Details
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Suggestion
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Resolution: Out of scope
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Not Evaluated
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None
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5.4.1
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None
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Visual Studio 2013, Windows 7
Description
While trying to upgrade our software from VS 2010 to VS 2013, I also upgraded Qt from 4.8.4 to 5.4.1. I noticed that parts of the application ran considerably slower. I found the cause of the problem in QVector, which is much slower in 5.4.1 than in 4.8.4 (I haven't tested any other 5.* releases).
Here are my test results:
| Loop over 500Mio int | Qt 4.8.4, VS 2010 | Qt 5.4.1, VS 2013 | Factor |
|---|---|---|---|
| QVector::iterators | 193ms | 729ms | 3.7 |
| QVector::operator[] | 417ms | 970ms | 2.3 |
| std::vector::iterators | 194ms | 132ms | 0.68 |
| std::vector::operator[] | 156ms | 130ms | 0.83 |
Looking at the code, every access to operator[] or constBegin() has much more overhead than it used to have in 4.8.4. Also compared to STL, performance is quite bad in this case. Is this bad QVector performance a known issue and planned to be resolved? Otherwise it seems we'll have to switch from QVector to std::vector.
Test Code:
const int _size = 500 * 1000 * 1000;
int _sum = 0;
QVector<int> _vector(_size, 0);
std::vector<int> _vector2(_size, 0);
myTimer _timer;
for (int i = 0; i < _size; i++)
{
_sum += _vector[i];
}
_timer.measure("QVector::operator[]");
for (auto _iter = _vector.constBegin(); _iter != _vector.constEnd(); _iter++)
{
_sum += *_iter;
}
_timer.measure("QVector::iterators");
for (int ii = 0; ii < _size; ii++)
{
_sum += _vector2[ii];
}
_timer.measure("std::vector::operator[]");
for (auto _iter2 = _vector2.cbegin(); _iter2 != _vector2.cend(); _iter2++)
{
_sum += *_iter2;
}
_timer.measure("std::vector::iterators");