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I wish to know how multi-threading in a uniprocessor system is helpful my doubt is
when you create the thread it is going to take the execution time slice from the main thread only and other thing is scheduling of threads (context switch between the threads) will also takes considerable amount of time (preemptive kernel) and at a time processor is going to execute only one thread.
Many processes have their speed bound by the slow speed of I/O devices such as disks. Using multiple threads, you can do useful work even while waiting for a slow disk access to complete. Of course, if your process is not I/O bound, then multi-threading on a single processor can cause slow-downs, rather than speed-ups - it's a question of horses for courses.
It can also be helpful to the user experience to use multiple threads, even if things don't actually run faster because of it.
Nothing worse than seeing an entire window refuse to repaint when an operation is going off in the background, especially when there's a progress bar which of course becomes useless.
Because sometimes threading is the most natural way to express your program. Threads provide a way for you to represent tasks that should conceptually run at the same time. Even though, on single processors they obviously can't run at the same time.
One common area to use threading is GUIs, for example. You don't want your GUI to be unresponsive just because there is a lot of work going on in another area of the program. So by splitting off the GUI into another thread, you can still have your GUI responsive despite a lot of computation somewhere else in your program.
If you put the heavy work in separate threads, the gui is still responsive.
Multithreading was invented because it was found that most of the time a program is waiting for I/O. If the processor is shared among other programs this idle time can be made use of. Even though some processor time is spent managing thread/processes this practice was found to be more productive than running one program at a time to the end in sequence.
It depends on the OS, but the scheduler usually considers thread priority as well. For example, for 'real-time' audio applications (e.g. recording the audio with some processing), the processing and recording is more important than the UI refreshment, since the audio signal is lost forever if you miss even a few samples.
Most "pro-grade" audio applications used multi-threading long before multi-core CPU became common-place.
With Uniprocessor systems, multithreading helps in sharing the CPU among multiple tasks so that no one task hogs the CPU till it gets completed.
A good example is a game, where you have to do many things concurrently.
The common approach is to have a main loop where you process events, game logic, physics, graphics and sound; but if those task need to be interleaved in a non static-deterministic way, because some of them take more than one iteration to complete (for example, you're dropping some frames, but the game logic is still running) or you need to sample sound more frequently because otherwise glitches can be heard; the scheduler of you game is likely to become more and more complex...
In that case, you could just split your tasks in threads and let the OS to do the scheduling job for you. But you'll need to design that very carefully because it's very probable that all the threads have to read the same data (the world state) and one or two of them also write it (the game logic and physics) so it's imperative to stablish the proper locks.
Interestingly, when I tried a PLINQ sample (Parallel LINQ i.e. automatic multithreading expressed using LINQ expressions) on my uniprocessor PC, I still gained a roughly 2x speed increase. This baffles me, but my best guess is that it's to do with Hyperthreading. So a single-core CPU can apparently behave as though it is using simultaneous multithreaded execution. I don't really understand hyperthreading, but what I guess is happening is that a second thread is fitted into some time that the first thread would see as the CPU idling.
Worth experimenting.
Multi threading is useful in uniprocessors because a process can be run simultaneously on I/O devices and CPU with the help of multiple threads.
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Given a single core CPU, what is the benefit to coding using threads?
At least with the Java implementation, and it seems intuitive to naturally extend to any other language considering the single core restriction, you may have several threads performing various actions but the processes are time-limited and switched.
Given process A and process B:
What is the benefit of performing half of process A, finish process B, and then finish the second half of process A VS performing process A then B?
It seems that the switching between the threads would introduce time delays that would prolong the overall completion time of both processes VS not switching and just completing A then B.
The reason to use threads on a single-core system is simply to allow processes that would otherwise use all the CPU to be preempted by other tasks that need to get done sooner. The most common reason to make a system multi-threaded is to have a responsive user interface even while performing long calculations.
Of course, any operation can take a long time (reading a file, accessing a database, resizing a photo, recalculating a spreadsheet), and those operations can be performed on a separate thread to allow the thread responding to user input to operate the whole time.
