Execution time of C program

Question

I have a C program that aims to be run in parallel on several processors  I need to be able to record the execution time  which could be anywhere from 1 second to several minutes   I have searched for answers  but they all seem to suggest using the clock   function  which then involves calculating the number of clocks the program took divided by the Clocks per second value   I m not sure how the Clocks per second value is calculated    In Java  I just take the current time in milliseconds before and after execution    Is there a similar thing in C  I ve had a look  but I can t seem to find a way of getting anything better than a second resolution    I m also aware a profiler would be an option  but am looking to implement a timer myself   Thanks

User · Answer

You have to take into account that measuring the time that took a program to execute depends a lot on the load that the machine has in that specific moment.

Knowing that, the way of obtain the current time in C can be achieved in different ways, an easier one is:

#include <time.h>

#define CPU_TIME (getrusage(RUSAGE_SELF,&ruse), ruse.ru_utime.tv_sec + \
  ruse.ru_stime.tv_sec + 1e-6 * \
  (ruse.ru_utime.tv_usec + ruse.ru_stime.tv_usec))

int main(void) {
    time_t start, end;
    double first, second;

    // Save user and CPU start time
    time(&start);
    first = CPU_TIME;

    // Perform operations
    ...

    // Save end time
    time(&end);
    second = CPU_TIME;

    printf("cpu  : %.2f secs\n", second - first); 
    printf("user : %d secs\n", (int)(end - start));
}

Hope it helps.

Regards!

User · Answer

Comparison of execution time of bubble sort and selection sort I have a program which compares the execution time of bubble sort and selection sort  To find out the time of execution of a block of code compute the time before and after the block by    clock t start clock          clock t end clock     CLOCKS PER SEC is constant in time h library   Example code    include  lt stdio h gt   include  lt stdlib h gt   include  lt time h gt  int main        int a 10000  i j min temp     for i 0 i lt 10000 i               a i  rand   10000            The bubble Sort    clock t start end     start clock       for i 0 i lt 10000 i              for j i 1 j lt 10000 j                  if a i  gt a j                     int temp a i            a i  a j            a j  temp                          end clock       double extime  double   end-start  CLOCKS PER SEC     printf   n tExecution time for the bubble sort is  f seconds n   extime       for i 0 i lt 10000 i              a i  rand   10000          clock t start1 end1     start1 clock          The Selection Sort    for i 0 i lt 10000 i              min i       for j i 1 j lt 10000 j                  if a min  gt a j                     min j                       temp a min        a min  a i        a i  temp          end1 clock       double extime1  double   end1-start1  CLOCKS PER SEC     printf   n       printf   tExecution time for the selection sort is  f seconds n n   extime1      if extime1 lt extime       printf   tSelection sort is faster than Bubble sort by  f seconds n n   extime - extime1      else if extime1 gt extime       printf   tBubble sort is faster than Selection sort by  f seconds n n   extime1 - extime      else      printf   tBoth algorithms have the same execution time n n

User · Answer

CLOCKS PER SEC is a constant which is declared in  lt time h gt   To get the CPU time used by a task within a C application  use   clock t begin   clock        here  do your time-consuming job     clock t end   clock    double time spent    double  end - begin    CLOCKS PER SEC    Note that this returns the time as a floating point type  This can be more precise than a second  e g  you measure 4 52 seconds   Precision depends on the architecture  on modern systems you easily get 10ms or lower  but on older Windows machines  from the Win98 era  it was closer to 60ms   clock   is standard C  it works  everywhere   There are system-specific functions  such as getrusage   on Unix-like systems   Java s System currentTimeMillis   does not measure the same thing  It is a  wall clock   it can help you measure how much time it took for the program to execute  but it does not tell you how much CPU time was used  On a multitasking systems  i e  all of them   these can be widely different

User · Answer

Some might find a different kind of input useful  I was given this method of measuring time as part of a university course on GPGPU-programming with NVidia CUDA  course description   It combines methods seen in earlier posts  and I simply post it because the requirements give it credibility   unsigned long int elapsed  struct timeval t start  t end  t diff  gettimeofday  amp t start  NULL       perform computations      gettimeofday  amp t end  NULL   timeval subtract  amp t diff   amp t end   amp t start   elapsed    t diff tv sec 1e6   t diff tv usec   printf  GPU version runs in   lu microsecs n   elapsed     I suppose you could multiply with e g  1 0   1000 0 to get the unit of measurement that suits your needs

User · Answer

A lot of answers have been suggesting clock   and then CLOCKS PER SEC from time h   This is probably a bad idea  because this is what my  bits time h file says      ISO IEC 9899 1990 7 12 1   lt time h gt  The macro  CLOCKS PER SEC  is the number per second of the value returned by the  clock  function        CAE XSH  Issue 4  Version 2   lt time h gt  The value of CLOCKS PER SEC is required to be 1 million on all XSI-conformant systems        define CLOCKS PER SEC  1000000l     if  defined   STRICT ANSI    amp  amp   defined   USE XOPEN2K    Even though CLOCKS PER SEC has such a strange value CLK TCK presents the real value for clock ticks per second for the system          include  lt bits types h gt  extern long int   sysconf  int       define CLK TCK     clock t    sysconf  2       2 is  SC CLK TCK       endif   So CLOCKS PER SEC might be defined as 1000000  depending on what options you use to compile  and thus it does not seem like a good solution

