Threading Issues (Thread Cancellation)

Summary

This lecture delves into thread cancellation in multi-threading, explaining its definition and common scenarios like searching databases and web page loading. It distinguishes between asynchronous and deferred cancellation, highlighting the challenges of resource allocation and data integrity when threads are abruptly terminated. The speaker advocates for deferred cancellation as a safer method, allowing threads to terminate safely, thus preventing threading issues.

Takeaways

• 🧐 Thread cancellation is the process of terminating a thread before its execution is complete.
• 🔎 Examples of thread cancellation include multiple threads searching a database where one thread finding the result leads to the cancellation of the others, and a user stopping a web page from loading, which cancels all threads involved in the page load.
• 📌 A thread that is to be cancelled is referred to as the 'target thread'.
• 🔄 There are two scenarios for thread cancellation: asynchronous cancellation, where a thread is immediately terminated by another, and deferred cancellation, where the target thread periodically checks if it should terminate itself.
• 🚫 Asynchronous cancellation can be problematic as it does not allow the target thread to release resources or complete data updates safely.
• 🔄 Deferred cancellation provides a safer approach, allowing the target thread to check for cancellation safety before terminating, thus preventing issues with resource allocation and data integrity.
• 🛡️ Difficulty with cancellation often lies in the management of resources and shared data when a thread is cancelled unexpectedly.
• 💡 The operating system may not always be able to reclaim all resources from a cancelled thread, especially in the case of asynchronous cancellation.
• 🔑 Deferred cancellation gives the target thread the ability to defer cancellation until it is safe to do so, preventing potential issues with incomplete data updates and resource allocation.
• 📚 The lecture emphasizes the importance of choosing the right cancellation technique to avoid threading issues, highlighting deferred cancellation as the preferred method.
• 👋 The speaker concludes by hoping the lecture on thread cancellation and its relation to threading issues was clear, inviting viewers to the next lecture.

Q & A

What is thread cancellation?

Thread cancellation is the process of terminating a thread before its execution is complete, effectively stopping the thread from finishing its intended task.

Why might thread cancellation be necessary?

Thread cancellation might be necessary when a task is no longer required, such as when one thread has already found the result of a search, making the continuation of other threads redundant.

Can you provide an example where thread cancellation is beneficial?

An example of beneficial thread cancellation is when multiple threads are searching a database for a specific value. If one thread finds the value, the remaining threads can be cancelled to save resources.

What is a real-life scenario where thread cancellation occurs?

A real-life scenario is when a user presses the cancel button in a web browser to stop a web page from loading further, which results in all threads involved in loading the page being cancelled.

What is the difference between a target thread and a regular thread?

A target thread is a specific thread that is designated to be cancelled. It is the thread that other threads may terminate or that may terminate itself under certain conditions.

What are the two scenarios in which thread cancellation can occur?

The two scenarios are asynchronous cancellation, where one thread immediately terminates the target thread, and deferred cancellation, where the target thread periodically checks whether it should terminate itself.

How does asynchronous cancellation differ from deferred cancellation?

Asynchronous cancellation involves immediate termination of a target thread by another thread without any prior notice. Deferred cancellation, on the other hand, allows the target thread to check periodically if it should terminate itself, providing an opportunity for an orderly shutdown.

What issues can arise from thread cancellation?

Issues can arise when a cancelled thread is holding resources that the operating system may not be able to reclaim, or when a thread is in the midst of updating shared data and its cancellation leaves the data in an incomplete state.

Why is deferred cancellation considered a safer approach than asynchronous cancellation?

Deferred cancellation is safer because it allows the target thread to check for safe cancellation points, ensuring that it does not leave resources unreleased or data in an inconsistent state.

How does the operating system handle resource allocation for a cancelled thread?

The operating system will typically reclaim system resources from a cancelled thread, but there may be cases where it is unable to reclaim all resources, especially if the cancellation is asynchronous and the thread was holding critical resources.

What is the importance of understanding thread cancellation in concurrent programming?

Understanding thread cancellation is crucial for managing resources effectively and ensuring data consistency in concurrent programming, preventing issues such as resource leaks or corruption of shared data.