Twenty years ago, for example, it was rare to have a multi-CPU system or an OS that allowed multi-threading, so nearly every program was single-threaded and there were many frameworks created to allow systems to have UIs and still do I/O. The standard mechanism for this is an event loop, where all events (UI, network, timers, etc.) are processed in a big loop.
This type of system means that the UI is held up during things like file I/O and calculations. In order to not hold up the UI too much, you have to do the I/O in chunks (say, read the file 4k at a time), processing any incoming UI events between chunks. This is really just a hack to keep the system running, but it's hard to make the system run smoothly like this because you don't know how often you need to process events.
The solution is to have a separate thread to recalculate your spreadsheet or write your file. That way the OS can give those threads fair timeslices while still preempting them to run the UI, allowing the UI to always be responsive.
An executing thread is not necessarily doing anything useful. The canonical example is reading from disk -- that data isn't going to be there for another few milliseconds, during which time the processor would be sitting unused. Threads allow one piece of the program to use the CPU while other pieces of the program are waiting for operations to complete.
There are many reasons. Wikipedia gives a decent overview on its page about threads.
Here's a few OTOH:
I/O bound tasks benefit from threading (especially network applications).
Hyperthreaded processors may speed up multithreaded applications even on a single core.
Threads can be instructed to wait (block) and wake up on specific events, enabling responsive event-driven programming.
If your program has to do several things "at the same time" then threads are a good way to go, particularly is some of those tasks are quite long running. Otherwise you find yourself writing code that looks like an operating system scheduler inside your program, which is always a waste of time if the OS underneath you has a perfectly good one already. You'd find that your source code was mostly 'scheduler' and not much 'program', which is very inelegant. A good threaded program can be very elegant and economic in source code, which makes oneself look good and saves time.
Some run times get/got it wrong. In the early days of Ada the runtime environment would do its own thread scheduling, and it was never very satisfactory. That was partly due to the fact that whilst the Ada language spec included the concept of threads, the OSes we had back then quite often didn't provide them. Ada got a lot better when the compiler writers started using the underlying OS threads instead.
Similarly Python doesn't really properly use the underlying OS threads; it spoils it with the Global Interpreter Lock. Python has sidestepped the whole issue by going for multiprocessing instead (not necessarily a good thing on Windows hosts...).
Early versions of Windows didn't do threads either, they did cooperative multitasking. This depended on each process in the whole machine calling any OS routine at least now and then. Each OS routine would first consult the 'scheduler' to see if anything else was waiting to run before getting on with whatever it was supposed to be doing on behalf of the program. There were many terrible programs back then that wouldn't play ball and hogged the entire machine. You couldn't get on with playing a game of Solitaire when something else embarked on a length calculation.
What's the mental model of your program?
IF it depends on multiple external inputs that can happen in unpredictable orders, and if what you want to do in response to those inputs is not simple and can overlap in time ...
THEN it makes sense to devote a separate thread to each input request, and have that thread perform the response needed by that request.
So, for example, if your program is waiting for input requests from an external channel, and each request must trigger its own protocol of outgoing and incoming messages, it can very much simplify the code to create a new thread (or re-use an old one) for each request.
Somehow people seem to enter the workforce thinking that threads are only there for speed (through parallelism).
That's one use, provided it allows multiple CPU chips to get cranking,
but it is by no means the only use.
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Hi i am an embedded programmer. Recently we came across a project where we are forced to use multi threading. I have used the same in java but i could not implement it my embedded code for 8051. Could any body please help me?
Threading requires that there be some mechanism to switch threads, typically called a scheduler.
Broadly speaking, there are two types of threading: cooperative, and pre-emptive.
In cooperative threading, each thread does some work and then transfers control back to the scheduler. This is almost like having a grand while(1) {} loop as a program structure, only with more independence (only during development) of the tasks. It still suffers from the risk of one task hogging the CPU, or even locking up and preventing anything else from running. In effect, the independence between tasks is only an illusion or organizational abstraction for the developer.
In pre-emptive multi-tasking, the scheduler (likely driven from a timer interrupt) periodically forces a change of tasks by grabbing execution out of one thread, saving its state, and restarting a different frozen thread. This is a little trickier to set up, but a lot more reliable.
Often with either scheme, you would not write the infrastructure from scratch, but instead would use a primitive operating system or at least scheduler routine developed by others.
