QStandardItemModel* from non-GUI thread didn`t emit itemChanged signal - multithreading

I have started database loading in non-GUI thread with QtCuncurrent::run.
In this nonGui thread I have to create QStandardItemModel* and after that I received model in GUI thread with
model = modelWatcher.result();
on QFutureWatcher finished() signal. It works pretty (UI is builded successfully), but itemChanged() signal is not emitted on item data changes (checkbox state changed). When I creates the model in GUI thread, there are no collisions. Connect works without assert fails:
bool ok = connect(model, SIGNAL(itemChanged(QStandardItem*)), this, SLOT(onFolderStateChanged(QStandardItem*)), static_cast<Qt::ConnectionType>(Qt::UniqueConnection));
Q_ASSERT(ok);
As I can see in that thread (there are no code samples and I misunderstood the main idea) I can't to create model (part of Qt5Gui) in nonGui thread. But it works for me! Ui is builded) Also I have to declare sended type with:
qRegisterMetaType<QStandardItemModel*>("QStandardItemModel*");
And my other sends like:
qRegisterMetaType<QList<QTreeWidgetItem*> >("QList<QTreeWidgetItem*>");
works good (though its also Qt5Gui part).
I dont understand how can I **get the model from nonGui thread with full functionality** like itemChanged signals?
Its something like emit mysignal(QStandardItemModel*); ?
In that case, why other tasks works fine without any emit`s ? including currentChanged signals etc.

When I send model from nonGui thread, some signals have been lost, because model pointer and related data is incorrect if I'm not mistaken. Also QStandardItemModel - is a part of Qt5Gui and its not threadsafe. It means, that model and view should be collected in GUI thread, and data must be sended from worker thread and binded to the model in GUI thread. Its not simple in my situation - I have big tree and I don't want to create my own struct for parsing the tree - but it is a "true way").
I'm using more simple decision - I just sends the parentItem pointer to the new model in GUI thread - and it works.
If anybody knows how to send tree structure from worker thread simple - please tell it in this thread)
In any case - using text-represented data transfer between threads - its more preferred method (like JSON/XML...any yourself representation, based on indexes of items, listed in ``QList>` etc...)

Have you tried this ?
bool ok = connect(model, SIGNAL(itemChanged(QStandardItem*)), this,
SLOT(onFolderStateChanged(QStandardItem*)),
static_cast<Qt::ConnectionType>(Qt::AutoConnection | Qt::UniqueConnection));

Related

Vulkan theaded application get error message on queue submissions under mutex

I have an application with Vulkan for rendering and glfw for windowing. If I start several threads, each with a different window, I get errors on threading and queue submission even though ALL vulkan calls are protected by a common mutex. The vulkan layer says:
THREADING ERROR : object of type VkQueue is simultaneously used in thread 0x0 and thread 0x7fc365b99700
Here is the skeleton of the loop under which this happens in each thread:
while (!finished) {
window.draw(...);
std::this_thread::sleep_for(std::chrono::milliseconds(10));
}
The draw function skeleton looks like:
draw(Arg arg) {
static std::mutex mtx;
std::lock_guard lock{mtx};
// .... drawing calls. Including
device.acquireNextImageKHR(...);
// Fill command bufers
graphicsQueue.submit(...);
presentQueue.presentKHR(presentInfo);
}
This is C++17 which slightly simplifies the syntax but is otherwise irrelevant.
Clearly everything is under a mutex. I also intercept the call to the debug message. When I do so, I see that one thread is waiting for glfw events, one is printing the vulkan layer message and the other two threads are trying to acquire the mutex for the lock_guard.
I am at a loss as to what is going on or how to even figure out what is causing this.
I am running on linux, and it does not crash. However on Mac OS X, after a random amount of time, the code will crash in a queue submit call of MoltenVK and when the crash happens, I see a similar situation of the threads. That is to say no other thread is inside a Vulkan call.
I'd appreciate any ideas. My next move would be to move all queue submissions to a single thread, though that is not my favorite solution.
PS: I created a complete MCVE under the Vookoo framework. It is at https://github.com/FunMiles/Vookoo/tree/lock_guard_queues and is the example 00-parallelTriangles
To try it, do the following:
git clone https://github.com/FunMiles/Vookoo.git
cd Vookoo
git checkout lock_guard_queues
mkdir build
cd build
cmake ..
make
examples/00-parallelTriangles
The way you call the draw is:
window.draw(device, fw.graphicsQueue(), [&](){//some lambda});
The insides of draw is protected by mutex, but the fw.graphicsQueue() isn't.
fw.graphicsQueue() million abstraction layers below just calls vkGetDeviceQueue. I found executing vkGetDeviceQueue in parallel with vkQueueSubmit causes the validation error.
So there are few issues here:
There is a bug in layers that causes multiple initialization of VkQueue state on vkGetDeviceQueue, which is the cause of the validation error
KhronosGroup/Vulkan-ValidationLayers#1751
Thread id 0 is not a separate issue. As there are not any actual previous accesses, thread id is not recorded. The problem is the layers issue the error because the access count goes into negative because it is previously wrongly reset to 0.
Arguably there is some spec issue here. It is not immediatelly obvious from the text that VkQueue is not actually accessed in vkGetDeviceQueue, except the silent assumption that it is the sane thing to do.
KhronosGroup/Vulkan-Docs#1254

