I'm taking my first steps in GTK+ (C++ and gtkmm more specificaly) and I have a rather conceptual doubts about how to best structure my program. Right now I just want my GUI to show what is happening in my C++ program by printing several values, and since my main thread is halted while the GUI window is running, I've come across solutions that separated both the processing/computing operations and the graphical interface in separate threads. Is this commonly accepted as the best way to do it, not at all, or not even relevant?
Unless you have a good reason, you are generally better off not creating new threads. Synchronization is hard to get right.
GUI programming is event driven (click on a button and something happens). So you will probably need to tie your background processing into the GUI event system.
In the event that your background processing takes a long time, you will need to break it into a number of fast chunks. At the end of each chunk, you can update a progress bar and schedule the next chunk.
This will mean you will need to probably use some state machine patterns.
Also make sure that any IO is non-blocking.
Here's an example of lengthy operation split in smaller chunks using the main loop without additional threads. Lazy Loading using the main loop.
Yes, absolutely! (in response to your title)
The GUI must be run in a separate thread. If you have ever come across those extremely annoying interfaces that lock up while an operation is in progress1, you'd know why it's very important to have the GUI always running regardless of operation happening.
It's a user experience thing.
1 I don't mean the ones that disable some buttons during operation (that's normal), but the ones that everything seems frozen.
This is the reverse: the main thread should be the Gtk one, and the long processing/computing tasks should be done in threads.
The documentation gives a clear example:
https://pygobject.readthedocs.io/en/latest/guide/threading.html
Related
I have a question concerning long calculations:
While executing some tasks of my GUI long calculations might be done. This is not a problem, it just takes a while – everything works fine (at least the results are fine).
What bothers me is that after a certain time my GUI doesn't seem to respond: For example my ProcessBar that is shown during calculations will not be displayed and in the title bar of my GUI the text “keine Rückmeldung” is added (which means something like busy, crashed, etc - sorry I don't know the correct translation which makes it hard for me to find anything in the internet about that issue).
Is there a possibility to stop that behavior?
Thank you.
You should outsource your expensive, long-lasting calculations from the GUI-Thread to a worker thread to prevent your GUI from freezing.
Qt-Documentation: Threading Basics
Good explanation of QThread-usage I found useful: How To Really, Truly Use QThreads
The GUI itself cannot be changed from a worker thread. You have to notify your main-thread about a data-change and update your GUI from there.
You have two options. The more efficient one is to put your calculations into another thread (or multiple threads, there are very few single core CPUs in modern PCs). JSilver's answer has a few links for you.
However, with threads come multitude of threading related things you must learn and take into account. There's a lot of potential for subtle bugs, if you don't know what you're doing. So I would recommend alternative approach as first step, single-threaded. As a bonus, it'll make moving to multi-threaded solution much easier later.
Create a plain sublclass of QObject. Into this QObject, put the state of your calculation as member variables.
Write a slot method into above class, which does a small piece of the calculation, then returns. It should do it's thing at most around 50 ms for good user experience. You can just use a fixed number of iterations in your loop, or use QElapsedTimer to measure time, or whatever. And then, when called again, the method should continue the calculation again for another 50ms. When calculation completes, the method can for example emit a signal with the results.
Add a QTimer with interval 0. Connect the timeout to the slot method described above. Interval 0 here effectively means, Qt will call the method as often as it can. You want this, because you want the calculation to finish as quickly as possible of course. However, since the method returns very soon, then Qt can do other stuff (update GUI etc), before calling your method again.
Once this works, in single thread, you can then learn to do Qt threading and move the worker object to live in another thread, for potentially increased performance. Also then you will have a single-threaded baseline version to compare to, in case you run into threading problems.
When making a program with Qt, we can have long recursive process,
If so, after a while, windows show the "Dont answer" message next to the window title.
This message could lead the user to think the program don't work which is not true.
How can I do to avoid this message in Qt?
In order to remain responsive to the system and user input, put a long running task into its own thread. You might also want to provide feedback to the user, like a progress bar, so he sees the program is still doing some job he requested.
See also Threading Basics for an introduction on using threads with Qt and Threading and Concurrent Programming Examples for some examples.
If your process takes a long time because of loops (or recursive functions), you can call QCoreApplication::processEvents() in your loop to ask your application to treat events.
If you have only one instruction that take a long time (such as copy a large file), you may use QThread or QtConcurrent.
While Olaf's answer is good, a simpler approach would be to sprinkle QCoreApplication::processEvents() in your code.
From the docs:
Processes all pending events for the calling thread according to the
specified flags until there are no more events to process. You can
call this function occasionally when your program is busy performing a
long operation (e.g. copying a file).
I'm working on a game server, written in C++, and I'm trying to decide how many threads to use and what tasks to thread. The basic server skeleton consists of keyboard I/O and output to a console, accepting incoming connects, sending outgoing connects, and doing the game "stuff".