For a very small embedded system though, you can also consider that interrupt service routines can themselves provide something akin to alternate threads for handling certain brief and/or urgent tasks. If your serial interrupt fires, you grab some character(s) and store them for later interpretation at a convenient time by something else. Many tasks can be implemented by using interrupts to deal with the immediate part, and then doing resulting work at a later point in a while(1) {} type program structure.
Some might properly laugh at the idea of a scheduler running on an 8051 - though for an oddity of reasons, inexpensive little 8051-equivalent cores end up in some fairly complicated special purpose chips today (typically accessorized by huge amounts of banked memory, and powerful peripheral engines to do the real work), so it's actually not uncommon to see multithreading solutions with dynamic task creation implemented on them in order to manage everything which the device does.
The architecture of the 8051 is not amenable to any reasonable preemptive scheduling. At least the stack, and probably more, in the on-chip RDATA/IDATA has to swapped out to XDATA and it gets very messy.
8051 is good for toaster/washing-machine controllers.
If you want/need such functionality as a premptive scheduler, move to ARM.
I don’t want to make this subjective...
If I/O and other input/output-related bottlenecks are not of concern, then do we need to write multithreaded code? Theoretically the single threaded code will fare better since it will get all the CPU cycles. Right?
Would JavaScript or ActionScript have fared any better, had they been multithreaded?
I am just trying to understand the real need for multithreading.
I don't know if you have payed any attention to trends in hardware lately (last 5 years) but we are heading to a multicore world.
A general wake-up call was this "The free lunch is over" article.
On a dual core PC, a single-threaded app will only get half the CPU cycles. And CPUs are not getting faster anymore, that part of Moores law has died.
In the words of Herb Sutter The free lunch is over, i.e. the future performance path for computing will be in terms of more cores not higher clockspeeds. The thing is that adding more cores typically does not scale the performance of software that is not multithreaded, and even then it depends entirely on the correct use of multithreaded programming techniques, hence multithreading is a big deal.
Another obvious reason is maintaining a responsive GUI, when e.g. a click of a button initiates substantial computations, or I/O operations that may take a while, as you point out yourself.
The primary reason I use multithreading these days is to keep the UI responsive while the program does something time-consuming. Sure, it's not high-tech, but it keeps the users happy :-)
Most CPUs these days are multi-core. Put simply, that means they have several processors on the same chip.
If you only have a single thread, you can only use one of the cores - the other cores will either idle or be used for other tasks that are running. If you have multiple threads, each can run on its own core. You can divide your problem into X parts, and, assuming each part can run indepedently, you can finish the calculations in close to 1/Xth of the time it would normally take.
By definition, the fastest algorithm running in parallel will spend at least as much CPU time as the fastest sequential algorithm - that is, parallelizing does not decrease the amount of work required - but the work is distributed across several independent units, leading to a decrease in the real-time spent solving the problem. That means the user doesn't have to wait as long for the answer, and they can move on quicker.
10 years ago, when multi-core was unheard of, then it's true: you'd gain nothing if we disregard I/O delays, because there was only one unit to do the execution. However, the race to increase clock speeds has stopped; and we're instead looking at multi-core to increase the amount of computing power available. With companies like Intel looking at 80-core CPUs, it becomes more and more important that you look at parallelization to reduce the time solving a problem - if you only have a single thread, you can only use that one core, and the other 79 cores will be doing something else instead of helping you finish sooner.
Much of the multithreading is done just to make the programming model easier when doing blocking operations while maintaining concurrency in the program - sometimes languages/libraries/apis give you little other choice, or alternatives makes the programming model too hard and error prone.
Other than that the main benefit of multi threading is to take advantage of multiple CPUs/cores - one thread can only run at one processor/core at a time.
No. You can't continue to gain the new CPU cycles, because they exist on a different core and the core that your single-threaded app exists on is not going to get any faster. A multi-threaded app, on the other hand, will benefit from another core. Well-written parallel code can go up to about 95% faster- on a dual core, which is all the new CPUs in the last five years. That's double that again for a quad core. So while your single-threaded app isn't getting any more cycles than it did five years ago, my quad-threaded app has four times as many and is vastly outstripping yours in terms of response time and performance.
