Timers

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** sample code <code> ** sample code <code>
#define CPUFREQ <INSERT FREQUENCY IN Hz HERE> #define CPUFREQ <INSERT FREQUENCY IN Hz HERE>
 + unsigned long long start_time = 0;
 + unsigned long long finish_time = 0;
 + unsigned long long total_time = 0;
 + start_time = rdtsc();
 + //do stuff
 + finish_time = rdtsc();
total_time = finish_time - start_time; total_time = finish_time - start_time;
printf ("It took %.8lf seconds.\n", total_time / CPUFREQ );</code> printf ("It took %.8lf seconds.\n", total_time / CPUFREQ );</code>

Revision as of 14:58, 7 August 2011

This page serves as a gateway for information related to computing the elapsed time between two positions in your software.

C/C++

1- cycle counting (rdtsc cycle counter) - source code

  • precise (CS)
  • cpu frequency required
    • get the cpu frequency with
   cat /proc/cpuinfo | grep -i "cpu MHZ" -m 1 | awk '{print $4}'
    • sample code
   #define CPUFREQ <INSERT FREQUENCY IN Hz HERE> 
   unsigned long long start_time = 0;
   unsigned long long finish_time = 0;
   unsigned long long total_time = 0;
   start_time = rdtsc();
   //do stuff
   finish_time = rdtsc();
   total_time = finish_time - start_time;  
   printf ("It took %.8lf seconds.\n", total_time / CPUFREQ );
  • low cost and resolution (OS)

2- MPI_WTime()

  • May offer a high resolution than 1 second.(CS)

3- MPI_WTick()

  • can be queried to determine resolution. (CS)

4- getTimeOfDay() and friends (sys/time.h)

  • portable (BM)
  • not always precise or accurate (BM)
  • may be relatively expensive on IBM BG (compared to cycle counting) (OS)

5- time.h/ctime.h functions (Reference : http://www.cppreference.com/wiki/c/date/start )

  • Have a higher resolution than 1 second (CS)
  • clock function returns elapsed CPU cycles (CS)
  • Needs cpu frequency which may be inaccurate on POSIX compilers (CS)

Note: Feel free to make corrections -- MM

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