Qt signals reach UI thread with noticeable delay

Question

I have a worker thread in my application, which occasionally recieves information that should be quickly displayed in QML UI. When I have such portion of information, I emit signal, which is received by object that lives on UI thread.

I want this process to be very responsive, so that changes are displayed in QML UI as quickly as possible (this matters because worker thread handles external controller, and I want the shortest "critical path" between user interaction with controller and UI change).

However I discovered, that the time difference between emit signal() and slot called in UI thread is always 20-40 milliseconds. Why so?

What can I do to speed up this? I tried calling QCoreApplication::processEvents() in worker thread after signal is emitted, but this barely changes anything.

Some thoughts:

• Can I call processEvents but for UI thread somehow?
• Use event with high priority instead of signal. Will it help?

OS: Win8.1, Qt 5.5

Answer 1

When you emit a signal from a worker thread to the UI thread, it is put into the UI event queue, and delivered when the event queue is pumped and reaches that message. If your worker thread is a higher priority than your UI thread, then the UI thread will have to wait until the worker thread blocks. If the worker thread is the same priority it will complete it's time quanta, which may be 20ms. You can make your signal connect Direct rather than Queued, and then you will need to take care of thread safety yourself.