I want to find out how much time a certain function takes in my C++ program to execute on Linux. Afterwards, I want to make a speed comparison . I saw several time function but ended up with this from boost. Chrono:
process_user_cpu_clock, captures user-CPU time spent by the current process
Now, I am not clear if I use the above function, will I get the only time which CPU spent on that function?
Secondly, I could not find any example of using the above function. Can any one please help me how to use the above function?
P.S: Right now , I am using std::chrono::system_clock::now() to get time in seconds but this gives me different results due to different CPU load every time.
It is a very easy-to-use method in C++11. You have to use std::chrono::high_resolution_clock from <chrono> header.
Use it like so:
#include <chrono>
/* Only needed for the sake of this example. */
#include <iostream>
#include <thread>
void long_operation()
{
/* Simulating a long, heavy operation. */
using namespace std::chrono_literals;
std::this_thread::sleep_for(150ms);
}
int main()
{
using std::chrono::high_resolution_clock;
using std::chrono::duration_cast;
using std::chrono::duration;
using std::chrono::milliseconds;
auto t1 = high_resolution_clock::now();
long_operation();
auto t2 = high_resolution_clock::now();
/* Getting number of milliseconds as an integer. */
auto ms_int = duration_cast<milliseconds>(t2 - t1);
/* Getting number of milliseconds as a double. */
duration<double, std::milli> ms_double = t2 - t1;
std::cout << ms_int.count() << "ms\n";
std::cout << ms_double.count() << "ms\n";
return 0;
}
This will measure the duration of the function long_operation.
Possible output:
150ms
150.068ms
Working example: https://godbolt.org/z/oe5cMd
Here's a function that will measure the execution time of any function passed as argument:
#include <chrono>
#include <utility>
typedef std::chrono::high_resolution_clock::time_point TimeVar;
#define duration(a) std::chrono::duration_cast<std::chrono::nanoseconds>(a).count()
#define timeNow() std::chrono::high_resolution_clock::now()
template<typename F, typename... Args>
double funcTime(F func, Args&&... args){
TimeVar t1=timeNow();
func(std::forward<Args>(args)...);
return duration(timeNow()-t1);
}
Example usage:
#include <iostream>
#include <algorithm>
typedef std::string String;
//first test function doing something
int countCharInString(String s, char delim){
int count=0;
String::size_type pos = s.find_first_of(delim);
while ((pos = s.find_first_of(delim, pos)) != String::npos){
count++;pos++;
}
return count;
}
//second test function doing the same thing in different way
int countWithAlgorithm(String s, char delim){
return std::count(s.begin(),s.end(),delim);
}
int main(){
std::cout<<"norm: "<<funcTime(countCharInString,"precision=10",'=')<<"\n";
std::cout<<"algo: "<<funcTime(countWithAlgorithm,"precision=10",'=');
return 0;
}
Output:
norm: 15555
algo: 2976
In Scott Meyers book I found an example of universal generic lambda expression that can be used to measure function execution time. (C++14)
auto timeFuncInvocation =
[](auto&& func, auto&&... params) {
// get time before function invocation
const auto& start = std::chrono::high_resolution_clock::now();
// function invocation using perfect forwarding
std::forward<decltype(func)>(func)(std::forward<decltype(params)>(params)...);
// get time after function invocation
const auto& stop = std::chrono::high_resolution_clock::now();
return stop - start;
};
The problem is that you are measure only one execution so the results can be very differ. To get a reliable result you should measure a large number of execution.
According to Andrei Alexandrescu lecture at code::dive 2015 conference - Writing Fast Code I:
Measured time: tm = t + tq + tn + to
where:
tm - measured (observed) time
t - the actual time of interest
tq - time added by quantization noise
tn - time added by various sources of noise
to - overhead time (measuring, looping, calling functions)
According to what he said later in the lecture, you should take a minimum of this large number of execution as your result.
I encourage you to look at the lecture in which he explains why.
Also there is a very good library from google - https://github.com/google/benchmark.