What I'd like to know is which things should be given a separate thread. Should each connect have its own thread? I know this is variable, it depends on the project or so, but I would like it to support a pretty decent number of players (somewhere in the hundreds if possible).
The standard answer should always be: Try it the simplest way first, and only look for ways to improve performance if the simple way isn't good enough. However, re-architecting a large C++ program can be a painful experience, so some guesses about performance in advance may be appropriate.
Theoretically, hundreds of threads are probably OK on modern machines. The NPTL implementation for Linux was tested with tens of thousands of threads, as I recall. If that's the easiest way for you to implement, it may be the right answer.
However, high-performance web servers and similar typically use event-driven models instead. Consider a library like libevent. I'm sure there are C++ libraries for the same purpose.
I personally believe that languages without first-class continuations, or at least coroutines, are poor choices for this kind of work, but the C language family is how we get work done today, so off we go. :-)
A good solution could be to use a Thread pool.
Idea is to let the main thread dispatch equitably all connexions in a fixed number of threads.
With a good design, you can easily set the number of thread on runtime.
You can find more informations here.
Create more threads than you have CPU cores is not productive, and adding too threads decrease performances due to time taken for switching between threads.
By example, for compiling a large project (it's not exactly the same thing, but it's valid for both case), it's often recommended to use no more thread than number of CPU cores + 1.
A very common technique is to have the game server run on one thread to monitor several connections (i.e. sockets) by using a select on each socket. When data is available, grab the data and enqueue it in a producer/consumer type model for the game engine to pick up.
This is by no means the be-all-end-all implementation, but it should be enough to get you started. Sounds like a cool project. Good luck!
If you setup the connections and utilize them in a manner that cause the thread to block waiting on IO then you should be able to service all of the connections and the keyboard on one thread. You may not want to put the console output on that same thread, as I've seen cases (on windows at least), where the speed of writing to the console is actually a bottleneck (i.e. if the console window is minimized the process runs considerably faster).
If the work of your game engine parallelizes well then you probably want to set use as many threads as there are CPUs less one (for the OS and the other two threads). If you expect the client to run on the same machine the server will want to detect that and scale back the number of threads it uses.
Usually developing applications I am used to print to console in order to get useful debugging/tracing information. The application I am working now since it is multi-threaded sometimes I see my printf overlapping each other.
I tried to synchronize the screen using a mutex but I end up in slowing and blocking the app. How to solve this issue?
I am aware of MT logging libraries but in using them, since I log too much, I slow ( a bit ) my app.
I was thinking to the following idea..instead of logging within my applications why not log outside it? I would like to send logging information via socket to a second application process that actually print out on the screen.
Are you aware of any library already doing this?
I use Linux/gcc.
thanks
afg
You have 3 options. In increasing order of complexity:
Just use a simple mutex within each thread. The mutex is shared by all threads.
Send all the output to a single thread that does nothing but the logging.
Send all the output to a separate logging application.
Under most circumstances, I would go with #2. #1 is fine as a starting point, but in all but the most trivial applications you can run in to problems serializing the application. #2 is still very simple, and simple is a good thing, but it is also quite scalable. You still end up doing the processing in the main application, but for the vast majority of applications you gain nothing by spinning this off to it's own, dedicated application.
Number 3 is what you're going to do in preformance-critical server type applications, but the minimal performance gain you get with this approach is 1: very difficult to achieve, 2: very easy to screw up, and 3: not the only or even most compelling reason people generally take this approach. Rather, people typically take this approach when they need the logging service to be seperated from the applications using it.
Which OS are you using?
Not sure about specific library's, but one of the classical approaches to this sort of problem is to use a logging queue, which is worked by a writer thread, who's job is purely to write the log file.
You need to be aware, either with a threaded approach, or a multi-process approach that the write queue may back up, meaning it needs to be managed, either by discarding entries or by slowing down your application (which is obviously easier if it's the threaded approach).
It's also common to have some way of categorising your logging output, so that you can have one section of your code logging at a high level, whilst another section of your code logs at a much lower level. This makes it much easier to manage the amount of output that's being written to files and offers you the option of releasing the code with the logging in it, but turned off so that it can be used for fault diagnosis when installed.
As I know critical section has less weight.
Critical section
Using critical section
If you use gcc, you could use atomic accesses. Link.
Frankly, a Mutex is the only way you really want to do that, so it's always going to be slow in your case because you're using so many print statements.... so to solve your question then, don't use so many print_f statements; that's your problem to begin with.
Okay, is your solution using a mutex to print? Perhaps you should have a mutex to a message queue which another thread is processing to print; that has a potential hang up, but I think will be faster. So, use an active logging thread that spins waiting for incoming messages to print. The networking solution could work too, but that requires more work; try this first.
