How to write entire QVector to a binary file?

[CJha]

@JonB Thanks, this might just work for me. I will try it out now and see what's the difference in speed of writing using this method and iterative method and will post it back here.

[CJha]

@J-Hilk That's a good point, but it's not the case for me as the data write and data read happens at different times. Also, SSD lifetime doesn't matter as these researchers have lots of funding and SSD is a cheap item for them. My job is to give them what they ask for, and if they ruin their SSD in 200 days that is up to them (of course I will tell them that it can ruin their SSD fast but that's all I can do).

[J.Hilk]

@CJha Fair enough,
The customer gets, what the customer wants.

I can't remember the amount of times I had to do implement stuff I whole heartily disagreed with ...🙈

[JonB]

@CJha said in How to write entire QVector to a binary file?:

Also, SSD lifetime doesn't matter as these researchers have lots of funding and SSD is a cheap item for them.

LOL :)

[JonB]

@CJha
BTW. When you have gotten it working with that file.write(), which is going to be as good as it gets. Since speed seems to be such an issue, and you're going to be doing ~1,000,000 points, and you goal is going to be to access the data array and write it out raw. Then my thought would be: why use a Qt QVector<> at all? For best efficiency/memory usage, would this be a case where simply creating a C++ array of doubles of sufficient size and storing into that directly/writing out to file would be simpler than wrapping it in QVector<> overheads: even if that is small, what's the point?

And P.S.
If you stick with QVector<>, do make sure you use QVector::resize/reserve(int size) appropriately early (once if possible), I think. What you do not want is to have the QVector keep reallocating/moving existing data as your million points keep arriving....

[CJha]

@JonB I agree that a simple C++ array would be faster and easier as that is the format in which data is generated in the buffer from the acquisition device.

However, if I write the data to a file in the same thread (in the same callback function where the data is deposited in the buffer from the acquisition device or in a different function), then since writing takes a long time it blocks the entire thread, this (once in a while) blocks the callback function which is called each time the required number of data samples is generated by the acquisition device resulting in an error.

To solve this problem, I write data to a binary file in a different thread. Now, if I pass the address of the same buffer in which data is deposited then it defeats the purpose of having multiple threads as I am accessing the same buffer in which data is deposited from the acquisition device just from a different thread instead of the main one. To overcome this I write the incoming data from the acquisition device's buffer to a QVector<double> then send this vector over a Qt::QueuedConnection to my "Writer" thread and I write it there. I am not so good with C++ arrays and so I am not quite confident on how to achieve this without involving QVector in the process. If you have any idea on how can I simplify this process I will be very grateful :)

[jsulm]

@CJha said in How to write entire QVector to a binary file?:

then since writing takes a long time it blocks the entire thread

You could do double-buffering with two arrays :-)

[JonB]

@CJha
My simple answer would be: mutexes. How that compares to queued signals I do not know; I am not suggesting mutexes, only answering the question.

QVector<double> then send this vector over a Qt::QueuedConnection

Wouldn't mind just seeing how you send it, do you use const QVector<> &?

[CJha]

@jsulm I already use double buffering i.e. I assign twice the amount of memory for the buffer than needed. But I cannot use two separate buffers as these buffers are controlled by the C based specialized functions which are specific to the acquisition device. All I can do is assign the size of the memory to the buffer. The program flow is as follows:

• Assign buffer size
• Start acquisition
• C based function puts data in the buffer and alerts my application through a callback function
• I retrieve data from the buffer to a C array (I cannot retrieve directly to a vector as this step is also controlled by device-specific C function which only accepts a pointer to a C array)
• Now I can do whatever I want with acquired data

So, there is not much choice in terms of the buffer.

Regarding the use of the C array that I use to get data out of the buffer, it is generated on the heap and deleted at the end of the callback function in which I get the data from the buffer.

I could use two different vectors to store data and achieve so-called 'double buffering' from my application's point of view, and I have tried that. But in this case as well the thread is blocked for the time period of writing data to a file.

[CJha]

@JonB Yeah I could use mutexes, but I prefer Queued Connection as there is just one QVector to be sent to a different thread. I do not use const QVector<double>& because I want to depend on Qt's implicit sharing i.e. if the QVector<double> is changed while I am still using the previous QVector to write data in my binary file then it would not affect my "Writer" thread. If I would use const QVector<double>& then it would refer to the original QVector in the main thread and then I would have to use QMutex to protect read and write operations. This is my function in the Writer class which inherits QObject and is run in a different thread:

void Writer::writeData(QVector<double> vec)
{
    ++sweepCount_; // Increament the count to keep track of number of times data vector is written

    if(isBin_){ // If user selects file type as .bin
        for(int ii = 0; ii < vec.length(); ++ii)
            binOut_ << vec[ii]; // binOut_ is a QDataStrem, assigned to a file when the user clicks on Start button
    }
    else{ // if the user selects file type as .csv
        if(vec.length() > 1){
            outStream_ << vec[0];
            for(int ii = 1; ii < vec.length(); ++ii)
                outStream_ << seperator_ << vec[ii]; // seperator_ = ',' or ';' depending on QLocale
            outStream_ << '\n';
        }
    }
}

[SGaist]

Hi,

In what format do you get the data in the callback ?

[J.Hilk]

@CJha said in How to write entire QVector to a binary file?:

I do not use const QVector<double>& because I want to depend on Qt's implicit sharing i.e. if the QVector<double> is changed while I am still using the previous QVector to write data in my binary file then it would not affect my "Writer" thread

So that you know, when passing your QVector through Qt::QueuedConnection - which is the default and correct one across threads - your QVector will be copied auto automatically, there will be no share until write. There will be a copy inside your thread

[CJha]

@SGaist Hi, It is double. The data is placed in a C Array from where I retrieve it.

[CJha]

@J-Hilk Thanks, one thing which I am not sure about: What would happen if I use const QVector<double>& instead of QVector<double>, will it still copy the data if the connection type is Qt:QueuedConnection or will it just copy the reference for the vector?

[KroMignon]

@CJha said in How to write entire QVector to a binary file?:

will it still copy the data if the connection type is Qt:QueuedConnection or will it just copy the reference for the vector?

In short: Yes they will be copied.
For more details take a look at this => https://www.embeddeduse.com/2013/06/29/copied-or-not-copied-arguments-signals-slots/

[CJha]

@KroMignon Thanks :)

[JonB]

@CJha
According to @KroMignon's link

The conclusion from the above results is that we should pass arguments to signals and slots by const reference and not by value. This advice is true for both direct and queued connections. Even if the sender of the signal and the receiver of the slot are in different threads, we should still pass arguments by const reference.

If you have both signal & slot declared with QVector<double> vec and you change both to have const QVector<double>&, you reduce 3 copies to 1.

Having said that: I do get lost as to what gets copied, aren't we only talking about the QVector structure and not the data it references??

[CJha]

@JonB If it's a copy then I assume it is always going to be the data because the structure is not that significant in terms of memory usage, but I am not sure about it either.

[jsulm]

@CJha said in How to write entire QVector to a binary file?:

If it's a copy then I assume it is always going to be the data

No, because Qt containers use copy-on-write. See https://doc.qt.io/qt-5/implicit-sharing.html