Your question would be valid had we only had single cores. The things is though, we mostly have multicore CPU's these days. If you have a quadcore and write a single threaded program, you will have three cores which is not used by your program.
So actually you will have at most 25% of the CPU cycles and not 100%. Since the technology today is to add more cores and less clockspeed, threading will be more and more crucial for performance.
That's kind of like asking whether a screwdriver is necessary if I only need to drive this nail. Multithreading is another tool in your toolbox to be used in situations that can benefit from it. It isn't necessarily appropriate in every programming situation.
Here are some answers:
You write "If input/output related problems are not bottlenecks...". That's a big "if". Many programs do have issues like that, remembering that networking issues are included in "IO", and in those cases multithreading is clearly worthwhile. If you are writing one of those rare apps that does no IO and no communication then multithreading might not be an issue
"The single threaded code will get all the CPU cycles". Not necessarily. A multi-threaded code might well get more cycles than a single threaded app. These days an app is hardly ever the only app running on a system.
Multithreading allows you to take advantage of multicore systems, which are becoming almost universal these days.
Multithreading allows you to keep a GUI responsive while some action is taking place. Even if you don't want two user-initiated actions to be taking place simultaneously you might want the GUI to be able to repaint and respond to other events while a calculation is taking place.
So in short, yes there are applications that don't need multithreading, but they are fairly rare and becoming rarer.
First, modern processors have multiple cores, so a single thraed will never get all the CPU cycles.
On a dualcore system, a single thread will utilize only half the CPU. On a 8-core CPU, it'll use only 1/8th.
So from a plain performance point of view, you need multiple threads to utilize the CPU.
Beyond that, some tasks are also easier to express using multithreading.
Some tasks are conceptually independent, and so it is more natural to code them as separate threads running in parallel, than to write a singlethreaded application which interleaves the two tasks and switches between them as necessary.
For example, you typically want the GUI of your application to stay responsive, even if pressing a button starts some CPU-heavy work process that might go for several minutes. In that time, you still want the GUI to work. The natural way to express this is to put the two tasks in separate threads.
Most of the answers here make the conclusion multicore => multithreading look inevitable. However, there is another way of utilizing multiple processors - multi-processing. On Linux especially, where, AFAIK, threads are implemented as just processes perhaps with some restrictions, and processes are cheap as opposed to Windows, there are good reasons to avoid multithreading. So, there are software architecture issues here that should not be neglected.
Of course, if the concurrent lines of execution (either threads or processes) need to operate on the common data, threads have an advantage. But this is also the main reason for headache with threads. Can such program be designed such that the pieces are as much autonomous and independent as possible, so we can use processes? Again, a software architecture issue.
I'd speculate that multi-threading today is what memory management was in the days of C:
it's quite hard to do it right, and quite easy to mess up.
thread-safety bugs, same as memory leaks, are nasty and hard to find
Finally, you may find this article interesting (follow this first link on the page). I admit that I've read only the abstract, though.
Separating different parts of a program into different processes seems (to me) to make a more elegant program than just threading everything. In what scenario would it make sense to make things run on a thread vs. separating the program into different processes? When should I use a thread?
Edit
Anything on how (or if) they act differently with single-core and multi-core would also be helpful.
You'd prefer multiple threads over multiple processes for two reasons:
Inter-thread communication (sharing data etc.) is significantly simpler to program than inter-process communication.
Context switches between threads are faster than between processes. That is, it's quicker for the OS to stop one thread and start running another than do the same with two processes.
Example:
Applications with GUIs typically use one thread for the GUI and others for background computation. The spellchecker in MS Office, for example, is a separate thread from the one running the Office user interface. In such applications, using multiple processes instead would result in slower performance and code that's tough to write and maintain.
Well apart from advantages of using thread over process, like:
Advantages:
Much quicker to create a thread than
a process.
Much quicker to switch
between threads than to switch
between processes.
Threads share data
easily
Consider few disadvantages too:
No security between threads.
One thread can stomp on another thread's
data.
If one thread blocks, all
threads in task block.
As to the important part of your question "When should I use a thread?"