How to sync Delphi event while running DB operations in a background thread?

Using Delphi 7 & UIB, I'm running database operations in a background thread to eliminate problems like:
Timeout
Priority
Immediate Force-reconnect after network-loss
Non-blocked UI
Keeping an opened DB connection alive
User canceling
I've read ALL related topics here, and realized: using while isMyThreadStillRuning and not UserCanceled do sleep(100); end; isn't the recommended way to do this, but rather using TEvent.WaitFor(3000)....
The solutions here are either about sending signals FROM or TO... the thread, or doing it with messages, but never both ways.
Reading the help file, I've also found TSimpleEvent, which seems to be easier to use.
So what is the recommended way to communicate between Main-UI + DB-Thread in both ways?
Should I simply create 2+2 TSimpleEvent?
to start a new transaction (thread should stop sleeping)
force-STOP execution
to signal back if it's moved to a new stage (transaction started / executed / commited=done)
to signal back if there is any error happened
or should there be only 1 TEvent?
Update 2:
First tests show:
2x TSimpleEvent is enough (1 for Thread + 1 for Gui)
Both created as public properties of the background thread
Force-terminating the thread does not work. (Too many errors impossible to handle..)
Better to set a variable like (Stop_yourself) and let it cancel and free itself, (while creating a new instance from the same class and try again.)
(still work in progress...)
You should move the query to a TThread. Unfortunately, anonymous threads are not available in D7 so you need to write your own TThread derived class. Inside, you need its own DB connection to prevent shared resources. From the caller method, you can wait for the thread to end. The results should be stored somewhere in the caller class. Ensure that the access to parameters of the query and for storing the result of the query is handled thread-safe by using a TMutex or TMonitor.

NSURLSession dataTaskWithURL

I am using NSURLSession dataTaskWithURL:completionHandler. It looks like completionHandler is executed in a thread which is different than the thread(in my case, it's the main thread) which calls dataTaskWithURL. So my question is, since it is asynchronized, is it possible that the main thread exit, but the completionHandler thread is still running since the response has not come back, which is the case I am trying to avoid. If this could happen, how should I solve the problem? BTW, I am building this as a framework, not an application.Thanks.
In the first part of your question you seem un-sure that the completion handler is running on a different thread. To confirm this let's look at the NSURLSession Class Reference. If we look at the "Creating a Session" section we can see in the description for the following method the answer.
+ sessionWithConfiguration:delegate:delegateQueue:
Swift
init(configuration configuration: NSURLSessionConfiguration,
delegate delegate: NSURLSessionDelegate?,
delegateQueue queue: NSOperationQueue?)
Objective-C
+ (NSURLSession *)sessionWithConfiguration:(NSURLSessionConfiguration *)configuration
delegate:(id<NSURLSessionDelegate>)delegate
delegateQueue:(NSOperationQueue *)queue
In the parameters table for the NSOperationQueue queue parameter is the following quote.
An operation queue for scheduling the delegate calls and completion handlers. The queue need not be a serial queue. If nil, the session creates a serial operation queue for performing all delegate method calls and completion handler calls.
So we can see the default behavior is to provide a queue whether from the developer or as the default class behavior. Again we can see this in the comments for the method + sessionWithConfiguration:
Discussion
Calling this method is equivalent to calling
sessionWithConfiguration:delegate:delegateQueue: with a nil delegate
and queue.
If you would like a more information you should read Apple's Concurrency Programming Guide. This is also useful in understanding Apple's approach to threading in general.
So the completion handler from - dataTaskWithURL:completionHandler: is running on a different queue, with queues normally providing their own thread(s). This leads the main component of your question. Can the main thread exit, while the completion handler is still running?
The concise answer is no, but why?
To answer this answer this we again turn to Apple's documentation, to a document that everyone should read early in their app developer career!
The App Programming Guide
The Main Run Loop
An app’s main run loop processes all user-related events. The
UIApplication object sets up the main run loop at launch time and uses
it to process events and handle updates to view-based interfaces. As
the name suggests, the main run loop executes on the app’s main
thread. This behavior ensures that user-related events are processed
serially in the order in which they were received.
All of the user interact happens on the main thread - no main thread, no main run loop, no app! So the possible condition you question mentions should never exist!
Apple seems more concerned with you doing background work on the main thread. Checkout the section "Move Work off the Main Thread"...
Be sure to limit the type of work you do on the main thread of your
app. The main thread is where your app handles touch events and other
user input. To ensure that your app is always responsive to the user,
you should never use the main thread to perform long-running or
potentially unbounded tasks, such as tasks that access the network.
Instead, you should always move those tasks onto background threads.
The preferred way to do so is to use Grand Central Dispatch (GCD) or
NSOperation objects to perform tasks asynchronously.
I know this answer is long winded, but I felt the need to offer insight and detail in answering your question - "the why" is just as important and it was good review :)
NSURLSessionTasks always run in background by default that's why we have completion handler which can be used when we get response from Web service.
If you don't get any response explore your request URL and whether HTTPHeaderFields are set properly.
Paste your code so that we can help it
I just asked the same question. Then figured out the answer. The thread of the completion handler is setup in the init of the NSURLSession.
From the documentation:
init(configuration configuration: NSURLSessionConfiguration,
delegate delegate: NSURLSessionDelegate?,
delegateQueue queue: NSOperationQueue?)`
queue - A queue for scheduling the delegate calls and completion handlers. If nil, the session creates a serial operation queue for performing all delegate method calls and completion handler calls.*
My code that sets up for completion on main thread:
var session = NSURLSession(configuration: configuration, delegate:nil, delegateQueue:NSOperationQueue.mainQueue())
(Shown in Swift, Objective-C the same) Maybe post more code if this does not solve.

