<time.h> / <ctime> are not counting ticks - c++

EDIT: It appears to be functioning now. The code has been updated to show my revisions. Thank you all for your help.
I imagine I'm just stupid, but I'm attempting to use ctime to count CPU ticks through my entire program. I'm writing an encryption algorithm for a school project and I'm trying to include a timer so that I can add noise processes, equalizing the amount of time among different key/plaintext combinations.
Here is a little test for ctime:
#include <iostream>
#include <string>
#include <ctime>
int main (int arc, char **argv)
{
double elapsedTime;
const clock_t start = clock ();
int uselessInt = 0;
for (int i = 0; i <= 200; i++)
{
uselessInt = uselessInt * 2 / 3 + i;
std::cout << uselessInt << std::endl;
}
clock_t end = clock();
elapsedTime = static_cast<double>(end - start);
std::cout << elapsedTime << " CPU ticks have elapsed since this application's initiation." << std::endl;
return (0);
}
which prints:
0
1
2
4
/* ... long list of numbers ... */
591
594
0 CPU ticks have elapsed since this application's initiation.
[smalltock#localhost Desktop]$
I am using GCC (G++) and it appears that ctime/time.h simply isn't counting ticks like I want it to. Can anybody identify the problem? I'm a relative amateur in this language.

My two cents. When you do cin.get(), it waits for your to input something on the console, did you do anything or simply typed enter?
I did run your code without typing any text but simply press enter, it gave the following output:
Test Text
It's a stone, Luigi... you didn't make it.
0 CPU ticks have elapsed since this application's initiation.
Real 0m0.700s
User 0m0.000s
Sys 0m0.061s
It may be because the precision of CLOCKS_PER_SEC is kind of "big" (in seconds) compared to the CPU time used by your program
Meanwhile, a syntax error in duration line, you either missed another ) or should delete the first (
BTW:
Real is wall clock time - time from start to finish of the call.
User is the amount of CPU time spent in user-mode code (outside the kernel) within the process. This is only actual CPU time used in executing the process.
Sys is the amount of CPU time spent in the kernel within the process.
So you basically have 0 CPU time since you are keep waiting for I/O, no CPU computation.

elapsedTime in your program is a measure of time in seconds, not a count of clock ticks. If you want ticks, use duration.
Since your program (presumably) spends the vast majority of its time blocked on I/O, not very many seconds are going to have gone by.

Related

how to write a measurement function for multithreaded function [duplicate]

I am running a .cpp code (i) in sequential style and (ii) using OpenMP statements. I am trying to see the time difference. For calculating time, I use this:
#include <time.h>
.....
main()
{
clock_t start, finish;
start = clock();
.
.
.
finish = clock();
processing time = (double(finish-start)/CLOCKS_PER_SEC);
}
The time is pretty accurate in sequential (above) run of the code. It takes about 8 seconds to run this. When I insert OpenMP statements in the code and thereafter calculate the time I get a reduction in time, but the time displayed is about 8-9 seconds on the console, when actually its just 3-4 seconds in real time!
Here is how my code looks abstractly:
#include <time.h>
.....
main()
{
clock_t start, finish;
start = clock();
.
.
#pragma omp parallel for
for( ... )
for( ... )
for (...)
{
...;
}
.
.
finish = clock();
processing time = (double(finish-start)/CLOCKS_PER_SEC);
}
When I run the above code, I get the reduction in time but the time displayed is not accurate in terms of real time. It seems to me as though the clock () function is calculating each thread's individual time and adding up them up and displaying them.
Can someone tell the reason for this or suggest me any other timing function to use to measure the time in OpenMP programs?
Thanks.
It seems to me as though the clock () function is calculating each thread's individual time and adding up them up and displaying them.
This is exactly what clock() does - it measures the CPU time used by the process, which at least on Linux and Mac OS X means the cumulative CPU time of all threads that have ever existed in the process since it was started.
Real-clock (a.k.a. wall-clock) timing of OpenMP applications should be done using the high resolution OpenMP timer call omp_get_wtime() which returns a double value of the number of seconds since an arbitrary point in the past. It is a portable function, e.g. exists in both Unix and Windows OpenMP run-times, unlike gettimeofday() which is Unix-only.
I've seen clock() reporting CPU time, instead of real time.
You could use
struct timeval start, end;
gettimeofday(&start, NULL);
// benchmark code
gettimeofday(&end, NULL);
delta = ((end.tv_sec - start.tv_sec) * 1000000u +
end.tv_usec - start.tv_usec) / 1.e6;
To time things instead
You could use the built in omp_get_wtime function in omp library itself. Following is an example code snippet to find out execution time.
#include <stdio.h>
#include <omp.h>
int main(){
double itime, ftime, exec_time;
itime = omp_get_wtime();
// Required code for which execution time needs to be computed
ftime = omp_get_wtime();
exec_time = ftime - itime;
printf("\n\nTime taken is %f", exec_time);
}
Well yes, that's what clock() is supposed to do, tell you how much processor time the program used.
If you want to find elapsed real time, instead of CPU time, use a function that returns wall clock time, such as gettimeofday().
#include "ctime"
std::time_t start, end;
long delta = 0;
start = std::time(NULL);
// do your code here
end = std::time(NULL);
delta = end - start;
// output delta