Well you should consider few facts that a threads should not alter the semantics of a program. They simply change the timing of operations. As a result, they are almost always used as an elegant solution to performance related problems. Here are some examples of situations where you might use threads:
Doing lengthy processing: When a windows application is calculating it cannot process any more messages. As a result, the display cannot be updated.
Doing background processing: Some
tasks may not be time critical, but
need to execute continuously.
Doing I/O work: I/O to disk or to
network can have unpredictable
delays. Threads allow you to ensure
that I/O latency does not delay
unrelated parts of your application.
I assume you already know you need a thread or a process, so I'd say the main reason to pick one over the other would be data sharing.
Use of a process means you also need Inter Process Communication (IPC) to get data in and out of the process. This is a good thing if the process is to be isolated though.
You sure don't sound like a newbie. It's an excellent observation that processes are, in many ways, more elegant. Threads are basically an optimization to avoid too many transitions or too much communication between memory spaces.
Superficially using threads may also seem like it makes your program easier to read and write, because you can share variables and memory between the threads freely. In practice, doing that requires very careful attention to avoid race conditions or deadlocks.
There are operating-system kernels (most notably L4) that try very hard to improve the efficiency of inter-process communication. For such systems one could probably make a convincing argument that threads are pointless.
I would like to answer this in a different way. "It depends on your application's working scenario and performance SLA" would be my answer.
For instance threads may be sharing the same address space and communication between threads may be faster and easier but it is also possible that under certain conditions threads deadlock and then what do you think would happen to your process.
Even if you are a programming whiz and have used all the fancy thread synchronization mechanisms to prevent deadlocks it certainly is not rocket science to see that unless a deterministic model is followed which may be the case with hard real time systems running on Real Time OSes where you have a certain degree of control over thread priorities and can expect the OS to respect these priorities it may not be the case with General Purpose OSes like Windows.
From a Design perspective too you might want to isolate your functionality into independent self contained modules where they may not really need to share the same address space or memory or even talk to each other. This is a case where processes will make sense.
Take the case of Google Chrome where multiple processes are spawned as opposed to most browsers which use a multi-threaded model.
Each tab in Chrome can be talking to a different server and rendering a different website. Imagine what would happen if one website stopped responding and if you had a thread stalled due to this, the entire browser would either slow down or come to a stop.
So Google decided to spawn multiple processes and that is why even if one tab freezes you can still continue using other tabs of your Chrome browser.
Read more about it here
and also look here
I agree to most of the answers above. But speaking from design perspective i would rather go for a thread when i want set of logically co-related operations to be carried out parallel. For example if you run a word processor there will be one thread running in foreground as an editor and other thread running in background auto saving the document at regular intervals so no one would design a process to do that auto saving task separately.
In addition to the other answers, maintaining and deploying a single process is a lot simpler than having a few executables.
One would use multiple processes/executables to provide a well-defined interface/decoupling so that one part or the other can be reused or reimplemented more easily than keeping all the functionality in one process.
Came across this post. Interesting discussion. but I felt one point is missing or indirectly pointed.
Creating a new process is costly because of all of the
data structures that must be allocated and initialized. The process is subdivided into different threads of control to achieve multithreading inside the process.
Using a thread or a process to achieve the target is based on your program usage requirements and resource utilization.
What are some concrete examples of applications that need to be multi-threaded, or don't need to be, but are much better that way?
Answers would be best if in the form of one application per post that way the most applicable will float to the top.
There is no hard and fast answer, but most of the time you will not see any advantage for systems where the workflow/calculation is sequential. If however the problem can be broken down into tasks that can be run in parallel (or the problem itself is massively parallel [as some mathematics or analytical problems are]), you can see large improvements.
If your target hardware is single processor/core, you're unlikely to see any improvement with multi-threaded solutions (as there is only one thread at a time run anyway!)
Writing multi-threaded code is often harder as you may have to invest time in creating thread management logic.
Some examples
Image processing can often be done in parallel (e.g. split the image into 4 and do the work in 1/4 of the time) but it depends upon the algorithm being run to see if that makes sense.
Rendering of animation (from 3DMax,etc.) is massively parallel as each frame can be rendered independently to others -- meaning that 10's or 100's of computers can be chained together to help out.