This library is very simple to use and powerful. You can checkout some lectures of Chandler Carruth on youtube where he is using this library in practice. For example CppCon 2017: Chandler Carruth “Going Nowhere Faster”;
Example usage:
#include <iostream>
#include <chrono>
#include <vector>
auto timeFuncInvocation =
[](auto&& func, auto&&... params) {
// get time before function invocation
const auto& start = high_resolution_clock::now();
// function invocation using perfect forwarding
for(auto i = 0; i < 100000/*largeNumber*/; ++i) {
std::forward<decltype(func)>(func)(std::forward<decltype(params)>(params)...);
}
// get time after function invocation
const auto& stop = high_resolution_clock::now();
return (stop - start)/100000/*largeNumber*/;
};
void f(std::vector<int>& vec) {
vec.push_back(1);
}
void f2(std::vector<int>& vec) {
vec.emplace_back(1);
}
int main()
{
std::vector<int> vec;
std::vector<int> vec2;
std::cout << timeFuncInvocation(f, vec).count() << std::endl;
std::cout << timeFuncInvocation(f2, vec2).count() << std::endl;
std::vector<int> vec3;
vec3.reserve(100000);
std::vector<int> vec4;
vec4.reserve(100000);
std::cout << timeFuncInvocation(f, vec3).count() << std::endl;
std::cout << timeFuncInvocation(f2, vec4).count() << std::endl;
return 0;
}
EDIT:
Ofcourse you always need to remember that your compiler can optimize something out or not. Tools like perf can be useful in such cases.
simple program to find a function execution time taken.
#include <iostream>
#include <ctime> // time_t
#include <cstdio>
void function()
{
for(long int i=0;i<1000000000;i++)
{
// do nothing
}
}
int main()
{
time_t begin,end; // time_t is a datatype to store time values.
time (&begin); // note time before execution
function();
time (&end); // note time after execution
double difference = difftime (end,begin);
printf ("time taken for function() %.2lf seconds.\n", difference );
return 0;
}
Easy way for older C++, or C:
#include <time.h> // includes clock_t and CLOCKS_PER_SEC
int main() {
clock_t start, end;
start = clock();
// ...code to measure...
end = clock();
double duration_sec = double(end-start)/CLOCKS_PER_SEC;
return 0;
}
Timing precision in seconds is 1.0/CLOCKS_PER_SEC
#include <iostream>
#include <chrono>
void function()
{
// code here;
}
int main()
{
auto t1 = std::chrono::high_resolution_clock::now();
function();
auto t2 = std::chrono::high_resolution_clock::now();
auto duration = std::chrono::duration_cast<std::chrono::microseconds>( t2 - t1 ).count();
std::cout << duration<<"/n";
return 0;
}
This Worked for me.
Note:
The high_resolution_clock is not implemented consistently across different standard library implementations, and its use should be avoided. It is often just an alias for std::chrono::steady_clock or std::chrono::system_clock, but which one it is depends on the library or configuration. When it is a system_clock, it is not monotonic (e.g., the time can go backwards).
For example, for gcc's libstdc++ it is system_clock, for MSVC it is steady_clock, and for clang's libc++ it depends on configuration.
Generally one should just use std::chrono::steady_clock or std::chrono::system_clock directly instead of std::chrono::high_resolution_clock: use steady_clock for duration measurements, and system_clock for wall-clock time.
Here is an excellent header only class template to measure the elapsed time of a function or any code block:
#ifndef EXECUTION_TIMER_H
#define EXECUTION_TIMER_H
template<class Resolution = std::chrono::milliseconds>
class ExecutionTimer {
public:
using Clock = std::conditional_t<std::chrono::high_resolution_clock::is_steady,
std::chrono::high_resolution_clock,
std::chrono::steady_clock>;
private:
const Clock::time_point mStart = Clock::now();
public:
ExecutionTimer() = default;
~ExecutionTimer() {
const auto end = Clock::now();
std::ostringstream strStream;
strStream << "Destructor Elapsed: "
<< std::chrono::duration_cast<Resolution>( end - mStart ).count()
<< std::endl;
std::cout << strStream.str() << std::endl;
}
inline void stop() {
const auto end = Clock::now();
std::ostringstream strStream;
strStream << "Stop Elapsed: "
<< std::chrono::duration_cast<Resolution>(end - mStart).count()
<< std::endl;
std::cout << strStream.str() << std::endl;
}
}; // ExecutionTimer
#endif // EXECUTION_TIMER_H
Here are some uses of it:
int main() {
{ // empty scope to display ExecutionTimer's destructor's message
// displayed in milliseconds
ExecutionTimer<std::chrono::milliseconds> timer;
// function or code block here
timer.stop();
}
{ // same as above
ExecutionTimer<std::chrono::microseconds> timer;
// code block here...