Simplest way to have the following timer

I am currently trying to re-write some software in C++ from an old python code.
In the python version I used to have timers like these:
from time import time, sleep
t_start = time()
while (time()-t_start < 5) # 5 second timer
# do some stuff
sleep(0.001) #Don't slam the CPU
sleep(1)
print time()-t_start # would print something like 6.123145 notice the precision!
However, in C++ when I try to use time(0) from < time.h > I can only get precision in seconds as an integer, not a float.
#include <time.h>
#include <iostream>
time_t t_start = time(0)
while (time(0) - t_start < 5) // again a 5 second timer.
{
// Do some stuff
sleep(0.001) // long boost sleep method.
}
sleep(1);
std::cout << time(0)-t_start; // Prints 6 instead of 6.123145
I have also tried gettimeofday(struct, NULL) from < sys/time.h > however whenever I sleep the program with boost::this_thread::sleep it doesn't count that time...
I hope somebody here has come across a similar problem and found a solution.
Also, I really do need the dt in at least millisecond precision, because in the
// Do some stuff
part of the code I may break out of the while loop early and I need to know how long I was inside, etc.
Thank you for reading!
gettimeofday() is known to have issues when there are discontinuous jumps in the system time.
For portable milisecond precision have a look at chrono::high_resolution_clock()
Here a little snippet:
chrono::high_resolution_clock::time_point ts = chrono::high_resolution_clock::now();
chrono::high_resolution_clock::time_point te = chrono::high_resolution_clock::now();
// ... do something ...
cout << "took " << chrono::duration_cast<chrono::milliseconds>(te - ts).count() << " millisecs\n";
Please note the real clock resolution is bound to the operating system. For instance, on windows you usually have a 15ms precision, and you can go beyond this constraint only by using platform dependent clocking systems.

Measure execution time in C++ OpenMP code

I am running a .cpp code (i) in sequential style and (ii) using OpenMP statements. I am trying to see the time difference. For calculating time, I use this:
#include <time.h>
.....
main()
{
clock_t start, finish;
start = clock();
.
.
.
finish = clock();
processing time = (double(finish-start)/CLOCKS_PER_SEC);
}
The time is pretty accurate in sequential (above) run of the code. It takes about 8 seconds to run this. When I insert OpenMP statements in the code and thereafter calculate the time I get a reduction in time, but the time displayed is about 8-9 seconds on the console, when actually its just 3-4 seconds in real time!
Here is how my code looks abstractly:
#include <time.h>
.....
main()
{
clock_t start, finish;
start = clock();
.
.
#pragma omp parallel for
for( ... )
for( ... )
for (...)
{
...;
}
.
.
finish = clock();
processing time = (double(finish-start)/CLOCKS_PER_SEC);
}
When I run the above code, I get the reduction in time but the time displayed is not accurate in terms of real time. It seems to me as though the clock () function is calculating each thread's individual time and adding up them up and displaying them.
Can someone tell the reason for this or suggest me any other timing function to use to measure the time in OpenMP programs?
Thanks.
It seems to me as though the clock () function is calculating each thread's individual time and adding up them up and displaying them.
This is exactly what clock() does - it measures the CPU time used by the process, which at least on Linux and Mac OS X means the cumulative CPU time of all threads that have ever existed in the process since it was started.
Real-clock (a.k.a. wall-clock) timing of OpenMP applications should be done using the high resolution OpenMP timer call omp_get_wtime() which returns a double value of the number of seconds since an arbitrary point in the past. It is a portable function, e.g. exists in both Unix and Windows OpenMP run-times, unlike gettimeofday() which is Unix-only.
I've seen clock() reporting CPU time, instead of real time.
You could use
struct timeval start, end;
gettimeofday(&start, NULL);
// benchmark code
gettimeofday(&end, NULL);
delta = ((end.tv_sec - start.tv_sec) * 1000000u +
end.tv_usec - start.tv_usec) / 1.e6;
To time things instead
You could use the built in omp_get_wtime function in omp library itself. Following is an example code snippet to find out execution time.
#include <stdio.h>
#include <omp.h>
int main(){
double itime, ftime, exec_time;
itime = omp_get_wtime();
// Required code for which execution time needs to be computed
ftime = omp_get_wtime();
exec_time = ftime - itime;
printf("\n\nTime taken is %f", exec_time);
}
Well yes, that's what clock() is supposed to do, tell you how much processor time the program used.
If you want to find elapsed real time, instead of CPU time, use a function that returns wall clock time, such as gettimeofday().
#include "ctime"
std::time_t start, end;
long delta = 0;
start = std::time(NULL);
// do your code here
end = std::time(NULL);
delta = end - start;
// output delta

