Profiling
- C++ course
- Expert
- Lesson 25/47
- 5 min
Learn performance measurement with profiling tools.
Profiling
Profiling measures where a program spends its time. Profile to act on data rather than guesses; often an unexpected function is the bottleneck.
In C++ time is measured with std::chrono::steady_clock: auto t0 = std::chrono::steady_clock::now(); for the start, auto t1 = std::chrono::steady_clock::now(); for the end. The elapsed time is the difference t1 - t0. On Linux use perf, with GCC use gprof, with Valgrind use callgrind.
Measuring with std::chrono
std::chrono::steady_clock is a monotonic clock; it never goes back. The now() call returns a time point; the difference between two points gives a duration.
To convert the duration to microseconds: auto us = std::chrono::duration_cast<std::chrono::microseconds>(t1 - t0).count();. Here .count() extracts the number. This idiom is placed inside multiplier (benchmark) loops to measure a function's average time.
Practice Measure a time point
Add #include <chrono>. In main write auto t0 = std::chrono::steady_clock::now(); and auto t1 = std::chrono::steady_clock::now();. Then write std::cout << (t1 >= t0);.
Starter code
#include <iostream>
#include <chrono>
int main() {
// Measure t0 and t1, compare and print
return 0;
}Sample solution
#include <iostream>
#include <chrono>
int main() {
auto t0 = std::chrono::steady_clock::now();
auto t1 = std::chrono::steady_clock::now();
std::cout << (t1 >= t0);
return 0;
}Expected output
1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does profiling measure?
auto t0 = std::chrono::steady_clock::now();
// ... work ...
auto t1 = std::chrono::steady_clock::now();- Where time is spent Correct answer
- Compile time
- Symbol count
- Memory size
Explanation: Profiling shows the bottleneck with data.
2 Complete the now() call
auto t0 = std::chrono::steady_clock::____();auto t0 = std::chrono::steady_clock::now();Explanation: now() returns the current time point.
3 Match each tool to its role
| Term | Meaning |
|---|---|
perf |
Linux profiler |
gprof |
GCC profiler |
callgrind |
Valgrind profiler |
std::chrono |
Duration measure |