First call to QIcon::pixmap takes 180 milliseconds

Youda008

I have a custom list view and model displaying user-specified filesystem entries (both files and directories are possible).

I wanted it to start displaying icons the OS uses for those filesystem entries, so i added this method, that is called from Model::data( role = Qt::DecorationRole )

static QHash< QString, QIcon > g_filesystemIconCache;

const QIcon & Entry::getIcon() const
{
    static QFileIconProvider iconProvider;  // only one initialization needed

    auto iter = g_filesystemIconCache.find( this->path );
    if (iter == g_filesystemIconCache.end())
    {
        iter = g_filesystemIconCache.insert( this->path, iconProvider.icon( entryInfo ) );
    }
    return iter.value();
}

But soon i noticed my application loading time has increased noticably.
My first idea was that it is caused by the high-res icons used by Windows, so i modified the inner code block creating the icon to

        QIcon origIcon = iconProvider.icon( this->path );
        QSize smallestSize = origIcon.availableSizes()[0];
        QPixmap pixmap = origIcon.pixmap( smallestSize );
        iter = g_filesystemIconCache.insert( this->path, QIcon( pixmap ) );

and measured the time it took for each line and found out that the origIcon.pixmap( smallestSize ) took 180ms for the first item and then less than 1ms for the rest of items.

This only happens on Windows and it only happens with the icons provided by QFileIconProvider. If i load an internal resource icon instead, the total icon loading time is few milliseconds.

Do you know what is causing QIcon::pixmap to take so long and is there a workaround?

Christian Ehrlicher

What exact Qt and Windows version do you use? Please provide a minimal, compilable example to reproduce your issue.
@Axel-Spoerl: Looks similar to QTBUG-110416 ?

Youda008

@Christian-Ehrlicher

Ok, here it is.

OS: Windows 10 Pro 22H2 19045.3086
Qt: 5.15.7

Minimal example: IconList.zip

Axel Spoerl

@Youda008
Good morning,
just ran the reproducer locally on Qt 5.15.14 and the latest bleeding edge (6.7 dev), both developer builds in debug mode.
The codebase of QFileIconProviderhasn't seen many changes and the stats don't show a big difference.
I use laptop computer, which is not incredibly fast. But it's got a fast SSD drive.
I don't want to exclude the possibility of a bug in Qt. However, if only the first call takes forever, I would find it worth checking for a local file caching issue.
I'd also consider changing to Qt 6 (in which case one line of code needs to be adapted, because QFileIconProvider::icon() doesn't like string arguments any more. Just call iconProvider.icon(entry);).
Cheers
Axel

08:16:45: Debugging /home/axel/QtDev/build-IconList-Qt_6_7_0_dev_build/Debug/IconList ...
#0 : QDirIterator::next()        took   0ms
#1 : QFileIconProvider::icon()   took  61ms
#2 : QIcon::pixmap()             took   0ms
#3 : QIcon::QIcon( QPixmap & )   took   0ms
08:17:00: Debugging of /home/axel/QtDev/build-IconList-Qt_6_7_0_dev_build/Debug/IconList has finished with exit code 0.

08:21:13: Debugging /home/axel/QtDev/build-IconList-Qt_5_15_14_dev_build/Debug/IconList ...
#0 : QDirIterator::next()        took   0ms
#1 : QFileIconProvider::icon()   took  79ms
#2 : QIcon::pixmap()             took   0ms
#3 : QIcon::QIcon( QPixmap & )   took   0ms
08:21:29: Debugging of /home/axel/QtDev/build-IconList-Qt_5_15_14_dev_build/Debug/IconList has finished with exit code 0.

Youda008

@Axel-Spoerl Thank you for looking into it.

Interestingly, in your results it's the QFileIconProvider::icon() that takes the most time, while in my tests it's the QIcon::pixmap().

#0 : QDirIterator::next()        took   0ms
#1 : QFileIconProvider::icon()   took   0ms
#2 : QIcon::pixmap()             took 179ms
#3 : QIcon::QIcon( QPixmap & )   took   0ms
#4 : QDirIterator::next()        took   1ms
#5 : QFileIconProvider::icon()   took   0ms
#6 : QIcon::pixmap()             took  12ms
#7 : QIcon::QIcon( QPixmap & )   took   0ms
#8 : QDirIterator::next()        took   0ms
#9 : QFileIconProvider::icon()   took   0ms
#10: QIcon::pixmap()             took  11ms
#11: QIcon::QIcon( QPixmap & )   took   0ms

It looks to me as if the QFileIconProvider::icon() or QIcon::pixmap() performed some heavy on-demand initialization. I am wondering what that could be and if it can be prevented.

Can you please add more files to the directory of the executable from which the code loads the icons, so that we can see what happens with the consequent calls?
Your time might not be as large as mine, but you might still see the same big difference between the first call and the other calls.

In the meantime i will try to investiate with ProcessMonitor, because i cannot debug the Qt's code, because i use pre-built packages.

Christian Ehrlicher

@Youda008 said in First call to QIcon::pixmap takes 180 milliseconds:

e Qt's code, because i use pre-built packages.

But even there you can install the debug symbols and sources.

Youda008

It seems to be DLL loading. Apparently when Windows is asked to provide the file icons, it starts loading a crapton of DLLs related to icons and thumbnails, and if the OS or drive is not in the best shape, it takes long time.