Fastest way to read part of 300 Gigabyte binary file

[JonB]

@J-Hilk
Out of interest: I hope you are right, but I don't see much in code which spends its time seeking and reading a few bytes out of an enormous file that will benefit from any code optimization. Presumably all the time is being taken in the OS calls themselves....

[J.Hilk]

@JonB said in Fastest way to read part of 300 Gigabyte binary file:

Presumably all the time is being taken in the OS calls themselves....

you mean, most time is lost during the network access calls? Possibly. But I would expect at least a couple of seconds improvements anyway :)

[JonB]

@J-Hilk
I would not, can't see how it would save anything here. But that aside, the OP wrote earlier:

@SGaist 15155 seconds (4 hours 12 min) it took to read these data.

Your "couple of seconds" is not going to be ground-breaking on that timing, is it? ;-)

OK, the OP has shown a newer, quicker timing. By all means try release optimization, worth a go :)

[Please_Help_me_D]

@J-Hilk Yes I did all the experiments in release mode

[Please_Help_me_D]

@Please_Help_me_D said in Fastest way to read part of 300 Gigabyte binary file:

uchar *memory = file.map(3608, file.size()-3608);

is it possible to represent *memory as a heap of type qint32 rather than uchar?

[jsulm]

@Please_Help_me_D said in Fastest way to read part of 300 Gigabyte binary file:

is it possible to represent *memory as a heap of type qint32 rather than uchar?

Sure, cast the pointer to qint32*

[JonB]

@jsulm
Your answer is in principle correct. However, should we warn the OP that I'm thinking this will only "work" if the return result from the QFile::map() he calls (given his offsets) is suitably aligned at a 32-bit boundary for qint32 * to address without segmenting?? I don't see the Qt docs mentioning whether this is the case for the normally-uchar * return result?

[jsulm]

@JonB Could be, I'm not sure

[J.Hilk]

@JonB
well if you take a look at the loop so far:

for(qint64 i = 0; i < N; i++){
            FFID[i] = memory[i*Nb];
            FFID[i+1] = memory[i*Nb+1];
            FFID[i+2] = memory[i*Nb+2];
            FFID[i+3] = memory[i*Nb+3];
        }

no checks inside the loop nor before, so it's going to hard crash any way, when the file is not int32_t aligned.

[JonB]

@J-Hilk
Umm, no, I don't see that. His current uchar *memory means it's only picking up bytes from there. And he made his FFID be QVector<uchar>. So he is copying one byte at a time (which is what I think he wants to get rid of), and current code won't have odd-boundary-memory-alignment issue. But new code with qint32* for uchar* could have problem....

If his offset is always like the example 7996 so it's divisible by 4 always then I would guess the return result from QFile::map() will not show any problem. This is an issue which does not arise when reading numbers from file, only from mapping, so just to be aware.

[J.Hilk]

@JonB
really? And what guarantees, that memory[i*Nb+3]; will be part of the valid memory ?

I assume this, is, what the OP wants to do

QVector<uchar> FFID(N*4); -> QVector<qint32> FFID(N);
uchar *memory -> qint32 *memory

and 
for(qint64 i = 0; i < N; i++){
            FFID[i] = memory[i*Nb];
        }

[Please_Help_me_D]

@jsulm thank you, that works!
@JonB @J-Hilk I think I see what you are discussing and I keep that in mind.
If I map the part of a file that is is not equal to N*4 (like in the code below) my program doesn't output any error or command line. Compiler says that it was succesfully built and application output throws that it started and one second later it is terminated.

#include <QCoreApplication>
#include <QFile>
#include <QVector>
//#include <QIODevice>
#include <armadillo>
using namespace arma;

int main()
{
    char segyFile[]{"C:/Users/tasik/Documents/Qt_Projects/raw_le.sgy"};
    QFile file(segyFile);
    if (!file.open(QIODevice::ReadOnly)) {
         //handle error
    }
    //qint32 *memory = new qint32;
    //(uchar*)&memory;
    uchar* memory = file.map(3608, file.size()-3607); // Here the mappable part file.size()-3607 has some remainder of the division by 4 
    (qint32*) memory;
    if (memory) {
        std::cout << "started..." << std::endl;
        wall_clock timer;
        qint64 fSize = file.size();
        qint64 N = 44861;
        qint64 Nb = 661*4;
        QVector<qint32> FFID(N);
        (uchar *)&FFID;
        timer.tic();
        for(qint64 i = 0; i < N; i++){
            FFID[i] = memory[i*Nb];
            /*FFID[i+1] = memory[i*Nb+1];
            FFID[i+2] = memory[i*Nb+2];
            FFID[i+3] = memory[i*Nb+3];*/
            std::cout << FFID[i] << std::endl;
        }
        double n0 = timer.toc();
        std::cout << n0 << std::endl;
        std::cout << "finished!" << std::endl;
    }
}

