Why the time taken by a thread is more than the total time taken by a process?

Question

I am executing the code to calculate the time taken by a Matrix multiplication Code.

I have created four threads and called the Calculate method like this:

std::thread t1( Calculate );
std::thread t2( Calculate );
std::thread t3( Calculate );
std::thread t4( Calculate );
t1.join();
t2.join();
t3.join();
t4.join();

This is the code where I am doing the matrix multiplication

void calculate()
{
clock_t starttime = clock();
//  some Code
clock_t endtime = clock();
cout << "Time Taken:"<<diffclock(endtime, starttime)<<"sec."<<endl;
}

This is the method to calculate time difference:

double diffclock(clock_t clock1,clock_t clock2)
{
double diffticks=clock1-clock2;
double diffms=(diffticks)/CLOCKS_PER_SEC;
return diffms;
}

After the execution the time taken by whole execution is displayed incorrectly. The time taken by the operation is around 22 seconds but the code is giving nearly 32 seconds as time taken. I have checked it from stopwatch and the output by this code is Incorrect.

This is the screen-shot of console output

As per the documentation of clock

In order to measure the time spent in a program, clock() should be called
at the start of the program and its return value subtracted from the value 
returned by subsequent calls. The value returned by clock() is defined for
compatibility across systems that have clocks with different resolutions.
To determine the time in seconds, the value returned by clock() should be 
divided by the value of the macro CLOCKS_PER_SEC. CLOCKS_PER_SEC is defined 
to be one million in <time.h>.

However the time returned by this time calculating code contradicts with the time provided by the IDE. I am using code::blocks here.

Am I missing some thing?

@MarcellFülöp so the problem is in clock(). Lets me check documentation and I will get beck to you soon. — Khushal Dave, Jul 13 '14 at 12:36
@MarcellFülöp COuld you please have a look at this http://pubs.opengroup.org/onlinepubs/7908799/xsh/clock.html — Khushal Dave, Jul 13 '14 at 12:37
The clock() returns the actual used CPU time. Related: http://stackoverflow.com/questions/2134363/the-c-clock-function-just-returns-a-zero — , Jul 13 '14 at 12:58
@DieterLücking how can Actual CPU time used by a thread be more than the total time taken by a process...?? — Khushal Dave, Jul 16 '14 at 17:58

score 1 · Accepted Answer · edited Jul 13 '14 at 14:23

1

Since you're on C++11, you can use std::chrono:

std::chrono::time_point<std::chrono :: system_clock> start, end;
start = std::chrono::system_clock::now();
// calculations here...
end = std::chrono::system_clock::now();
std::chrono::duration<double> elapsed = end-start;
std::cout << "Elapsed time: " << elapsed.count() << "s\n";

Note that you should also use an std::mutex when accessing cout. Your code looks fine, but cout is probably getting mixed up.

edited Jul 13 '14 at 14:23

Casey

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answered Jul 13 '14 at 12:48

kirbyfan64sos

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I am getting an error in the line `cout << "Elapsed time: " << elapsed.count << "s\n;` that `error: missing terminating character` – Khushal Dave Jul 13 '14 at 12:58
the last line has problem I replaced it by this. `cout << "Elapsed time: " << elapsed.count()< – Khushal Dave Jul 13 '14 at 13:08

Why the time taken by a thread is more than the total time taken by a process?

1 Answers1