Deleting QChart Causes 30+ Second Application Hang!

FleetingMemory

Hi, I have a QApplication with a decent amount of data being plotted in a QChart. Deleting the old QChart or QChartView (I have tried both) causes the application to hang for 30+ seconds, both when I switch charts and when I close the window/application. This is crazy since it only takes like 3 seconds to plot.
I am currently restricted to using QT5.15/QT6.7 for this project.

I have tried (with no success):

deleting the QChart
deleting the QChartView
Hiding chart/chartview prior to deleting
Removing all series prior to deleting
running QCoreApplication::procesEvents() before/after delete

Since the chart exists in the main thread, Windows sees this as the application being unresponsive. I cannot move the widget to a new thread to delete in the background... Any solutions?

JonB

@FleetingMemory
Can you reproduce this in a standalone, minimal example, including it producing whatever data points are necessary? Indeed, does the data content (e.g. empty) actually matter to the timing?

FleetingMemory

@JonB Data content definitely does make a difference. I delete the initial empty QChart/QChartView when I do my initial plot, which is pretty much instantaneous. If I reduce the data, it seems to delete much faster. If plotting 1/4 the data, the delete is pretty much 1 second. Half of the data, reduces the delete to about 10 seconds and 3/4 the data to about 15 seconds). All the data (30+seconds)... The total data is approx. 450,000 points.

I can't reproduce the data or project code. But there are approx. 450,000 points total being graphed. My program is already pretty minimal.

The ui is all done programmatically and I can lay it out:

A basic UI is made programmatically with chart options, a chart view, and a chart
When a chart option is selected:
- Data is queried from a db to QList of QList of QPointF (progress is shown in a progressbar)
- If a QChartView exists delete QChartView and make new QChartView/QChart
- Create and Set new X/Y axis
- For Every QList<QPoint> create and populate a QLineSeries with replace and assign Axes & assign to chart
- set range of axes to data ranges
- Add new Chart to UI

The time it takes to delete is longer than it takes to query AND plot...

JoeCFD

@FleetingMemory I guess you are not able to see all points at the same time for such big data set. Therefore, paint only points which are needed to be displayed dynamically.

JonB

@FleetingMemory
So it should be reproducible with half a million random points across, say, 10 series? I think it keeps them as a QMap, I'm not sure whether 30 seconds is to be expected for deleting them all, it does sound a bit long. As @JoeCFD says, you might have to thin your plotted points. If you want to investigate further, try creating the line series and then deleting them without even adding them to the chart, is that similar time?

FleetingMemory

@JonB I will try that. I also tried calling remove all series and then adding the new series into the same chart, but that seems to take 5-10 seconds longer than deleting...

Also, I have a way to manipulate the range of the X-Axis, which effectively zooms in, making all points viewable and necessary @JoeCFD

FleetingMemory

@JonB I tried storing all the series* in a QMap<int, series*> after building them instead of the chart. I then for-looped the Map and deleted the series* within (as well as the map itself). This deletion process took less than 1 second. So the issue is when clearing/deleting the chart

My computer is running 12/24T Xeon, with 128gb ddr4 memory (so it's not my system....)

Update - If I still attach to the chart and build the QMap above, and then loop through and delete it again takes ~30+ seconds to delete all the series from memory. There seems to be an issue with deleting them after they are added to the QChart...

FleetingMemory

So I have done a little digging. It seems no matter what I do, whether I delete all series or delete the chart, as it deletes, the chart loops through its update/repaint method removing one series at a time. I think these function calls are what is slowing it down. It needs to be updated so that it does not call repaint or loop through update when delete is called. I think this is a bug?

@JonB @JoeCFD

JonB

@FleetingMemory
Well I don't find anything like you do. Here is a sample I think you should try:

#ifndef WIDGET_H
#define WIDGET_H

#include <QtCharts>
#include <QWidget>

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();

private:
    QChart *chart;
    QChartView *chartView;

private slots:
    void deleteChart();
};
#endif // WIDGET_H

#include <QDebug>
#include <QElapsedTimer>
#include <QTimer>
#include <QVBoxLayout>

#include "widget.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 < 10; i++)
    {
        QLineSeries* series = new QLineSeries();
        for (int x = 0; x < 50000; x++)
            series->append(x, x * (i + 1));
        chart->addSeries(series);
    }

    QTimer::singleShot(2000, this, &Widget::deleteChart);
}

void Widget::deleteChart()
{
    QElapsedTimer elapsed;
    qDebug() << "Starting to delete";
    elapsed.start();
    layout()->removeWidget(chartView);
    delete chart;
    chart = nullptr;
    delete chartView;
    chartView = nullptr;
    qDebug() << "Finished deleting" << elapsed.elapsed();
}

Widget::~Widget() {}

500,000 points, 10 series. For me this takes 6 milliseconds to "delete", where you claim 30 seconds! Ubuntu 24.04, VirtualBox slow machine/not nearly as much memory as you, Qt 6.4.2 supplied with Ubuntu. How long does it take you?

Possible it could be slow if compiled for Debug, though that is actually how I ran mine, you might check whether Release makes a huger difference?

JoeCFD

@FleetingMemory Is it possible to block qchart plotting while series are deleted. Do update of qchart after all series are cleared?

JonB

@JoeCFD
If you read OP's first post I am relying that he has tried all this sort of stuff. I just take his first two points and delete the chart view and chart, which he claims is slow.

JoeCFD

@JonB In his first post, He did

deleting the QChart
deleting the QChartView
Hiding chart/chartview prior to deleting
Removing all series prior to deleting  <===he might not have tried to block update of qchart in deletion of series. 
running QCoreApplication::procesEvents() before/after delete

His issue:
as it deletes, the chart loops through its update/repaint method removing one series at a time.

If he is able to block update of qchart while series are deleted, one final update may be ok. I do not use QChart (not free) and am not sure if it is doable.

JonB

@JoeCFD
I'm still not with you. The first two lines says he deletes chart and view. I do no more or less than that in my code. It takes 6 milliseconds. He says it takes him 30 seconds. I don't know why or what other than mine he is doing or needs to do? (I checked my code with valgrind, no leaks reported.)

JoeCFD

@JonB Could be hardware related. I think he needs to try your code and see if he still has the same problem. If not, he has other issues in his code.

JonB

@JoeCFD
I am hoping he will (a) try my code and see the time for him, (b) criticise my code if I have done anything wrong (like a leak) and then (c) compare against whatever else he is doing if different.

JoeCFD

@JonB If I were him, I would have tried your code already.

JonB

He may not be on same timezone/working/reading! Hopefully later.

JoeCFD

@JonB Likely.

FleetingMemory

@JonB I think its not the number of points in a singular line, but the number of series (lines) in total causing the problem. My times I listed are from running on release.

JonB

@FleetingMemory
I asked you to produce a minimal example such as mine but you have not. I said earlier I was guessing 10 series to make 500,000 points. If you want help I don't see why you can't convey the necessary information, no point leaving me to create 10 series I mentioned and then say it's not the right number..... And you could always alter my code to whatever numbers you desire to test.....