GUI programming often helps to have at least two threads when doing something slow, e.g. processing large number of files - this allows the interface to remain responsive whilst the worker does the hard work (in C# the BackgroundWorker is an example of this)
GUI's are an interesting area as the "responsiveness" of the interface can be maintained without multi-threading if the worker algorithm keeps the main GUI "alive" by giving it time, in Windows API terms (before .NET, etc) this could be achieved by a primitive loop and no need for threading:
MSG msg;
while(GetMessage(&msg, hwnd, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
// do some stuff here and then release, the loop will come back
// almost immediately (unless the user has quit)
}
Servers are typically multi-threaded (web servers, radius servers, email servers, any server): you usually want to be able to handle multiple requests simultaneously. If you do not want to wait for a request to end before you start to handle a new request, then you mainly have two options:
Run a process with multiple threads
Run multiple processes
Launching a process is usually more resource-intensive than lauching a thread (or picking one in a thread-pool), so servers are usually multi-threaded. Moreover, threads can communicate directly since they share the same memory space.
The problem with multiple threads is that they are usually harder to code right than multiple processes.
There are really three classes of reasons that multithreading would be applied:
Execution Concurrency to improve compute performance: If you have a problem that can be broken down into pieces and you also have more than one execution unit (processor core) available then dispatching the pieces into separate threads is the path to being able to simultaneously use two or more cores at once.
Concurrency of CPU and IO Operations: This is similar in thinking to the first one but in this case the objective is to keep the CPU busy AND also IO operations (ie: disk I/O) moving in parallel rather than alternating between them.
Program Design and Responsiveness: Many types of programs can take advantage of threading as a program design benefit to make the program more responsive to the user. For example the program can be interacting via the GUI and also doing something in the background.
Concrete Examples:
Microsoft Word: Edit document while the background grammar and spell checker works to add all the green and red squiggle underlines.
Microsoft Excel: Automatic background recalculations after cell edits
Web Browser: Dispatch multiple threads to load each of the several HTML references in parallel during a single page load. Speeds page loads and maximizes TCP/IP data throughput.
These days, the answer should be Any application that can be.
The speed of execution for a single thread pretty much peaked years ago - processors have been getting faster by adding cores, not by increasing clock speeds. There have been some architectural improvements that make better use of the available clock cycles, but really, the future is taking advantage of threading.
There is a ton of research going on into finding ways of parallelizing activities that we traditionally wouldn't think of parallelizing. Even something as simple as finding a substring within a string can be parallelized.
Basically there are two reasons to multi-thread:
To be able to do processing tasks in parallel. This only applies if you have multiple cores/processors, otherwise on a single core/processor computer you will slow the task down compared to the version without threads.
I/O whether that be networked I/O or file I/O. Normally if you call a blocking I/O call, the process has to wait for the call to complete. Since the processor/memory are several orders of magnitude quicker than a disk drive (and a network is even slower) it means the processor will be waiting a long time. The computer will be working on other things but your application will not be making any progress. However if you have multiple threads, the computer will schedule your application and the other threads can execute. One common use is a GUI application. Then while the application is doing I/O the GUI thread can keep refreshing the screen without looking like the app is frozen or not responding. Even on a single processor putting I/O in a different thread will tend to speed up the application.
The single threaded alternative to 2 is to use asynchronous calls where they return immediately and you keep controlling your program. Then you have to see when the I/O completes and manage using it. It is often simpler just to use a thread to do the I/O using the synchronous calls as they tend to be easier.
The reason to use threads instead of separate processes is because threads should be able to share data easier than multiple processes. And sometimes switching between threads is less expensive than switching between processes.
As another note, for #1 Python threads won't work because in Python only one python instruction can be executed at a time (known as the GIL or Global Interpreter Lock). I use that as an example but you need to check around your language. In python if you want to do parallel calculations, you need to do separate processes.
Many GUI frameworks are multi-threaded. This allows you to have a more responsive interface. For example, you can click on a "Cancel" button at any time while a long calculation is running.