[JonB]

@Please_Help_me_D said in Fastest way to read part of 300 Gigabyte binary file:

and application output throws that it started and one second later it is terminated.

Yes, that was my point. You won't get a compilation error. You would get a run-time "crash" on something like line FFID[i] = memory[i*Nb];. Under Linux you'd get a core dump (if enabled), under Windoze I don't know but would have thought it would bring up a message box of some kind.

However, I haven't got time, I don't think the code you've written reflects this. For a start statements (qint32*) memory; and (uchar *)&FFID; are No-Ops (turn compiler warnings level up, you might get a warning of "no effect" for these lines, you should always develop with highest warning level you can). You haven't changed over the memory to qint32*, what you seem to think is how to do casts is wrong. This is C/C++ stuff. You'll want something more like

qint32* memory = static_cast<qint32*>(file.map(3608, file.size()-3607));

qint32* memory = reinterpret_cast<qint32*>(file.map(3608, file.size()-3607)); 

but I haven't got time to sort you out. And if you do that you need to understand how to then index it, it won't be the same offsets as you used when it was uchar*. Don't try to change to qint32* for your accesses if you don't know what you're doing cast-wise in C/C++! :)

[Please_Help_me_D]

@JonB said in Fastest way to read part of 300 Gigabyte binary file:

qint32* memory = static_cast<qint32*>(file.map(3608, file.size()-3607));

thank you but this sends me an error:

main.cpp:17:22: error: static_cast from 'uchar *' (aka 'unsigned char *') to 'qint32 *' (aka 'int *') is not allowed

[J.Hilk]

@Please_Help_me_D
@JonB meant to write reinterpret_cast not static_cast there are few uses for reinterpret_cast but this is one :)

[Please_Help_me_D]

@J-Hilk ok, now it works :)

[Please_Help_me_D]

@SGaist said in Fastest way to read part of 300 Gigabyte binary file:

Did you consider mapping only the parts that are pertinent to what you want to read ?

I don't know how to do that but I saw something like this in BOOST C++ documentation . Here is writen:
What is a memory mapped file?
File mapping is the association of a file's contents with a portion of the address space of a process. The system creates a file mapping to associate the file and the address space of the process. A mapped region is the portion of address space that the process uses to access the file's contents. A single file mapping can have several mapped regions, so that the user can associate parts of the file with the address space of the process without mapping the entire file in the address space, since the file can be bigger than the whole address space of the process (a 9GB DVD image file in a usual 32 bit systems). Processes read from and write to the file using pointers, just like with dynamic memory.

Maybe if I could map only regions of my file that I need to read then it would speed up my application? Does Qt provide something like that?

[SGaist]

Well... As already said, the map function takes an offset in your file and a size so you can map several regions of it with that. It's nowhere written that you have to passe an offset of zero and the full file size.

[Please_Help_me_D]

@SGaist I understand that I have offset and size parameters and actually I use them as a single valued numbers. If I want to map several regions of a file then I should use multiple offsets and multiple size but the example below doesn't work:

    qint64 offset[] = {100, 200, 300};
    qint64 size[] = {4, 4, 4};
    qint32* memory = reinterpret_cast<qint32*>(file.map(offset, size));

The error I get is:
main.cpp:19:57: error: cannot initialize a parameter of type 'qint64' (aka 'long long') with an lvalue of type 'qint64 [3]'
qfiledevice.h:127:23: note: passing argument to parameter 'offset' here

[SGaist]

You can't just replace an input type by an array of the same type. That's not how it's working. And in any case, the returned value of map is the address you'll have to pass to the unmap function.

You won't avoid using a form of loop or another.