Lamport's Mutual Exclusion Distributed Systems Synchronization
Summary
TLDRIn 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
🤖 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.
📨 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.
🔄 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.
Mindmap
Keywords
💡Mutual Exclusion
💡Distributed Systems
💡Message Passing
💡Lamport's Logical Clock
💡Timestamps
💡Critical Section
💡Non-Token Based Algorithms
💡Token Based Algorithms
💡Request, Reply, and Release Messages
💡Process ID (PID)
💡First-In-First-Out (FIFO)
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
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