User · Answer

include lt time h gt       include lt stdio h gt      int main    clock t begin clock         int i  for i 0 i lt 100000 i     printf   d  i      clock t end clock    printf  Time taken  lf   double  end-begin  CLOCKS PER SEC          This program will work like charm

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Thomas Pornin s answer as macros    define TICK X  clock t X   clock    define TOCK X  printf  time  s   g sec  n     X    double  clock   -  X     CLOCKS PER SEC    Use it like this   TICK TIME A   functionA    TOCK TIME A    TICK TIME B   functionB    TOCK TIME B     Output   time TIME A  0 001652 sec  time TIME B  0 004028 sec

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In plain vanilla C    include  lt time h gt   include  lt stdio h gt   int main         clock t tic   clock         my expensive function which can spawn threads         clock t toc   clock         printf  Elapsed   f seconds n    double  toc - tic    CLOCKS PER SEC        return 0

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All answers here are lacking  if your sysadmin changes the systemtime  or your timezone has differing winter- and sommer-times  Therefore      On linux use  clock gettime CLOCK MONOTONIC RAW   amp time variable   It s not affected if the system-admin changes the time  or you live in a country with winter-time different from summer-time  etc    include  lt stdio h gt   include  lt time h gt    include  lt unistd h gt     for sleep       int main         struct timespec begin  end      clock gettime CLOCK MONOTONIC RAW   amp begin        sleep 1           waste some time      clock gettime CLOCK MONOTONIC RAW   amp end        printf   Total time    f seconds n                end tv nsec - begin tv nsec    1000000000 0                end tv sec  - begin tv sec         man clock gettime states    CLOCK MONOTONIC               Clock  that  cannot  be set and represents monotonic time since some unspecified starting point   This clock is not affected by discontinuous jumps in the system time                e g   if the system administrator manually changes the clock   but is affected by the incremental adjustments performed by adjtime 3  and NTP

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You functionally want this    include  lt sys time h gt   struct timeval  tv1  tv2  gettimeofday  amp tv1  NULL      stuff to do     gettimeofday  amp tv2  NULL    printf   Total time    f seconds n             double   tv2 tv usec - tv1 tv usec    1000000             double   tv2 tv sec - tv1 tv sec      Note that this measures in microseconds  not just seconds

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Every solution s are not working in my system    I can get using    include  lt time h gt   double difftime time t time1  time t time0

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ANSI C only specifies second precision time functions  However  if you are running in a POSIX environment you can use the gettimeofday   function that provides microseconds resolution of time passed since the UNIX Epoch   As a side note  I wouldn t recommend using clock   since it is badly implemented on many if not all   systems and not accurate  besides the fact that it only refers to how long your program has spent on the CPU and not the total lifetime of the program  which according to your question is what I assume you would like to measure

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perf tool is more accurate to be used in order to collect and profile the running program  Use perf stat to show all information related to the program being executed

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I ve found that the usual clock    everyone recommends here  for some reason deviates wildly from run to run  even for static code without any side effects  like drawing to screen or reading files  It could be because CPU changes power consumption modes  OS giving different priorities  etc     So the only way to reliably get the same result every time with clock   is to run the measured code in a loop multiple times  for several minutes   taking precautions to prevent the compiler from optimizing it out  modern compilers can precompute the code without side effects running in a loop  and move it out of the loop   like i e  using random input for each iteration   After enough samples are collected into an array  one sorts that array  and takes the middle element  called median  Median is better than average  because it throws away extreme deviations  like say antivirus taking up all CPU up or OS doing some update   Here is a simple utility to measure execution performance of C C   code  averaging the values near median  https   github com saniv gauge  I m myself still looking for a more robust and faster way to measure code  One could probably try running the code in controlled conditions on bare metal without any OS  but that will give unrealistic result  because in reality OS does get involved   x86 has these hardware performance counters  which including the actual number of instructions executed  but they are tricky to access without OS help  hard to interpret and have their own issues   http   archive gamedev net archive reference articles article213 html    Still they could be helpful investigating the nature of the bottle neck  data access or actual computations on that data

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Most of the simple programs have computation time in milli-seconds  So  i suppose  you will find this useful    include  lt time h gt   include  lt stdio h gt   int main        clock t start   clock           Execuatable code     clock t stop   clock        double elapsed    double  stop - start    1000 0   CLOCKS PER SEC      printf  Time elapsed in ms   f   elapsed       If you want to compute the runtime of the entire program and you are on a Unix system  run your program using the time command like this time   a out

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If you are using the Unix shell for running  you can use the time command   doing     time   a out   assuming a out as the executable will give u the time taken to run this

[c] Execution time of C program

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