Deleting QChart Causes 30+ Second Application Hang!

JoeCFD

@FleetingMemory try to call
removeAllSeries() first. It seems this call will not trigger update of qchart.
and then update,
next delete chart.

FleetingMemory

@JoeCFD Will give this a try and let you know!

JonB

@FleetingMemory
In your first post of the steps you have tried you already included

Removing all series prior to deleting

FleetingMemory

Yeah, I am going to retry in release with elapsed() to see what is going on and give more info.
Query Size 450,000 points. Spread across 12,000 series.
populating the series after query takes 2,964 milliseconds
Gets stuck on remove all series (freezes main thread), takes 35,774 milliseconds to remove

JonB

@FleetingMemory
Yes please test. OOI how can you expect to display 12,000 series to a user, I just don't get it?

FleetingMemory

A little more testing - I used release mode, removed all series from QChart in a QChartView after a new query is run. During this test I re-used the same QChart -- after running removeAllSeries(), I used the points from the query to generate new series and add them to the chart. Points are formatted <millis since epoch, value>. Series have OpenGL turned on in this test.

1: A Query is run, Qlist<Qlist<QPointF>> generated. I return the Query Count
2: If QChart Exists, remove all series (this I timed)
3: for each Qlist<QPointF> generate a new series with replace(), append Axes and insert into chart (this I timed)

Query Size 450,000 points. Spread across 12,000 series.
populating 12,000 series after query takes 2.964 s
Gets stuck on remove all 12,000 series (freezes main thread), takes 35.774 seconds to remove

Query Size: 332,000 points
populating 10,600 series: 3.3s
populating 2nd time diff data: 3.4s
removing all series: 25.35s
removing all (2nd time diff data): 25.6s

Query Size: 247,900 points
populating 9300 series: 2.5s
removing all series: 19s

Query Size 250,000
Populating Series: 1.32 s
removing all 8,050 series: 14.33s
removing all 8,050 series (2nd run diff data): 13.4s

Query Size 223,200
Populating 8600 series: 2.4s
populating 8600 series(2nd run diff data): 2s
removing all series: 15.6s
removing all series (2nd run diff data): 15.6s

Query Size: 195,600
populating series: 1.7s
Removing 7820 series: 12.5s
removing all 7820 series (2nd run diff data): 12.9s

Query Size 158,543
populating the series after query: 0.7s
removing all 4,859 series: 4.9s

Query Size 21700
Removing 1240 series: 0.395s
populating series: 0.1s

Query Size: 9,818
populating series: 0.04s
removing all 735 series: 0.045s
removing all 735 series (2nd run diff data): 0.044s

Performance seems to get worse pretty quickly at a non-linear rate

If it helps, I am compiling this as a plugin loaded in a simple main application (though that shouldn't affect things).
I have tried with useOpenGL both on and off. As far as I can tell, it makes little difference.
The fact the removeAll/delete freezes the main thread, which essentially locks up the entire application it is running in is a huge issue for me...

JoeCFD

@FleetingMemory
look at source code here.
https://codebrowser.dev/qt5/qtcharts/src/charts/qchart.cpp.html
removeAllSeries func has a loop to call removeSeries(QAbstractSeries *series)
I guess it will trigger update for each removal. This is not efficient.

JonB

@FleetingMemory
Why can't you test exactly these figures, with whatever (hopefully tiny) code you use, against the skeleton I provided? You only have to alter the two loop counters. Then people have actual code they can test, so far as I know we really don't know for sure what your code reads. I seem to have been saying/suggesting this all along. If you want people to help, or even to report a bug, we have to have a complete, minimal, reproducible example.

FleetingMemory

@JonB I just tried your loop. I get similar results when increasing series count.

JoeCFD

@FleetingMemory I tried Jon's code with 10000 series + 10 points and tried different things:

disable update of QChartView
disable QChartView and QChart
hide QChartView and QChart
before chart is deleted. Nothing helps.

However, I am able to get quick clean-up of qchartview with

    chartView->setChart( new QChart() );
    chart->deleteLater();

Not sure if this helps you. If this piece of code does not help, you may try to delete the chart in a thread while charview is available for other uses. I guess your app will have issue at exit.

FleetingMemory

This post is deleted!

FleetingMemory

@JoeCFD I tried that as well, it is marginally faster to delete this way than removeAll. I have tried threading but because QChart calls GUI functions, I was not able to get it to work. I tried threading the delete as well as the removal of Series. Neither works. I usually run into errors like -- widget cannot be moved to a new thread -- accessing widget from wrong thread (usually pertains to the QChartData) -- sending signal accross thread, which results in an immediate crash. This is because the removeALLSeries and delete call on update()

SGaist

Hi,

Did you try calling setUpdateEnabled(false) on the chart ?

FleetingMemory

@SGaist QChart does not have updatesEnabled(false) only QChartView. Doesn't seem to make a difference. I also tried blocking signals.

JoeCFD

@FleetingMemory My results with Jon's code:

use deleteLater
Starting to delete
Finished deleting 771
original deletion
Starting to delete
Finished deleting 11663

Qt 6.8.2 + Ubuntu 22.04

FleetingMemory

@JoeCFD how many series? It still locks the main thread though... I did not see a noticeable difference between delete and deleteLater performance on my side

JoeCFD

@FleetingMemory 10000 series + 10 points

Jon's code and chartView is not deleted. Series are cleared quickly.

#include "chart.h"

// using namespace QtCharts;

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    setGeometry(100, 100, 800, 600);
    setLayout(new QVBoxLayout);

    this->chart = new QChart();
    this->chartView = new QChartView(chart, this);
    layout()->addWidget(chartView);
    chartView->setSizePolicy(QSizePolicy::MinimumExpanding, QSizePolicy::MinimumExpanding);

    for (int i = 0; i < 10000; i++)
    {
        QLineSeries* series = new QLineSeries();
        for (int x = 0; x < 10; x++)
            series->append(x, x * (i + 1));
        chart->addSeries(series);
    }
    
    chart->createDefaultAxes();
    chart->axes(Qt::Horizontal).first()->setTitleText("X Axis");
    chart->axes(Qt::Vertical).first()->setTitleText("Y Axis");
    QTimer::singleShot(5000, this, &Widget::deleteChart);
}

void Widget::deleteChart()
{
    QElapsedTimer elapsed;
    qDebug() << "Starting to delete";
    elapsed.start(); 

    chartView->setChart( new QChart() );
    chart->deleteLater();

    layout()->removeWidget(chartView);
    //chart->removeAllSeries();
    //delete chart;
    //chart = nullptr;
    //delete chartView;
    //chartView = nullptr;
    qDebug() << "Finished deleting" << elapsed.elapsed();
}

Widget::~Widget() {}

FleetingMemory

@JoeCFD Going back to my original code, I tried moving setchart(new QChart()) and deletelater() to before I populate the new chart with new series. Previously I was populating a new chart then setting it, then deleting a pointer to the old chart. Now, if I watch the memory, deleteLater still doesn't actually call delete until after the new chart is populated fully. So it looks like it deleted quickly but it still hangs after the new chart pops in with data. Doing it this way, however does take slightly less time to delete.

JoeCFD

@FleetingMemory Maybe try to delete it in a thread. Also a splash widget may be needed at exit.

FleetingMemory

@JoeCFD Can't delete in a thread, it calls on GUI functions because it loops across removeSeries(), which calls update. This causes a cross-thread exception in Qt. DeleteLater() doesn't actually delete immediately within your timer bounds in the code you posted above (that is just timing QT scheduling it), so it still freezes the program later on.