Outlines

00:00

🔍 Understanding Thread Cancellation

This paragraph introduces the concept of thread cancellation in computing, where a thread's execution is terminated before it naturally completes. The speaker uses examples such as multiple threads searching a database and a user-initiated cancellation of a web page load to illustrate scenarios leading to thread cancellation. The paragraph also differentiates between 'asynchronous cancellation', where a thread is abruptly terminated by another, and 'deferred cancellation', where the target thread periodically checks for a cancellation condition, allowing for a more controlled termination.

05:00

🚧 Challenges of Thread Cancellation

The second paragraph delves into the difficulties associated with thread cancellation, particularly when it comes to resource allocation and data integrity. It explains that a cancelled thread may still hold onto system resources, which the operating system might not always be able to reclaim, leading to potential resource wastage. Additionally, the paragraph highlights the risk of data inconsistency when a thread is cancelled in the middle of updating shared data, which can result in other threads accessing incomplete information. The speaker contrasts the issues of 'asynchronous cancellation' with the more controlled and safer 'deferred cancellation', which allows a thread to cancel itself at a safe point in its execution.

10:00

🛠️ Solutions to Threading Issues through Deferred Cancellation

The final paragraph wraps up the lecture by emphasizing the benefits of adopting deferred cancellation as a technique to address the issues arising from thread cancellation. It suggests that most problems related to threading can be mitigated by allowing threads to check for cancellation conditions and terminate safely, thus maintaining system stability and data integrity. The speaker concludes the lecture with a note of thanks and an invitation to the next session, indicating a structured and informative presentation.

Keywords

💡Thread Cancellation

Thread cancellation refers to the process of terminating a thread before it has completed its execution. It is a crucial concept in the video, as it sets the stage for discussing the issues that arise from this action. For example, when multiple threads are searching a database, and one finds the desired result, the others may be cancelled to save resources and prevent redundant work.

💡Asynchronous Cancellation

Asynchronous cancellation is a method where a thread is terminated immediately by another thread without any prior notice. It is a direct and immediate form of cancellation, leaving the target thread with no control over its termination. This is exemplified in the video when it discusses the scenario of a thread being cancelled while it is holding resources or in the midst of updating shared data.

💡Deferred Cancellation

Deferred cancellation allows a thread to periodically check whether it should terminate, giving it the opportunity to end its execution in an orderly fashion. This method provides the thread with some control over its cancellation, which can prevent issues related to resource allocation and data integrity. The video highlights this as a safer and more stable approach compared to asynchronous cancellation.

💡Target Thread

A target thread is the thread that is intended to be cancelled. The term is used throughout the video to denote the thread that is either being terminated immediately in asynchronous cancellation or checking for cancellation conditions in deferred cancellation. It is central to understanding the dynamics of thread cancellation.

💡Resource Allocation

Resource allocation in the context of the video refers to the assignment of system resources to a thread for its operation. The script discusses how a cancelled thread may still hold onto these resources, which can lead to issues if not properly managed, especially in the case of asynchronous cancellation.

💡Shared Data

Shared data is data that is accessible by multiple threads. The video points out the complications that can arise when a thread updating shared data is cancelled, potentially leaving the data in an inconsistent or incomplete state for other threads to access.

💡Operating System (OS)

The operating system plays a role in managing resources and threads. In the context of thread cancellation, the OS is responsible for reclaiming system resources from a cancelled thread. However, the video notes that in some cases, the OS may not be able to reclaim all resources, leading to potential issues.

💡Thread Safety

Thread safety is the property of correctly handling the execution of multiple threads in a way that avoids undesirable states or behaviors. The video discusses how deferred cancellation contributes to thread safety by allowing threads to check for cancellation at safe points, thus avoiding issues like resource leakage or inconsistent data.

💡Concurrency

Concurrency is the concept of multiple threads executing in parallel, potentially leading to increased efficiency. The video uses the example of multiple threads searching a database to illustrate concurrency, and how thread cancellation can be a part of managing concurrent operations.

💡Web Browser Cancellation

The video provides a real-life example of thread cancellation in the context of a web browser, where pressing a cancel button stops the loading of a web page. This action results in the cancellation of all threads involved in loading the page's elements, demonstrating a practical application of thread cancellation.

Highlights

Thread cancellation is defined as the termination of a thread before its execution is complete.

Thread cancellation can occur in scenarios such as multiple threads searching a database, where one thread finding the result leads to the cancellation of others.