timer.stop();
}
{ // same as above
ExecutionTimer<std::chrono::nanoseconds> timer;
// code block here...
timer.stop();
}
{ // same as above
ExecutionTimer<std::chrono::seconds> timer;
// code block here...
timer.stop();
}
return 0;
}
Since the class is a template we can specify real easily in how we want our time to be measured & displayed. This is a very handy utility class template for doing bench marking and is very easy to use.
If you want to safe time and lines of code you can make measuring the function execution time a one line macro:
a) Implement a time measuring class as already suggested above ( here is my implementation for android):
class MeasureExecutionTime{
private:
const std::chrono::steady_clock::time_point begin;
const std::string caller;
public:
MeasureExecutionTime(const std::string& caller):caller(caller),begin(std::chrono::steady_clock::now()){}
~MeasureExecutionTime(){
const auto duration=std::chrono::steady_clock::now()-begin;
LOGD("ExecutionTime")<<"For "<<caller<<" is "<<std::chrono::duration_cast<std::chrono::milliseconds>(duration).count()<<"ms";
}
};
b) Add a convenient macro that uses the current function name as TAG (using a macro here is important, else __FUNCTION__ will evaluate to MeasureExecutionTime instead of the function you wanto to measure
#ifndef MEASURE_FUNCTION_EXECUTION_TIME
#define MEASURE_FUNCTION_EXECUTION_TIME const MeasureExecutionTime measureExecutionTime(__FUNCTION__);
#endif
c) Write your macro at the begin of the function you want to measure. Example:
void DecodeMJPEGtoANativeWindowBuffer(uvc_frame_t* frame_mjpeg,const ANativeWindow_Buffer& nativeWindowBuffer){
MEASURE_FUNCTION_EXECUTION_TIME
// Do some time-critical stuff
}
Which will result int the following output:
ExecutionTime: For DecodeMJPEGtoANativeWindowBuffer is 54ms
Note that this (as all other suggested solutions) will measure the time between when your function was called and when it returned, not neccesarily the time your CPU was executing the function. However, if you don't give the scheduler any change to suspend your running code by calling sleep() or similar there is no difference between.
It is a very easy to use method in C++11.
We can use std::chrono::high_resolution_clock from header
We can write a method to print the method execution time in a much readable form.
For example, to find the all the prime numbers between 1 and 100 million, it takes approximately 1 minute and 40 seconds.
So the execution time get printed as:
Execution Time: 1 Minutes, 40 Seconds, 715 MicroSeconds, 715000 NanoSeconds
The code is here:
#include <iostream>
#include <chrono>
using namespace std;
using namespace std::chrono;
typedef high_resolution_clock Clock;
typedef Clock::time_point ClockTime;
void findPrime(long n, string file);
void printExecutionTime(ClockTime start_time, ClockTime end_time);
int main()
{
long n = long(1E+8); // N = 100 million
ClockTime start_time = Clock::now();
// Write all the prime numbers from 1 to N to the file "prime.txt"
findPrime(n, "C:\\prime.txt");
ClockTime end_time = Clock::now();
printExecutionTime(start_time, end_time);
}
void printExecutionTime(ClockTime start_time, ClockTime end_time)
{
auto execution_time_ns = duration_cast<nanoseconds>(end_time - start_time).count();
auto execution_time_ms = duration_cast<microseconds>(end_time - start_time).count();
auto execution_time_sec = duration_cast<seconds>(end_time - start_time).count();
auto execution_time_min = duration_cast<minutes>(end_time - start_time).count();
auto execution_time_hour = duration_cast<hours>(end_time - start_time).count();
cout << "\nExecution Time: ";
if(execution_time_hour > 0)
cout << "" << execution_time_hour << " Hours, ";
if(execution_time_min > 0)
cout << "" << execution_time_min % 60 << " Minutes, ";
if(execution_time_sec > 0)
cout << "" << execution_time_sec % 60 << " Seconds, ";
if(execution_time_ms > 0)
cout << "" << execution_time_ms % long(1E+3) << " MicroSeconds, ";
if(execution_time_ns > 0)
cout << "" << execution_time_ns % long(1E+6) << " NanoSeconds, ";
}
I recommend using steady_clock which is guarunteed to be monotonic, unlike high_resolution_clock.
