I have encountered a problem when using VTune to find hotspots in a large program I am developing. It appears that VTune is not capable of properly identifying names of functions comprising hotspots and their associated call stacks when the code is compiled with optimization enabled (e.g. /O2).
To illustrate the problem, I have generated a simple example:
#include <stdio.h>
#include <omp.h>
double largesum(int Ncores, long long int Nterms, double* result) {
double sum = 0;
double time;
omp_set_num_threads(Ncores);
time = omp_get_wtime();
#pragma omp parallel for reduction(+ : sum)
for (long long int i = 1; i <= Nterms; i++) {
double id=(double)i;
sum += 1.0 / (id * id);
}
time = omp_get_wtime() - time;
*result = sum;
return time;
}
int main()
{
double result, time,Ncores;
long long int Nterms;
Ncores = 1;
Nterms = (long long int)1e8;
time=largesum(Ncores, Nterms, &result);
printf("Result: %f\nTime: %f\n", result, time);
}
I.e. the program calculates the value of large convergent sum with 1e8 terms. The sum is calculated in the function largesum()
I can first compile the code (using Windows 10. and version 19.0/19.1 of the Intel Compiler) without optimization. E.g:
icl Speedtestconsole.cpp /Od /Qopenmp /Zi
When analysing the result in VTune "Hotspots", it can be seen that largesum() is recognized as a CPU-intensive function:
However, compiling the program with optimization:
icl Speedtestconsole.cpp /O2 /Qopenmp /Zi
results in this:
In this case, the function largesum() is never identified.
I also tried adding more of the recommended compiler settings (/MD, /debug:full or /debug:inline-debug-info ), but without any luck. I would really like to be able with optimizations enabled.
Does anybody know what the problem might be?
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Hi.
Looks like compiler inlined this function.
I've added
__declspec(noinline)
double largesum(int Ncores, long long int Nterms, double* result) {
....
To your sample and got the stack
Hm, I used the same 'icl Speedtestconsole.cpp /O2 /Qopenmp /Zi /debug:inline-debug-info'
Strange. I tried to use the exact same compiler command, and Vtune shows:
If I invoke the "__declspec(noinline)" directive and use the same compilation option, I do see the function:
Vtune version 2019 Update 6.
Hi Denis,
Ok, thank you for pointing that out. Is there some other recommended approach than using VTune then?
Regarding the function being CPU-intensive: Right, it is not in the presented version, but increase Nterms to, say, (long long int)1e10 or even higher, and it should start to be CPU-intensive, right?
BTW, I just tried to run with 1e11 terms in the sum and got this result:
It is interesting that the type cast is so CPU-intensive compared to the sum-increment line which both involves an addition, a division and a multiplication. However, when optimization is disabled, most of the workload is indeed in the sum-increment-line.