Real-life example of thread cancellation includes pressing a cancel button in a web browser to stop a web page from loading further.

A thread to be cancelled is often referred to as a 'target thread'.

There are two types of thread cancellation: asynchronous and deferred.

Asynchronous cancellation involves one thread immediately terminating the target thread.

Deferred cancellation allows the target thread to periodically check if it should terminate, providing an orderly cancellation process.

Difficulties with thread cancellation include issues with resources allocated to a cancelled thread.

A cancelled thread may hold resources that the operating system cannot always reclaim.

Thread cancellation while updating shared data can lead to incomplete updates and potential data inconsistencies.

Deferred cancellation is safer as it allows a thread to check for safe cancellation points before terminating.

Asynchronous cancellation may not free all system-wide resources if the thread is holding them during cancellation.

The target thread in deferred cancellation has the power to defer cancellation until it is safe to do so.

Adopting deferred cancellation can solve most threading issues related to thread cancellation.

The lecture provides clear insights into the complexities and solutions of thread cancellation in programming.

Understanding the different scenarios of thread cancellation is crucial for effective multithreading management.

The importance of orderly cancellation for maintaining data integrity and resource management in multithreaded applications.

Transcripts

as we continue discussing about trading

issues in this lecture we'll be studying

about threat cancellation so we will be

seeing what are the issues that we face

in training when threat cancellation is

involved so first let us try to

understand what do we actually mean by

threat cancellation so threat

cancellation is the task of terminating

a thread before it has completed so if a

thread gets cancelled before its

execution is complete then that is known

as thread cancellation so when we say a

thread is cancelled we mean that before

the thread could complete its execution

it was terminated now let us take some

examples to understand when does thread

cancellation occurs and what are the

issues that we can face when thread

cancellations occur so in the first

example if multiple threads are

concurrently searching through a

database and one thread returns a result

the remaining threads might be cancelled

so let us say that we have a database

and we are searching through the

database for a particular value or a

particular result so according to the

search algorithm many threads may be

involved in searching for that

particular value in that database now if

one of the thread finds that value in

the database then we have got the result

that means we got the result that we

were searching for so the remaining

threads can be cancelled or they might

be cancelled because they don't need to

continue searching because the result is

already obtained so that is one scenario

where thread cancellations can occur now

let's take another example when a user

presses a button on the web browser that

stops a web page from loading any

further all threads loading the pages

are cancelled this is a real-life

example that you must have seen in your

day-to-day life so when you are using a

web browser for surfing the internet you

know that there is a cancel button that

is there in the web browser so when a

web page is loading if you press on that

cancel button what happens is the entire

web page stops loading any further so

what happens is that when you try to

visit a web page when you click on any

web page as a web page loads there are

several threads involved in loading of

that web

so whenever paste loads you know that

there are so many things in their web

page the web page may contain images it

may contain text it may contain

hyperlinks it may contain so many things

now for loading each and every of these

elements different threads are

responsible all these threads work

together and they load the entire

webpage now in the process of loading

that web page if you press the button

that stops the loading of the webpage

then what happens is all the threats

that were involved in loading the

different elements like images or text

or whatever it may be

everything will be canceled all those

threats will be terminated before it has

completed some might have completed or

some may still be in the process of

completion it will depend upon when you

press that button so many threats will

be canceled before it completed its

execution so that is another example

where we can see cancellation of threats

now a thread that is to be cancelled is

often referred to as a target thread so

whenever we use the term target thread

keep in mind that this is a thread that

has to be cancelled now let us see what

are the different types of thread

cancellation that can occur or what are

the different scenarios in which thread

cancellations can occur and let us see

which one among them is better and which

one among them will provide a stable

computation so as I said we will now see

the different scenarios in which thread

cancellations can occur so here it's a

sad cancellation of a target thread may

occur in two different scenarios and let

us see what are those scenarios one by

one so the first scenario is

asynchronous cancellation so what is

this asynchronous cancellation one

thread immediately terminates the target

thread so this is very straightforward

and it is very direct what happens is

one thread immediately terminates the

target thread so what is a target thread

a target thread is the thread that is to

be cancelled and another thread that is

going to cancel that target thread

immediately cancels that target thread

so this target thread is like a victim