QThread execution freezes my GUI

I'm new to multithread programming. I wrote this simple multi thread program with Qt. But when I run this program it freezes my GUI and when I click inside my widow, it responds that your program is not responding .
Here is my widget class. My thread starts to count an integer number and emits it when this number is dividable by 1000. In my widget simply I catch this number with signal-slot mechanism and show it in a label and a progress bar.
Widget::Widget(QWidget *parent) :
QWidget(parent),
ui(new Ui::Widget)
{
ui->setupUi(this);
MyThread *th = new MyThread;
connect( th, SIGNAL(num(int)), this, SLOT(setNum(int)));
th->start();
}
void Widget::setNum(int n)
{
ui->label->setNum( n);
ui->progressBar->setValue(n%101);
}
and here is my thread run() function :
void MyThread::run()
{
for( int i = 0; i < 10000000; i++){
if( i % 1000 == 0)
emit num(i);
}
}
thanks!
The problem is with your thread code producing an event storm. The loop counts very fast -- so fast, that the fact that you emit a signal every 1000 iterations is pretty much immaterial. On modern CPUs, doing a 1000 integer divisions takes on the order of 10 microseconds IIRC. If the loop was the only limiting factor, you'd be emitting signals at a peak rate of about 100,000 per second. This is not the case because the performance is limited by other factors, which we shall discuss below.
Let's understand what happens when you emit signals in a different thread from where the receiver QObject lives. The signals are packaged in a QMetaCallEvent and posted to the event queue of the receiving thread. An event loop running in the receiving thread -- here, the GUI thread -- acts on those events using an instance of QAbstractEventDispatcher. Each QMetaCallEvent results in a call to the connected slot.
The access to the event queue of the receiving GUI thread is serialized by a QMutex. On Qt 4.8 and newer, the QMutex implementation got a nice speedup, so the fact that each signal emission results in locking of the queue mutex is not likely to be a problem. Alas, the events need to be allocated on the heap in the worker thread, and then deallocated in the GUI thread. Many heap allocators perform quite poorly when this happens in quick succession if the threads happen to execute on different cores.
The biggest problem comes in the GUI thread. There seems to be a bunch of hidden O(n^2) complexity algorithms! The event loop has to process 10,000 events. Those events will be most likely delivered very quickly and end up in a contiguous block in the event queue. The event loop will have to deal with all of them before it can process further events. A lot of expensive operations happen when you invoke your slot. Not only is the QMetaCallEvent deallocated from the heap, but the label schedules an update() (repaint), and this internally posts a compressible event to the event queue. Compressible event posting has to, in worst case, iterate over entire event queue. That's one potential O(n^2) complexity action. Another such action, probably more important in practice, is the progressbar's setValue internally calling QApplication::processEvents(). This can, recursively call your slot to deliver the subsequent signal from the event queue. You're doing way more work than you think you are, and this locks up the GUI thread.
Instrument your slot and see if it's called recursively. A quick-and-dirty way of doing it is
void Widget::setNum(int n)
{
static int level = 0, maxLevel = 0;
level ++;
maxLevel = qMax(level, maxLevel);
ui->label->setNum( n);
ui->progressBar->setValue(n%101);
if (level > 1 && level == maxLevel-1) {
qDebug("setNum recursed up to level %d", maxLevel);
}
level --;
}
What is freezing your GUI thread is not QThread's execution, but the huge amount of work you make the GUI thread do. Even if your code looks innocuous.
Side Note on processEvents and Run-to-Completion Code
I think it was a very bad idea to have QProgressBar::setValue invoke processEvents(). It only encourages the broken way people code things (continuously running code instead of short run-to-completion code). Since the processEvents() call can recurse into the caller, setValue becomes a persona-non-grata, and possibly quite dangerous.
If one wants to code in continuous style yet keep the run-to-completion semantics, there are ways of dealing with that in C++. One is just by leveraging the preprocessor, for example code see my other answer.