Sleep() function usage

This is a sample pgm to check the functionality of Sleep() function.This is a demo only since iam using this sleep() and clock() functions in my app developement.
// TestTicks.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include<iostream>
#include<iomanip>
#include <Windows.h>
int _tmain(int argc, _TCHAR* argv[])
{
int i, i2;
i = clock();
//std::cout<<" \nTime before Sleep() : "<<i;
Sleep(30000);
i2 = clock();
//std::cout<<" \nTime After Sleep() : "<<i2;
std::cout<<"\n Diff : "<<i2 -i;
getchar();
return 0;
}
in this code i am calculating the time using clock() before and after the sleep function.
Since iam using sleep(30000), the time diff would be atleast 30000.
I have run this prgm many times. and printed output as 30000, 30001, 30002. These are ok. But some times i am getting values like 29999 and 29997.How this possible, since i put 30000 sleep b/w the clock().
Please give me the reason for this.
According to http://msdn.microsoft.com/en-us/library/windows/desktop/ms686298(v=vs.85).aspx:
The system clock "ticks" at a constant rate. If dwMilliseconds is less than the resolution of the system clock, the thread may sleep for less than the specified length of time. If dwMilliseconds is greater than one tick but less than two, the wait can be anywhere between one and two ticks, and so on.
It just means that the Sleep function will never sleep exactly for the amount of time given, but as close as possible given the resolution of the scheduler.
The same page gives you a method to increase the timer resolution if you really need it.
There are also high resolution timers that may better fit your needs.
Unless you're using a realtime OS, that is very much expected.
The Operating System has to schedule and run many other processes, so waking yours' up may not match the exact time you wanted to sleep.
The clock() function tells how much processor time the calling process has used.
You may replace the use of clock() by the function GetSystemTimeAsFileTime
in order to measure the time more accurately.
Also you may try to use timeBeginPeriod with wPeriodMin returned by a call to timeGetDevCaps in order to obtail maximum interrupt frequency.
In order to synchronize the sleeps with the system interrupt period, I'd also suggest
to have a sleep(1) ahead of the first "time capture".
By doing so, the "too shorts" will disappear.
More information abount sleep can be found here
#include <iostream>
#include <time.h>
void wait(int seconds)
{
int endwait;
endwait = clock() + seconds * CLOCKS_PER_SEC ;
while (clock() < endwait){}
}
int main()
{
wait(2);
cout<<"2 seconds have passed";
}

How to get system time in C++?

In fact i am trying to calculate the time a function takes to complete in my program.
So i am using the logic to get system time when i call the function and time when the function returns a value then by subtracting the values i get time it took to complete.
So if anyone can tell me some better approach or just how to get system time at an instance it would be quite a help
The approach I use when timing my code is the time() function. It returns a single numeric value to you representing the epoch which makes the subtraction part easier for calculation.
Relevant code:
#include <time.h>
#include <iostream>
int main (int argc, char *argv[]) {
int startTime, endTime, totalTime;
startTime = time(NULL);
/* relevant code to benchmark in here */
endTime = time(NULL);
totalTime = endTime - startTime;
std::cout << "Runtime: " << totalTime << " seconds.";
return 0;
}
Keep in mind this is user time. For CPU, time see Ben's reply.
Your question is totally dependant on WHICH system you are using. Each system has its own functions for getting the current time. For finding out how long the system has been running, you'd want to access one of the "high resolution performance counters". If you don't use a performance counter, you are usually limited to microsecond accuracy (or worse) which is almost useless in profiling the speed of a function.
In Windows, you can access the counter via the 'QueryPerformanceCounter()' function. This returns an arbitrary number that is different on each processor. To find out how many ticks in the counter == 1 second, call 'QueryPerformanceFrequency()'.
If you're coding under a platform other than windows, just google performance counter and the system you are coding under, and it should tell you how you can access the counter.
Edit (clarification)
This is c++, just include windows.h and import the "Kernel32.lib" (seems to have removed my hyperlink, check out the documentation at: http://msdn.microsoft.com/en-us/library/ms644904.aspx). For C#, you can use the "System.Diagnostics.PerformanceCounter" class.
You can use time_t
Under Linux, try gettimeofday() for microsecond resolution, or clock_gettime() for nanosecond resolution.
(Of course the actual clock may have a coarser resolution.)
In some system you don't have access to the time.h header. Therefore, you can use the following code snippet to find out how long does it take for your program to run, with the accuracy of seconds.
void function()
{
time_t currentTime;
time(&currentTime);
int startTime = currentTime;
/* Your program starts from here */
time(&currentTime);
int timeElapsed = currentTime - startTime;
cout<<"It took "<<timeElapsed<<" seconds to run the program"<<endl;
}
You can use the solution with std::chrono described here: Getting an accurate execution time in C++ (micro seconds) you will have much better accuracy in your measurement. Usually we measure code execution in the round of the milliseconds (ms) or even microseconds (us).
#include <chrono>
#include <iostream>
...
[YOUR METHOD/FUNCTION STARTING HERE]
auto start = std::chrono::high_resolution_clock::now();
[YOUR TEST CODE HERE]
auto elapsed = std::chrono::high_resolution_clock::now() - start;
long long microseconds = std::chrono::duration_cast<std::chrono::microseconds>(elapsed).count();
std::cout << "Elapsed time: " << microseconds << " ms;