Note that there are other solutions for this (for example the program can pause the calculation every half-a-second to check whether you clicked on the Cancel button or not), but they do not offer the same level of responsiveness (the GUI might seem to freeze for a few seconds while a file is being read or a calculation being done).
All the answers so far are focusing on the fact that multi-threading or multi-processing are necessary to make the best use of modern hardware.
There is however also the fact that multithreading can make life much easier for the programmer. At work I program software to control manufacturing and testing equipment, where a single machine often consists of several positions that work in parallel. Using multiple threads for that kind of software is a natural fit, as the parallel threads model the physical reality quite well. The threads do mostly not need to exchange any data, so the need to synchronize threads is rare, and many of the reasons for multithreading being difficult do therefore not apply.
Edit:
This is not really about a performance improvement, as the (maybe 5, maybe 10) threads are all mostly sleeping. It is however a huge improvement for the program structure when the various parallel processes can be coded as sequences of actions that do not know of each other. I have very bad memories from the times of 16 bit Windows, when I would create a state machine for each machine position, make sure that nothing would take longer than a few milliseconds, and constantly pass the control to the next state machine. When there were hardware events that needed to be serviced on time, and also computations that took a while (like FFT), then things would get ugly real fast.
Not directly answering your question, I believe in the very near future, almost every application will need to be multithreaded. The CPU performance is not growing that fast these days, which is compensated for by the increasing number of cores. Thus, if we will want our applications to stay on the top performance-wise, we'll need to find ways to utilize all your computer's CPUs and keep them busy, which is quite a hard job.
This can be done via telling your programs what to do instead of telling them exactly how. Now, this is a topic I personally find very interesting recently. Some functional languages, like F#, are able to parallelize many tasks quite easily. Well, not THAT easily, but still without the necessary infrastructure needed in more procedural-style environments.
Please take this as additional information to think about, not an attempt to answer your question.
The kind of applications that need to be threaded are the ones where you want to do more than one thing at once. Other than that no application needs to be multi-threaded.
Applications with a large workload which can be easily made parallel. The difficulty of taking your application and doing that should not be underestimated. It is easy when your data you're manipulating is not dependent upon other data but v. hard to schedule the cross thread work when there is a dependency.
Some examples I've done which are good multithreaded candidates..
running scenarios (eg stock derivative pricing, statistics)
bulk updating data files (eg adding a value / entry to 10,000 records)
other mathematical processes
E.g., you want your programs to be multithreaded when you want to utilize multiple cores and/or CPUs, even when the programs don't necessarily do many things at the same time.
EDIT: using multiple processes is the same thing. Which technique to use depends on the platform and how you are going to do communications within your program, etc.
Although frivolous, games, in general are becomming more and more threaded every year. At work our game uses around 10 threads doing physics, AI, animation, redering, network and IO.
Just want to add that caution must be taken with treads if your sharing any resources as this can lead to some very strange behavior, and your code not working correctly or even the threads locking each other out.
mutex will help you there as you can use mutex locks for protected code regions, a example of protected code regions would be reading or writing to shared memory between threads.
just my 2 cents worth.
The main purpose of multithreading is to separate time domains. So the uses are everywhere where you want several things to happen in their own distinctly separate time domains.
HERE IS A PERFECT USE CASE
If you like affiliate marketing multi-threading is essential. Kick the entire process off via a multi-threaded application.
Download merchant files via FTP, unzipping the files, enumerating through each file performing cleanup like EOL terminators from Unix to PC CRLF then slam each into SQL Server via Bulk Inserts then when all threads are complete create the full text search indexes for a environmental instance to be live tomorrow and your done. All automated to kick off at say 11:00 pm.
BOOM! Fast as lightening. Heck you have so much time left you can even download merchant images locally for the products you download, save the images as webp and set the product urls to use local images.
Yep I did it. Wrote it in C#. Works like a charm. Purchase a AMD Ryzen Threadripper 64-core with 256gb memory and fast drives like nvme, get lunch come back and see it all done or just stay around and watch all cores peg to 95%+, listen to the pc's fans kick, warm up the room and the look outside as the neighbors lights flicker from the power drain as you get shit done.
Future would be to push processing to GPU's as well.
Ok well I am pushing it a little bit with the neighbors lights flickering but all else was absolutely true. :)