Another way is to use expression templates to get the C++ compiler to generate the code you want. You may want to leverage a template library here -- Boost spirit has a decent starting point of an implementation that can be reused even though you're not writing a parser.
The Windows Workflow Foundation also tackles the problem of how to write sequential style code yet have it run as short run-to-completion fragments. They resort to specifying the flow of control in XML. There's apparently no direct way of reusing standard C# syntax. They only provide it as a data structure, a-la JSON. It'd be simple enough to implement both XML and code-based WF in Qt, if one wanted to. All that in spite of .NET and C# providing ample support for programmatic generation of code...
The way you implemented your thread, it does not have its own event loop (because it does not call exec()). I'm not sure if your code within run() is actually executed within your thread or within the GUI thread.
Usually you should not subclass QThread. You probably did so because you read the Qt Documentation which unfortunately still recommends subclassing QThread - even though the developers long ago wrote a blog entry stating that you should not subclass QThread. Unfortunately, they still haven't updated the documentation appropriately.
I recommend reading "You're doing it wrong" on Qt Blog and then use the answer by "Kari" as an example of how to set up a basic multi-threaded system.
But when I run this program it freezes my GUI and when I click inside my window,
it responds that your program is not responding.
Yes because IMO you're doing too much work in thread that it exhausts CPU. Generally program is not responding message pops up when process show no progress in handling application event queue requests. In your case this happens.
So in this case you should find a way to divide the work. Just for the sake of example say, thread runs in chunks of 100 and repeat the thread till it completes 10000000.
Also you should have look at QCoreApplication::processEvents() when you're performing a lengthy operation.

QPointer in multi-threaded programs

According to http://doc.qt.io/qt-5/qpointer.html, QPointer is very useful. But I found it could be inefficient in the following context:
If I want to show label for three times or do something else, I have to use
if(label) label->show1();
if(label) label->show2();
if(label) label->show3();
instead of
if(label) { label->show1();label->show2();label->show3(); }
just because label might be destroyed in another thread after label->show1(); or label->show2();.
Is there a beautiful way other than three ifs to get the same functionality?
Another question is, when label is destroyed after if(label), is if(label) label->show1(); still wrong?
I don't have experience in multi-threaded programs. Any help is appreciated. ;)
I think the only safe way to do it is to make sure you only access your QWidgets from within the main/GUI thread (that is, the thread that is running Qt's event loop, inside QApplication::exec()).
If you have code that is running within a different thread, and that code wants the QLabels to be shown/hidden/whatever, then that code needs to create a QEvent object (or a subclass thereof) and call qApp->postEvent() to send that object to the main thread. Then when the Qt event loop picks up and handles that QEvent in the main thread, that is the point at which your code can safely do things to the QLabels.
Alternatively (and perhaps more simply), your thread's code could emit a cross-thread signal (as described here) and let Qt handle the event-posting internally. That might be better for your purpose.
Neither of your approaches is thread-safe. It's possible that your first thread will execute the if statement, then the other thread will delete your label, and then you will be inside of your if statement and crash.
Qt provides a number of thread synchronization constructs, you'll probably want to start with QMutex and learn more about thread-safety before you continue working on this program.
Using a mutex would make your function would look something like this:
mutex.lock();
label1->show();
label2->show();
label3->show();
mutex.unlock()
As long as your other thread is using locking that same mutex object then it will prevented from deleting your labels while you're showing them.

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