Rust file parsing significantly faster than Qt/C++ file parsing. Solutions for Qt implementation wanted. File size: 68.5 MB

[DerReisende]

@Christian-Ehrlicher It is just a port of @J-Hilk code to run with 6.4. I do not intend to rework it for QStringView - and my first attempt was incorrect anyways.

[Christian Ehrlicher]

@DerReisende said in Rust file parsing significantly faster than Qt/C++ file parsing. Solutions for Qt implementation wanted. File size: 68.5 MB:

It is just a port of @J-Hilk code to run with 6.4.

But the port is wrong - you're now creating useless QString objects ...

[DerReisende]

@Christian-Ehrlicher Maybe I don't get the problem:
.splitRef() does not exist anymore in 64. .split() returns a QStringList which is implicitly shared according to the doc. Where is the mistake and how would you come to a QStringRef/QStringView version on Qt 6.4?

[Christian Ehrlicher]

@DerReisende said in Rust file parsing significantly faster than Qt/C++ file parsing. Solutions for Qt implementation wanted. File size: 68.5 MB:

Where is the mistake and how would you come to a QStringRef/QStringView version on Qt 6.4?

QString::splitRef() - create a QList/Vector with references to parts of a QString, no QString objects are created, nothing gets copied
QString::split() - create a QList/Vector with newly create QString objects

To avoid the creation of the QString objects and get the same behavior as before, use QStringView::split()0

[TheLumbee]

@DerReisende I'm running on Ubuntu 22.04 64-bit. I'm currently creating the Rust lib. Got the shared lib and C-header file generated, but having an issue linking it all. So hopefully that will be resolved soon.

[TheLumbee]

@JonB Unfortunately, no. The solutions provided are by far leaps and bounds better than before, but the file sizes I'm expecting will definitely cause an issue if it's all done in Qt. I'm currently writing a Rust lib to resolve the problem for now, but even a 2-3x increase with Rust will make a monumental difference.

[DerReisende]

@Christian-Ehrlicher
Ok I modified it with the following:

        //const auto result = line.split(semicolon);
        const QStringView sv{line};
        const auto result = sv.split(semicolon);

Doesn't make a difference in runtime. And looking at the QString split code I am almost sure that line.split already does this optimization through implicit sharing through QStringList.

[Christian Ehrlicher]

@DerReisende said in Rust file parsing significantly faster than Qt/C++ file parsing. Solutions for Qt implementation wanted. File size: 68.5 MB:

I am almost sure that line.split already does this optimization through implicit sharing through QStringList.

No, implicit sharing of a container has nothing to do with creating new QString objects - QStringList is a list of string objects, not a list of QStringViews ...

[JoeCFD]

@TheLumbee That is what I did with Java code before. When I got any bottleneck in my Java apps, I tried to use C/C++ code to do the jobs. Good luck!

[TheLumbee]

Update

I successfully created the Rust lib and it's passing all the data to Qt. Rust converts the dateTime portion to mSecsSinceEpoch and then it passes that to Qt to create a DateTime object. Much faster that way.

This wasn't the preferred choice, but it does the job faster than I've ever seen. I do appreciate everyone's input and help. Hopefully in the future Qt would be able to match the performance of Rust with this particular issue.

Also, I'm going to run some tests with this Rust lib on Windows to see if it's still glacial compared to Unix.

[TheLumbee]

Windows Update

After testing on Windows, I'm actually getting the same performance as Linux. So, it must be a C++ issue with Windows. I've tested with MinGW and MSVC with parsing and it's nearly unusable.

[Christian Ehrlicher]

@TheLumbee said in Rust file parsing significantly faster than Qt/C++ file parsing. Solutions for Qt implementation wanted. File size: 68.5 MB:

So, it must be a C++ issue with Windows

Again - use plain C++ and not Qt - the QDateTime parsing is painful slow...

[JonB]

@TheLumbee said in Rust file parsing significantly faster than Qt/C++ file parsing. Solutions for Qt implementation wanted. File size: 68.5 MB:

I've tested this with versions 512, 5.15, 6.0, 6.2.4, and 6.4. Never noticed a major difference between them regarding this issue.

Do you have access to both Qt 6.4 and C++20? If so, can you try combining
https://doc.qt.io/qt-6/qdatetime.html#fromStdTimePoint-1 (QDateTime QDateTime::fromStdTimePoint(const std::chrono::local_time<std::chrono::milliseconds> &time))
https://en.cppreference.com/w/cpp/chrono/parse (std::chrono::parse())
to see whether the datetime conversion part now matches Rust's?

