Threading Issues (Thread Cancellation)
Summary
TLDRThis 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
🔍 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.
🚧 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.
🛠️ 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.
Mindmap
Keywords
💡Thread Cancellation
💡Asynchronous Cancellation
💡Deferred Cancellation
💡Target Thread
💡Resource Allocation
💡Shared Data
💡Operating System (OS)
💡Thread Safety
💡Concurrency
💡Web Browser 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]
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