which has no power when some other

thread cancels it it has no other choice

than to be cancelled

so that is what we mean by asynchronous

cancellation now let's look at the other

scenario now the other scenario is known

as deferred cancellation in deferred

cancellation the target thread

periodically checks whether it should

terminate allowing it an opportunity to

terminate itself in an orderly fashion

so unlike the asynchronous cancellation

where the target thread was immediately

cancelled by another thread in deferred

cancellation what happens is that the

target thread will periodically check

whether it should terminate itself or

not so nobody can just come and

terminate it immediately without its

permission so the target thread will

periodically check whether it should

terminate and hence that allows it an

opportunity for it to terminate itself

in an orderly fashion so here we can say

that the thread is having some power it

will not be canceled directly by any

other thread but it will cancel itself

in an orderly fashion after making sure

that it can be cancelled so that is what

we mean by deferred cancellation so now

the question arises where the difficulty

with cancellation lies we have discussed

about thread cancellation and we have

seen the different scenarios in which a

thread can be cancelled but the question

is where is a difficulty with

cancellation what is the problem or

issue that we face with thread

cancellation so let's take a look into

that so here is where the difficulty

lies in situations where resources have

been allocated to a cancelled thread

that means a thread was under execution

and it was allocated certain resources

of our system because threads may

require different resources and

resources are allocated to the threads

so when a thread is holding some

resources and it gets cancelled then

there could be a problem so we will see

what is that problem but before that let

us look at the second point also

so it says thread is cancelled while in

the midst of updating data it is sharing

with other threads so this is again a

serious problem so there may be data

that is shared between different threads

so different threads can be writing or

updating into that shared region of data

now

when a thread is updating that shared

data if it gets canceled in the midst of

that obligation then it could be a

problem because when it is updating

something and it gets canceled then that

updation was not complete and it becomes

a incomplete data and then when other

threats are going to share that they may

be reading some incomplete data from

there so that is a situation where the

difficulty of cancellation lies now let

us come back to the first point here I

said resources have been allocated to a

canceled threat so what is a problem

with this so the thing is that when

resources are allocated to a thread

often the operating system will reclaim

the system resources from a cancelled

thread but will not reclaim all the

resources so when a thread is allocated

resources and if it gets cancelled then

the OS will by default reclaim the

system resources from the canceled

thread but in some cases it may not be

able to reclaim all the resources so

canceling a thread a synchronously may

not free unnecessary system-wide

resource so we already discuss about

asynchronous cancellation that is when a

thread is immediately cancelled by some

other thread without any further notice

that canceled thread or the target

thread may be holding some necessary

system-wide resource now only when the

thread complete is execution it will

release her resource but before it

completes its execution while it is

still holding the resource if it gets a

synchronously cancelled it has no option

than being cancelled and it may not

actually release that resource and hence

that resource may become unavailable so

that is one problem that we have with

asynchronous cancelation but in most

cases the OS will reclaim the system

resources from a canceled thread but in

some cases it will not be able to do so

so in those cases asynchronous

cancellation is not a very good

technique so what is the other technique

that we have it is deferred cancellation

so with deferred cancellation one thread

indicates that a target thread is to be

cancelled but cancellation occurs only

after the target thread has checked a

flag to determine if it should be

cancelled or not

this allows a threat to check whether it

should be cancelled at a point when it

can be canceled safely so this is a much

safer and a much better way of

cancellation instead of directly going

and cancelling a threat immediately as

in the case of a synchronous

cancellation with deferred cancellation

what happens is when one thread that is

a target trend has to be cancelled the

thread that is going to cancel it will

indicate that this target thread has to

be cancelled but will it be cancelled

immediately no the cancellation will

occur only after the target thread has

checked itself and has determined that

it is in a position to be cancelled so

if it is holding some system-wide

resource or if it is in some critical

situation where it is not safe to be

cancelled then it will not be canceled

the thread has the power to defer the

cancellation so it will only cancel at a

point when it can be canceled safely so

that is the better way of thread

cancellation that we have which is

differe cancellation so we see that the

issue that we face in thread

cancellation and we saw the two

scenarios in which it can be cancelled

and we see that most of the problem in

threading issues that is phased due to

thread cancellation can be solved if we

adopt this deferred cancellation

technique so I hope this lecture about

thread cancellation which leads to

threading issues was clear to you thank

you for watching and see you in the next

one

[Applause]

you

[Music]