved. In the video, it is assumed that messages are delivered in FIFO order, which is essential for the proper functioning of Lampard's mutual exclusion algorithm as it relies on the order of timestamps.

Highlights

Mutual exclusion is a concept that ensures only one process can access a critical section or shared resource at a time.

In distributed systems, the lack of shared memory and synchronized clocks requires alternative approaches to mutual exclusion, such as message passing.

Lamport's mutual exclusion algorithm is based on message passing and timestamps to achieve synchronization.

There are two types of algorithms for mutual exclusion in distributed systems: non-token based and token-based algorithms.

Lamport's algorithm and the Ricart-Agrawala algorithm are examples of non-token based algorithms that use timestamps.

Token-based algorithms, such as Raymen's tree algorithm and Suzuki-Kasami algorithm, use a 'token' to determine which process can enter the critical section.

The token is analogous to a ticket, granting access to the critical section to the process that holds it.

Three types of messages are involved in achieving mutual exclusion: request, reply, and release.

Each process maintains a queue to store timestamps, assuming messages are delivered in FIFO order.

Lamport's logical clock is used to assign timestamps to critical section requests, determining the priority based on the smallest timestamp.

The execution of critical section requests is in the order of their timestamps, with the smallest timestamp being executed first.

A process sends a request message with its timestamp to all other processes when it wants to enter the critical section.

Reply messages are sent by processes not executing the critical section, indicating permission to enter.

A process can enter the critical section if its timestamp is at the top of the queue and it has received replies from all other processes.

After executing the critical section, a process sends a release message to allow other processes to use the critical section.

The performance of Lamport's mutual exclusion is 3n-1 messages per critical section invocation, where n is the number of processes.

The algorithm ensures that mutual exclusion is achieved in a distributed system without relying on shared memory or synchronized clocks.

