Performance issue with big file reading/writing in C++ - c++

Question

I'm developing an application over Qt.
In this application the main thread is a web server. Another thread sometimes read data from big files (250mb) and write them in a output file (~2gb).
This thread performs high I/O operation on file, and CPU iowait is around 70%.
My problem is that when writing into the file, the web server is not responding quickly. What i understood is that the server's qt socket (on Linux) is represented by a system socket connected to the poll or select event system. So Qt send signal to my application only when poll emit event.
What i think is that too huge io operation with file writing may block the poll system, so my qt server doesn't receive socket event. When the thread has finished to write its data, everything become normal.
The file writing look like this:
while(dataToRead){
// context has the list of files to read and current step
dataToRead = extractData(context, &pBuffer, &sizeBuf);
fwrite (pBuffer, 1, sizeBuf, pOutFile);
free(pBuffer);
pBuffer = NULL;
// usleep(100000);
}
If i add a break with usleep function, this help to avoid the problem but not completely if i don't use a big enough sleep. But too big sleep destroy the performance, and i was the file generated as fast as possible.
What i'm doing wrong? Is it safe to read/write into a file as fast as possible? Is a sleep is mandatory in the above function? But how can we know the good timeslice?
I'm working on Mint LMDE, Linux 3.2.0 64 bits with Intel Core i5 2500 and standard HDD drive.
Edit:
A sample program that reproduce the problem is available here: https://bugreports.qt-project.org/secure/attachment/30436/TestQtBlocked.zip. Need qt's qmake to compile it. If you run it, it will create an empty 3GB file, the worker thread will be launched at startup and will create the file during few seconds. During this time, if you try to connect to http:// localhost:8081/ and you run many F5 to refresh the page, you will see that sometime it's not responding quickly.
If could be helpful if someone can reproduce my problem with my sample program, and let me know.

Answer 1

If you are starving the main thread's select calls, create a separate thread to do the file I/O. When the event comes from Qt, trigger some kind IPC that wakes up your worker thread to do the big file I/O and return from your event handler immediately.
(This assumes that writing to the file asynchronously makes sense to your program logic. Only you can figure out if that is true.)

Answer 2

from the man page:
size_t fwrite(const void *ptr, size_t size, size_t nmemb,
FILE *stream);
You want to write sizeBuf , 1 element.
You may want to tune buffering with setvbuf.
setvbuf(pOutfile, NULL, _IONBF, 0) - to disable buffering.
Complete example at:
http://publib.boulder.ibm.com/infocenter/iseries/v7r1m0/index.jsp?topic=%2Frtref%2Fsetvbuf.htm
better switch to work with file descritors, not file streams.
Using file descriptors you can use sendfile and slice.
man sendfile
man slice