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Details

Type: Bug
Resolution: Unresolved
Priority: P2: Important
Fix Version/s: None
Affects Version/s: 5.10.1, 5.12.1
Component/s: GUI: Window management, Widgets: Main Window, Widgets: Widgets and Dialogs
Labels:
None
Environment:
Qt Creator 4.8.2, MacBook Pro Retina (2014) and MacBook 12 (2017) on macOS 10.14.4

Platform/s:

macOS

Description

In my audio application I am trying to continuously update a widget that is part of a custom QDockWidget (peak meter). Think of the Qt demo application "Audio Input Example", just encapsuled in a dock widget. On Windows this is working perfectly fine (CPU load <1% on the same machine). But on macOS it is causing a serious performance impact (10%-95% depending on MainWindow size and CPU power).

This is probably a bug. Any help is appreciated. See my investigations below.

I've built this basic example in order to reproduce this issue (see attachment):

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QDockWidget>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connect(&m_timer, SIGNAL(timeout()), this, SLOT(onTimeout()));
    m_timer.start(30);
    m_projectsDock = new QDockWidget(tr("Test"), this);
    addDockWidget(Qt::LeftDockWidgetArea, m_projectsDock);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::onTimeout()
{
   m_projectsDock->update();
}

I've already profiled the application. It seems like the complete background (backing store) is copied every time an update is issued.

void QImage::detach()
{
    if (d) {
        if (d->is_cached && d->ref.load() == 1)
            QImagePixmapCleanupHooks::executeImageHooks(cacheKey());

        if (d->ref.load() != 1 || d->ro_data)
            *this = copy();

        if (d)
            ++d->detach_no;
    }
}

Somehow d is refereced more than once.

Heaviest Stack Trace:

  32  6150.0  PerformanceTest (12139) :0
  31  6147.0  Main Thread  0x33d05 :0
  30 libdyld.dylib 6147.0  start
  29 PerformanceTest 6147.0  main PerformanceTest/main.cpp:10
  28 QtCore 6147.0  QCoreApplication::exec()
  27 QtCore 6147.0 QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>)
  26 libqcocoa.dylib 6147.0
QCocoaEventDispatcher::processEvents(QFlags<QEventLoop::ProcessEventsFlag>)
  25 AppKit 6147.0  -[NSApplication run]
  24 AppKit 6147.0  -[NSApplication(NSEvent)
_nextEventMatchingEventMask:untilDate:inMode:dequeue:]
  23 AppKit 6147.0  _DPSNextEvent
  22 HIToolbox 6147.0 _BlockUntilNextEventMatchingListInModeWithFilter
  21 HIToolbox 6147.0  ReceiveNextEventCommon
  20 HIToolbox 6146.0  RunCurrentEventLoopInMode
  19 CoreFoundation 6144.0  CFRunLoopRunSpecific
  18 CoreFoundation 6087.0  __CFRunLoopRun
  17 CoreFoundation 6067.0  __CFRunLoopDoSources0
  16 CoreFoundation 6066.0  __CFRunLoopDoSource0
  15 CoreFoundation 6066.0
__CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__
  14 libqcocoa.dylib 6047.0
QCocoaEventDispatcherPrivate::postedEventsSourceCallback(void*)
  13 libqcocoa.dylib 6047.0
QCocoaEventDispatcherPrivate::processPostedEvents()
  12 QtCore 6047.0 QCoreApplicationPrivate::sendPostedEvents(QObject*,
int, QThreadData*)
  11 QtCore 6047.0  QCoreApplication::notifyInternal2(QObject*, QEvent*)
  10 QtWidgets 6047.0  QApplication::notify(QObject*, QEvent*)
   9 QtWidgets 6047.0  QApplicationPrivate::notify_helper(QObject*,
QEvent*)
   8 QtWidgets 6046.0  QMainWindow::event(QEvent*)
   7 QtWidgets 6046.0  QWidget::event(QEvent*)
   6 QtWidgets 6046.0  QWidgetBackingStore::doSync()
   5 QtGui 5658.0  QBackingStore::beginPaint(QRegion const&)
   4 libqcocoa.dylib 5657.0  QRasterBackingStore::beginPaint(QRegion
const&)
   3 QtGui 5588.0  QPainter::begin(QPaintDevice*)
   2 QtGui 5578.0  QImage::detach()
   1 QtGui 5578.0  QImage::copy(QRect const&) const
   0 libsystem_platform.dylib 5577.0 _platform_memmove$VARIANT$Haswell