What you can do is to have one queue per thread, and have the logging thread routinely go through each of these and post the message somewhere.
This is fairly easy to set up and the amount of contention can be very low (just a pointer swap or two, which can be done w/o locking anything).
I have a small architecture doubt about organizing code in separate functional units (most probably threads?). Application being developed is supposed to be doing the following tasks:
Display some images on a screen (i.e. slideshow)
Read the data from external device through the USB port
Match received data against the corresponding image (stimulus)
Do some data analysis
Plot the results of data analysis
My thoughts were to organize the application into the following modules:
GUI thread (+ image slideshow)
USB thread buffering the received data
Thread for analyzing/plotting data (main GUI thread should not be blocked while plotting the data which might consume some more time)
So, what do you generally think about this concept? Is there anything else you think that might be a better fit in this particular scenario?
You can probably get away with combining 1 & 2, since the slide-show feature is essentially gui oriented anyway.
For #3, you may be able to make do with some kind of asynchronous I/O methodology, so that you don't need to dedicate a polling thread. Not sure if you can do this with USB, but you can certainly get async I/O with serial and network interfaces, so it's worth looking into.
It's probably a good idea to move heavy-weight tasks like 4 & 5 to their own thread. If you aren't doing the analysis and plotting concurrently, maybe one thread for them both. However, you should really consider how much cpu time these activities will need. If the worst-case analyze-and-plot takes much less than half a second, you might even just perform these actions with a call from the gui. Conversely, if there are cases where this will take longer than that, a separate thread is favorable b/c your users won't like a laggy gui.
Just bear in mind that the dark side of threads lies in the inevitable challenge of coordinating them.
Because of the way the Windows API works, especially with regard to user input and window ownership. You can really only do UI on a single thread. If you try and use multiple threads, they just end up locking each other out and only 1 thread runs at a time. There are some specialized exceptions, but you have to be a real master of the API to pull it off.
So.
GUI thread, owns the Window, and handles all user input.
USB listening thread, you would know better than I whether this makes sense
Thread(s) for analyzing/plotting data, once again, I can't speak to this, but I'm skeptical that they will really both be running at the same time. It seems more likely this it would be analyze then plot so 1 thread.
Thread for rendering frames for a slideshow.
I'm not sure how plotting isn't the same thing as the slideshow, but I do think you can have a background thread for drawing the slideshow as long as it doesn't display the images.
You can render (i.e. draw to a bitmap or DirectX surface) in a background thread, you just can't show it in a window. But you could hand completed bitmaps off to the GUI thread and have it do the actual displaying of the bitmap. This is essentially how a lot of video playback code works.
A lot of this depends on how much is involved in performing 3 (Do some data analysis.) and 4 (Plot analyzed data.)
My instincts would be:
Definitely have a separate thread for reading the data off the USB. Assuming for a moment that 3 is dependent on reading the data, then I would do 3 in the same thread as reading the data. This will simplify your signaling to the GUI when the data is ready. This also assumes the processing is quick, and won't block the USB port (How is that being read? IO completion ports?). If the processing takes time then you need a separate thread.
Likewise if image slide processing show takes a long time, this should be done in a separate thread. If this can be quickly recalculated depending say in a paint function, I would keep it as part of the main GUI.
There is some overhead with context switch of threads, and for each thread added complexity of signaling. So I would only add a thread to solve blocking of the GUI and the USB port. It may be possible to do all of this in just two threads.
4 and 5 are definitely good ideas. That being said, avoid using low-level threads unless you absolutely must.
I'd check out Boost and Boost::Thread. Not only does it make your code more portable, but I haven't worked with an easier library for threading.
If you are using Builder 2009, you should look at TThread. It has some stuff to simplify thread coding.
I can't help thinking that you may be going a bit overboard here. A USB port can't really deliver data terribly quickly -- it's theoretical bandwidth is only 480 Mbits/second, and realistically, it's a pretty rare USB device that can really get very close to that.
Unless the analysis you've mentioned is quite a bit more complex than you've implied, my guess is that a single thread is probably entirely adequate. I'd think hard about using overlapped I/O to read the data, and MsgWaitForMultipleObjects for the main message loop.
It seems to me that the main place you stand a good chance of gaining a lot is in plotting the data after it's processed. It might be worth considering something like OpenGL or DirectX Graphics to do the drawing. Especially if you're producing quite a bit of output, this can give a really substantial speed improvement. In an ideal situation, multiple threads might multiply your speed by the number of available cores -- typically 2 or 4 on today's machines. Drawing the output is likely to be the slowest part of the job, and hardware acceleration can easily speed that up by a considerably larger factor -- 10x is at the low end of what you can typically expect, and 100x is fairly common.