#include <iostream>
#include <chrono>
using namespace std;
unsigned int stopwatch()
{
static auto start_time = chrono::steady_clock::now();
auto end_time = chrono::steady_clock::now();
auto delta = chrono::duration_cast<chrono::microseconds>(end_time - start_time);
start_time = end_time;
return delta.count();
}
int main() {
stopwatch(); //Start stopwatch
std::cout << "Hello World!\n";
cout << stopwatch() << endl; //Time to execute last line
for (int i=0; i<1000000; i++)
string s = "ASDFAD";
cout << stopwatch() << endl; //Time to execute for loop
}
Output:
Hello World!
62
163514
Since none of the provided answers are very accurate or give reproducable results I decided to add a link to my code that has sub-nanosecond precision and scientific statistics.
Note that this will only work to measure code that takes a (very) short time to run (aka, a few clock cycles to a few thousand): if they run so long that they are likely to be interrupted by some -heh- interrupt, then it is clearly not possible to give a reproducable and accurate result; the consequence of which is that the measurement never finishes: namely, it continues to measure until it is statistically 99.9% sure it has the right answer which never happens on a machine that has other processes running when the code takes too long.
https://github.com/CarloWood/cwds/blob/master/benchmark.h#L40
You can have a simple class which can be used for this kind of measurements.
class duration_printer {
public:
duration_printer() : __start(std::chrono::high_resolution_clock::now()) {}
~duration_printer() {
using namespace std::chrono;
high_resolution_clock::time_point end = high_resolution_clock::now();
duration<double> dur = duration_cast<duration<double>>(end - __start);
std::cout << dur.count() << " seconds" << std::endl;
}
private:
std::chrono::high_resolution_clock::time_point __start;
};
The only thing is needed to do is to create an object in your function at the beginning of that function
void veryLongExecutingFunction() {
duration_calculator dc;
for(int i = 0; i < 100000; ++i) std::cout << "Hello world" << std::endl;
}
int main() {
veryLongExecutingFunction();
return 0;
}
and that's it. The class can be modified to fit your requirements.
C++11 cleaned up version of Jahid's response:
#include <chrono>
#include <thread>
void long_operation(int ms)
{
/* Simulating a long, heavy operation. */
std::this_thread::sleep_for(std::chrono::milliseconds(ms));
}
template<typename F, typename... Args>
double funcTime(F func, Args&&... args){
std::chrono::high_resolution_clock::time_point t1 =
std::chrono::high_resolution_clock::now();
func(std::forward<Args>(args)...);
return std::chrono::duration_cast<std::chrono::milliseconds>(
std::chrono::high_resolution_clock::now()-t1).count();
}
int main()
{
std::cout<<"expect 150: "<<funcTime(long_operation,150)<<"\n";
return 0;
}
This is a very basic timer class which you can expand on depending on your needs. I wanted something straightforward which can be used cleanly in code. You can mess with it at coding ground with this link: http://tpcg.io/nd47hFqr.
