Description:

Since Qt (qt5-base and others) was updated to 5.7 the version of qjackctl started spiking CPU usage. It keeps one thread maxed to 100%. This is independent of jackd itself. It can be triggered by simply touching a few interface elements in qjackctl.

I downloaded PKGBUILD for qjackctl and built it on my local machine against the lastest available libraries and the problem persisted.

The current workaround I have found, short of downgrading Qt to 5.6 which would probably be hard because many other applications are compiled against it, is to enable Qt4 via a switch to configure in PKGBUILD and compile qjackctl against that. The application returns to its stable normal behavior and successfully used Qt4 (as its older versions did).


Additional info:

* qjackctl 0.4.2
* qt5-base (and others) 5.7.0

Steps to reproduce:

1. Start qjackctl compiled against Qt 5.7.0 available in Arch repos as of 2016-07-03.

2. Don't start jackd or do anything else, just to be sure.

3. Click on the 'Messages' button.

4. The 'Messages' box first shows a few interface related errors from Qt and then keeps printing 'update()' endlessly.
4a. If you press the 'Start' button to start jackd, or do many other UI tasks, the same happens.

5. One CPU thread is maxed out at 100% as long as qjackctl is kept running.

Workaround:

In PKGBUILD's build() hook change the exports to -qt4 version and supply --enable-qt4 to configure script.

Thoughts:

I think this is an upstream issue which will catch up with others as they update to newer Qt. That, or it is a Qt issue, which is less likely. I'll be grateful if someone with access to SourceForge also files a bug there. I don't have an account there and, honestly, don't wish to touch SF but the project's main development is hosted there apparently.