Limit CPU usage of fwrite operations - c++

Question

I'm developing a program with several threads that manages the streaming from several cameras. I have to write every raw images on SSD disk. I'm using fwrite to put the image in a binary file. Something like:
FILE* output;
output = fopen(fileName, "wb");
fwrite(imageData, imageSize, 1, output);
fclose(output);
The procedure seems to runs fast enough to save all images with the given cameras throughput. The problem is that the save procedure is CPU consuming, and I start to have sync issues when the save process is enabled, due to the CPU usage of the save threads.
Is there any way to reduce the CPU load of fwrite operations? Like playing with buffering, better DMA settings, ...?
Thanks!
MIX
-- UPDATE 1
Forgetting the multithreading software, here is a simple file writer software:
#include <stdio.h>
#include <stdlib.h>
const unsigned int TOT_DATA = 1280*2*960;
int main(int argc, char* argv[])
{
if(argc != 2)
{
printf("Usage:\n");
printf(" %s totWrite\n\n", argv[0]);
return -1;
}
char* imageData;
FILE* output;
char fileName[256];
unsigned int totWrite;
totWrite = atoi(argv[1]);
imageData = new char[TOT_DATA];
printf("Write imageData[%u] on file %u times.\n", TOT_DATA, totWrite);
for(unsigned int i = 0; i < totWrite; i++)
{
sprintf(fileName, "image_%06u.raw", i);
output = fopen(fileName, "wb");
fwrite(imageData, TOT_DATA, 1, output);
fclose(output);
}
printf("DONE!\n");
delete [] imageData;
return 0;
}
A char buffer will be created, and it will be written on file totWrite times. No overwrites, since each cycle writes on a new file. (of course, one have to remove files written by previous run...)
Running top (I'm on Linux) while the program is running I see that ~50% of the CPU (that means 50% of one of the 4 cores) is used. I suppose fwrite is the bottleneck regarding the CPU usage since it is the "slower" operation in the cycle, so the one "more probably" running when top update its stats. Even "more probable" if TOT_DATA will be increased, say, 100 times.
Any further consideration about what can reduce CPU usage in such program?

Answer 1

If you consider playing with DMA settings you're way out of the scope of the standard C library. It will be nowhere near portable - and then you don't have any benefits of using portable functions.
The first step you probably should use (after you've confirmed that it's CPU that is the bottleneck) is to use lower level functions like for example open/write (or whatever your OS calls them).
Basically what can happen with fwrite is that the program first copies the data to another place in memory (the FILE* buffer) before actually writing the data to disc. This operation certainly is CPU bound and if data transfer by the CPU is slower than the data transfer to the SSD it could be a case where CPU power is consumed for no good reason.
Also one should note that using multiple threads have it's drawbacks. First if it were not an SSD multiple threads writing to disk could result in redundant head movements which is not bad, but even SSD may suffer somewhat as you might fragment the layout of the data.
There's also a problem in loading the entire file as you seem to do in the example, especially if you do it in multiple threads. It will simply consume a lot of memory (which could result in that swapping is required). If possible you should write the data to the file as the data arrives.