class local_timer {
private:
std::chrono::_V2::system_clock::time_point start_time;
std::chrono::_V2::system_clock::time_point stop_time;
std::chrono::_V2::system_clock::time_point stop_time_temp;
std::chrono::microseconds most_recent_duration_usec_chrono;
double most_recent_duration_sec;
public:
local_timer() {
};
~local_timer() {
};
void start() {
this->start_time = std::chrono::high_resolution_clock::now();
};
void stop() {
this->stop_time = std::chrono::high_resolution_clock::now();
};
double get_time_now() {
this->stop_time_temp = std::chrono::high_resolution_clock::now();
this->most_recent_duration_usec_chrono = std::chrono::duration_cast<std::chrono::microseconds>(stop_time_temp-start_time);
this->most_recent_duration_sec = (long double)most_recent_duration_usec_chrono.count()/1000000;
return this->most_recent_duration_sec;
};
double get_duration() {
this->most_recent_duration_usec_chrono = std::chrono::duration_cast<std::chrono::microseconds>(stop_time-start_time);
this->most_recent_duration_sec = (long double)most_recent_duration_usec_chrono.count()/1000000;
return this->most_recent_duration_sec;
};
};
The use for this being
#include <iostream>
#include "timer.hpp" //if kept in an hpp file in the same folder, can also before your main function
int main() {
//create two timers
local_timer timer1 = local_timer();
local_timer timer2 = local_timer();
//set start time for timer1
timer1.start();
//wait 1 second
while(timer1.get_time_now() < 1.0) {
}
//save time
timer1.stop();
//print time
std::cout << timer1.get_duration() << " seconds, timer 1\n" << std::endl;
timer2.start();
for(long int i = 0; i < 100000000; i++) {
//do something
if(i%1000000 == 0) {
//return time since loop started
std::cout << timer2.get_time_now() << " seconds, timer 2\n"<< std::endl;
}
}
return 0;
}
I'm trying to get the linux timestamp from X milliseconds ago.
I know you can do something like this for seconds:
time_t old_time = difftime(time(nullptr), num_seconds)
However, I'm trying to get the timestamp (aka time from beginning of epoch) in milliseconds, and unfortunately it seems that difftime/time(nullptr) only works in increments of seconds.
I also saw some ideas around using std::chrono::system_clock but I wasn't able to get that to work, something around the way I was casting/etc. is wrong I think.
Here is an example of what I tried (my very first step, doesn't even compile):
Clock::time_point time = Clock::now();
std::chrono::system_clock::duration diff =
std::chrono::duration<int,std::milli>(100);
time = time - diff;
time_t final_timestamp = std::chrono::duration_cast<std::chrono::milliseconds>(time.time_since_epoch()).count();
Does anyone know of an easy solution for this issue? Thanks in advance!
#include <chrono>
#include <iostream>
int
main()
{
using namespace std::chrono;
auto old_time = time_point_cast<milliseconds>(system_clock::now()) - 100ms;
std::cout << old_time.time_since_epoch().count() << "ms\n";
}
This gets the current time UTC, truncates it to milliseconds precision, and then subtracts 100ms from that time point.
It then extracts the underlying value and prints it out (milliseconds since 1970-01-01 00:00:00.000 UTC).
This should help out
#include <iostream>
#include <time.h>
using namespace std;
int main(int argc,char** argv){
clock_t start_time = clock();
const long int REPEAT_CYCLES = 500000000;
int result = 0;
for(long int i=0;i<REPEAT_CYCLES;i++)
result = (result > 1000) ? result-1 : result+1;
clock_t finish_time = clock();
cout << "WORK EXECUTION TIME: " << (double)(finish_time - start_time)/1000 << "ms" << endl;
return EXIT_SUCCESS;
}
I'm clearly too stupid to use the C++17 <chrono> library. Compiling the following...
#include <chrono>
#include <iostream>
int main() {
using clock = std::chrono::steady_clock;
using duration = std::chrono::duration<double, std::chrono::seconds>;
using timepoint = std::chrono::time_point<clock, duration>;
timepoint t0 = clock::now();
for (int i = 0; i < 1000; i++) {
timepoint t = clock::now();
duration d = t-t0;
double seconds = d.count();
std::cout << seconds << std::endl;
}
}
I get...
/usr/include/c++/8/chrono:319:16: error: static assertion failed:
period must be a specialization of ratio
static_assert(__is_ratio<_Period>::value,
^~~~~~~~~~~~~~~~~~~
Any ideas?
The second type parameter to std::chrono::duration needs to be a ratio (ticks per second), not another duration (see https://en.cppreference.com/w/cpp/chrono/duration). std::chrono::seconds is a duration. You'd want this instead:
using duration = std::chrono::duration<double, std::ratio<1> >;
FYI std::chrono::seconds is basically a std::chrono::duration<some integer type, std::ratio<1> >; your duration type is sort of like seconds but with a floating point number instead of an integer.