For right or for wrong, I have appended a post into https://bugreports.qt.io/browse/QTBUG-97489 for this whole datetime parsing to QDateTime issue, as I am concerned it is a "show-stopper" if you have a large amountof string datetime data you need to get into Qt's QDateTime.

[Christian Ehrlicher]

Ok, some c++20 magic to use c++ instead c, but not really optimized

testFile.open(QFile::ReadOnly);
instr.tickList.clear();
instr.tickList.reserve(1000000);
QElapsedTimer parseTimer1;
parseTimer1.start();
const QByteArrayList allData = testFile.readAll().split('\n');
for (const auto &line : allData)
{
    const QByteArrayList data = line.split(';');
    std::string str = data.at(0).data();   // TODO: use data.at(0).toStdString()
    str[15] = '.';                         // sadly needed for correct msec parsing
    std::istringstream stream(str);
    std::chrono::sys_time<std::chrono::milliseconds> tTimePoint;
    std::chrono::from_stream(stream, "%Y%m%d %H%M%S", tTimePoint);
    instr.tickList.push_back({
        QDateTime::fromMSecsSinceEpoch(tTimePoint.time_since_epoch().count()),
        data.at(1).toDouble(),
        data.at(2).toDouble(),
        data.at(3).toDouble(),
        data.at(4).toInt()
    });
}
qDebug().noquote() << QString("Qt parse time: %1ms").arg(parseTimer1.elapsed());```

compared to @J-Hilk 's version:

compiled with debug:
Qt parse time: 32125ms
Qt parse time: 154511ms

compiled with release:
Qt parse time: 17926ms
Qt parse time: 157748ms

Attention: Qt debug libs, the emplace_back() doesn't help much.

[TheLumbee]

@Christian-Ehrlicher Before I ever used Qt, I was facing the same issue with C++. I initially believed it was a filesystem difference and posted in a forum here: https://cplusplus.com/forum/general/254030/

Near the end of the thread you'll see that others noticed the same issue with Windows. I just find it odd that this blatant difference has never been noticed, at least in a major way.

[JonB]

@TheLumbee said in Rust file parsing significantly faster than Qt/C++ file parsing. Solutions for Qt implementation wanted. File size: 68.5 MB:

Before I ever used Qt, I was facing the same issue with C++.

I do think this thread is getting confused. You certainly seem to talk in this thread about various different aspects of your speed with Rust/C++/Qt/Windows/file I/O all mixed into one. One has to deal with these separately. The issue @Christian-Ehrlicher and I, at least, are discussing now is specifically what to do about QDateTime::fromString(), which is by far the major contributor to your efficiency compared to Rust, other items are minor. The proposal is if one has Qt 6.4+ and C++ 20 then std::chrono can be used to parse the string input to a "naive datetime" (and I have a hunch that is what Rust uses) and that converted to a QDateTime in condirably better time that QDateTime::fromString().

This is quite distinct from e.g. the time taken to read the large file under Windows.

[TheLumbee]

@JonB said in Rust file parsing significantly faster than Qt/C++ file parsing. Solutions for Qt implementation wanted. File size: 68.5 MB:

The issue @Christian-Ehrlicher and I, at least, are discussing now is specifically what to do about QDateTime::fromString(), which is by far the major contributor to your efficiency compared to Rust, other items are minor. The proposal is if one has Qt 6.4+ and C++ 20 then std::chrono can be used to parse the string input to a "naive datetime" (and I have a hunch that is what Rust uses) and that converted to a QDateTime in condirably better time that QDateTime::fromString().

Apologies. I don't have C++20, but I can set up an environment to test it. But even without the DateTime, Rust is parsing the file 2-3x faster than C++, with just floats and ints. Maybe C++20 has some improvements in that domain, but I'll set up an environment to test this.

[JonB]

@TheLumbee said in Rust file parsing significantly faster than Qt/C++ file parsing. Solutions for Qt implementation wanted. File size: 68.5 MB:

But even without the DateTime, Rust is parsing the file 2-3x faster than C++, with just floats and ints.

I do understand this. But I suggest this is a separate issue from the QDateTime. You started with 40x faster. Dealing with QDateTime is the first priority. File reading or parsing ints and floats is a separate issue requiring its own solution.

[TheLumbee]

@JonB I agree. I'll do some testing with this today and provide an update.

Thanks again for the help.

[TheLumbee]

To provide an update, the solution provided by @JonB 's "final offering" in this post parses files a little quicker. Although better than my initial approach, still not nearly as fast as the Rust solution. For now, I'm sticking with the Rust lib I wrote but I do think this points out some performance enhancements that can be made on the C++ side of things.