Question

I just made some benchmarks for this super question/answer Why is my program slow when looping over exactly 8192 elements?

I want to do benchmark on one core so the program is single threaded. But it doesn’t reach 100% usage of one core, it uses 60% at most. So my tests are not acurate.

I’m using Qt Creator, compiling using MinGW release mode.

Are there any parameters to setup for better performance ? Is it normal that I can’t leverage CPU power ? Is it Qt related ? Is there some interruptions or something preventing code to run at 100%…

Here is the main loop

//  horizontal sums for first two lines
for(i=1;i<SIZE*2;i++){ 
    hsumPointer[i]=imgPointer[i-1]+imgPointer[i]+imgPointer[i+1];
}
// rest of the computation
for(;i<totalSize;i++){ 
    // compute horizontal sum for next line
    hsumPointer[i]=imgPointer[i-1]+imgPointer[i]+imgPointer[i+1];
    // final result
    resPointer[i-SIZE]=(hsumPointer[i-SIZE-SIZE]+hsumPointer[i-SIZE]+hsumPointer[i])/9; 
}

This is run 10 times on an array of SIZE*SIZE float with SIZE=8193, the array is on the heap.

Answer 1

There could be several reasons why Task Manager isn’t showing 100% CPU usage on 1 core:

1. You have a multiprocessor system and the load is getting spread across multiple CPUs (most OSes will do this unless you specify a more restrictive CPU affinity);
2. The run isn’t long enough to span a complete Task Manager sampling period;
3. You have run out of RAM and are swapping heavily, meaning lots of time is spent waiting for disk I/O when reading/writing memory.

Or it could be a combination of all three.

Also Let_Me_Be’s comment on your question is right — nothing here is QT’s fault, since no QT functions are being called (assuming that the objects being read and written to are just simple numeric data types, not fancy C++ objects with overloaded operator=() or something). The only activities taking place in this region of the code are purely CPU-based (well, the CPU will spend some time waiting for data to be sent to/from RAM, but that is counted as CPU-in-use time), so you would expect to see 100% CPU utilisation except under the conditions given above.