EDIT: Works just fine, I messed up in another place of my code.
I am trying to increase an Integer once every minute, using C++11 chrono library. For some reasons, the comparison does not work as it should: It just returns true every time. Is something wrong with the cast to seconds? Shouldn't the result be an int, containing the difference of both time points in seconds?
Would really appreciate your help! Here's the Code:
std::chrono::time_point<std::chrono::system_clock> starttime = std::chrono::system_clock::now();
int timeLine = 0;
int main() {
while (true) {
std::chrono::time_point<std::chrono::system_clock> now = std::chrono::system_clock::now();
int seconds = timeLine * 60;
if ((std::chrono::duration_cast<std::chrono::seconds>(starttime - now)).count() + seconds <= 0) {
timeLine++;
nextConstellation();
cout << "Timeline: " << timeLine << endl;
}
}
}
Here is a safer, and more readable way to write this code:
std::chrono::time_point<std::chrono::system_clock> starttime = std::chrono::system_clock::now();
int timeLine = 0;
int main() {
while (true) {
std::chrono::time_point<std::chrono::system_clock> now = std::chrono::system_clock::now();
std::chrono::seconds seconds = timeLine * std::chrono::minutes{1};
if (starttime - now + seconds <= std::chrono::seconds{0}) {
timeLine++;
nextConstellation();
std::cout << "Timeline: " << timeLine << std::endl;
}
}
}
In a nutshell, stay within the chrono-type system, and trust it to do the units conversions for you, implicitly wherever possible.
Or perhaps even more simply:
// ...
auto timelimit = timeLine * std::chrono::minutes{1};
if (now - starttime >= timelimit) {
// ...
And if in C++14, add using namespace std::chrono_literals and:
auto timelimit = timeLine * 1min;
How can I get current date time of the systems in seconds in C++?
I tried this one:
struct tm mytm = { 0 };
time_t result;
result = mktime(&mytm);
printf("%lld\n", (long long) result);
but I got: -1?
/* time example */
#include <stdio.h>
#include <time.h>
int main ()
{
time_t seconds;
seconds = time (NULL);
printf ("%ld seconds since January 1, 1970", seconds);
return 0;
}
C++11 version, which ensures that the representation of ticks is actually an integral:
#include <iostream>
#include <chrono>
#include <type_traits>
std::chrono::system_clock::rep time_since_epoch(){
static_assert(
std::is_integral<std::chrono::system_clock::rep>::value,
"Representation of ticks isn't an integral value."
);
auto now = std::chrono::system_clock::now().time_since_epoch();
return std::chrono::duration_cast<std::chrono::seconds>(now).count();
}
int main(){
std::cout << time_since_epoch() << std::endl;
}
Try this:
I hope it will work for you.
#include <iostream>
#include <ctime>
using namespace std;
int main( )
{
// current date/time based on current system
time_t now = time(0);
// convert now to string form
char* dt = ctime(&now);
cout << "The local date and time is: " << dt << endl;
// convert now to tm struct for UTC
tm *gmtm = gmtime(&now);
dt = asctime(gmtm);
cout << "The UTC date and time is:"<< dt << endl;
}
possibly a simpler version of example provided by #Zeta
time_t time_since_epoch()
{
auto now = std::chrono::system_clock::now();
return std::chrono::system_clock::to_time_t( now );
}
I'm using below function with few minor enhancements but as other have suggested definition of epoch might not be portable. For GCC it returns seconds as double value since Unix epoch but in VC++ it returns value since machine boot time. If your goal is just to get some value to do diff between two timestamps without persisting and sharing them, this should be ok however. If you need to persist or share timestamps then I would suggest to explicitly subtract some epoch from now() to get the duration object to be portable.
//high precision time in seconds since epoch
static double getTimeSinceEpoch(std::chrono::high_resolution_clock::time_point* t = nullptr)
{
using Clock = std::chrono::high_resolution_clock;
return std::chrono::duration<double>((t != nullptr ? *t : Clock::now() ).time_since_epoch()).count();
}
This will give the current date/time in seconds,
#include <time.h>
time_t timeInSec;
time(&timeInSec);
PrintLn("Current time in seconds : \t%lld\n", (long long)timeInSec);