Transcripts

hello everyone welcome to simplified

computer science concepts by professor

rutu shakatam

today we will be learning lampard's

mutual exclusion algorithm so let us

start

so what is this mutual exclusion in

distributed systems prior to that let us

see what is mutual exclusion

mutual exclusion uh is a concept where

in a critical section or we call as a

shared resource

it is shared among various processes now

already that particular shared resource

is used by a particular process at the

same time some other process let us say

p1 wants to enter into that critical

section however if the shared resource

or that critical section if utilized by

some other process

p1 cannot enter into the critical

section so the concept of mutual

exclusion says that for at any given

point of time

only one process can

execute into the critical section or

only one process can utilize the

critical section now how about the

mutual exclusion in distributed systems

so let us say this is a distributed

systems or let us say this is a system

wherein it has a own clock and it has

its own shared memory

but in distributed systems we do not

have shared memory and a same clock

because the systems are

geographically apart from each other

that is why the concept of shared memory

and a local clock does not work

so

obviously the the mutual exclusion uh

techniques such as semaphores will not

work

in distributed systems as it requires

shared memory concept

hence an approach based on message

passing is used to solve mutual

exclusion problem because the inter

process communication that is used in

distributed system is message passing

so

we have to follow a approach which is

based on message passing

so what all solutions are there for

mutual exclusion or to achieve mutual

exclusion in distributed systems so

obviously the solution will be based on

message passing

so there are basically two types of

algorithms that are there to achieve the

mutual exclusion the first one is

non-token based algorithms wherein the

message passing or the synchronization

takes place with the help of time stamps

and these time stamps are passed using

messages so the algorithms are lampard's

mutual exclusion algorithm and recart

agrawala algorithm

and in token based algorithms there is a

raymen's tree algorithm and suzuki

kasami algorithm

wherein

a token is nothing but a

holder or we can we can say it as a in a

real-time example it is a ticket

which is given to a process who is

already into the critical section once

it exits the critical section this

particular token will be transferred to

any other process which is requesting

for that particular critical section so

the process who has that token is the

one who will enter into the critical

section

this concept is mapped to a real-time

concept of movie tickets so

if the if the person has the movie

ticket then only he is allowed to enter

into the cinema hall

so let us see the general system model

uh before starting lampard's mutual

exclusion algorithm so basically there

are three messages that are uh involved

to achieve mutual exclusion so that is

request reply and release request is the

one

is the process who will

who wants to enter into the critical

section

that particular process will request all

other processes in the network saying

that i want to enter into the critical

section

with its own timestamp reply

will be sent by the other

sites or other processes in the network

who

who is not executing the critical

section

uh

saying that

uh

we do not have problem you are entering

into the critical section and the last

one is release message the release

message

is once the critical section is

used

it should be released so that some other

process can execute

into that critical section so these are

the three messages that are required for

synchronization

uh via mutual exclusion

so each process will maintain a queue

and it will store its timestamps into it

the assumption is that the messages that

are delivered are delivered in fifa

order first in first out order that is

why a queue is maintained

then a time stamp is given to

critical section request

and it is given with the help of

lampard's logical clock which we have

already discussed in the earlier class

then

timestamp is used to determine priority

of critical section request smaller the

timestamps gets higher priority

so if timestamp is one

and other other process timestamp is

true then which one which time stamp is

smaller one so one will get the higher

priority and it will enter into the

critical section

now the execution of critical section

request is always in the order of their

timestamp so whichever timestamp is the

smaller timestamp will be executed first

now let us look at this illustration and

then let us map the algorithm so these

are the three processes that ah

are into communication with each other

and we need to achieve synchronization

for three processes so initially a

process p3 wants to enter into the

critical section so first what it will

do it will send a request message to all

the other sides that is process p2 and

p3 with its own timestamp

now this timestamp

and this timestamp is maintained in a

queue each process will have a queue

this time this queue

has two fields one is the timestamp and

the other one is pid that is the process

id ts stands for timestamp pid stands

from process id now what is maintained

in the queue of q3 it says that at time

stamp one process three is requesting

for the critical section now the same

queue

contents are passed to process p2 and

process p1 saying that the current uh

current request for the critical section

is done at timestamp1 by process p3

now after some time process p1 also

wants to enter into the critical

section

right so p1 will also send request

message with its timestamp

now it will uh

it will append its own timestamp

to the queues so see initially at its

own

queue it will have timestamp 2 and

process id 1 because it is requested by

the critical section is requested by p1

and this stamp will be sent to p2 and p3

now when it comes to p2 already the

queue had timestamp 1 and pid3 so its

own p2 p1 timestamp will be

appended in the q2's q

process p 2 is q and at the same time

time stamp 2 and

process id 1 also gets appended or

concatenated at the process p 3 skew

now what happens is that

a reply message see after sending the

request a reply message is necessary a

reply message is sent by the other

processes so p2 which is not involved in

accessing the critical section

immediately sends a reply message that

yes you can use the critical section

from my end because i am not

going to use the critical section in

your future

same happens with p1 however p1 has

already requested for the critical

section but it checks in its own queue

that yes uh

i also want to enter into the critical

section so why should i send reply

message but it checks in the queue and

checks that times

process p3 has timestamp one which is

smaller one and that means it has the

highest priority so i have to send a

reply message hence process p1 sends a

reply message to process p3

again

uh

for the uh process p1 also gets reply

from q p3 and p2 p2 sends same thing

that i am not involved in the critical

section so you can use the critical

section

at the same time p3 also sends

the process

the p3 also sends the reply

but then now if both the processors are

requ which are requesting for the

critical section are getting replies

from the other processes which one will

enter so for this there is a uh there

are two requirements to enter into the

critical section so what is the

requirement the first requirement is

that in the queue

the the timestamp should be on the top

the process which wants to enter into

the critical section

that particular processes

timestamp should be on the top of the

queue and the second requirement is that

it should re receive the reply messages

from all the other processes so here

which are the all the other processes

that is process p2 and p1 if we consider

p3 so p3 is getting responses from p2

and p1 and its timestamp is at the top

in the queue that is why the critical

section will be allocated to process p3

right once the critical section is

allocated to process p3 it executes it

after execution it sends the release

message

why release message so that some other

process which is already requesting it

can use that critical section

and

release it later on so

after

p3 completes or process 3 completes its

execution it removes its time stamp from

the queue and now the updated queue is

left with only

process id 1 which means that the

critical section must be allocated to

process id 1. now again here we will

check for the condition that uh

if it wants to enter into the critical

section two conditions are there the

timestamp of

process one is at the top of the queue

and it has received the reply from other

processes so it has received here reply

from p3 and here it has received the

reply from p2

so

hence

q1 will go into the critical section

after q1 enters into the critical

section and utilizes it util

utilizes it it will

release the critical section

and now the queue contents are empty so

here if you see q1 q2 and q3 are now

empty which means no process is

requesting for the critical section

so this is the algorithm

so to enter into the critical section

first a process has to make a request

along with the timestamp and its process

id

that it has to be maintained in a queue

then after receiving the request message

a reply message needs to be sent to the

particular process and again that

timestamp is maintained in the queue to

execute a particular critical section

uh a particular process two requirements

are necessary that a particular process

should receive the message from all the

other sites and its timestamps is at the

top of the queue

and to release the party uh release the

critical section

uh so whenever a process ah

exists that particular critical section

then it has to remove its own timestamp

from the queue and send release messages

to all the other sites

so this was the algorithm

and the performance of lampard's mutual

exclusion is 3 n minus 1 messages per

critical section invocation which means

that by 3 n minus 1 so n minus 1 request

n minus 1 reply and n minus 1 release

messages so ok so that is why this 1 two

three so three n minus one messages per

cs invocation why so

so see if i want to invoke one critical

section

so uh this n is nothing but the number

of processes so how many number of

processes involved in our illustration

three so three minus one two so for each

critical section two request two reply

and two release messages so we will go

back and just see

so for each critical section uh that it

wants to enter there are two requests

two replies coming

and two release messages that are going

out

so that is why three n minus one is the

performance of

lampard's mutual exclusion algorithm

so that's it about lampard's mutual

exclusion thank you for watching this

video

uh please like share and subscribe to

the channel simplified computer science

concepts are professor rutuja

for any doubts you can post your

comments post in the comments

thank you for watching