Timers

From ScorecWiki

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

Contents 1 cycle counting (rdtsc cycle counter) 2 MPI_WTime() 3 getTimeOfDay() and friends (sys/time.h) 4 time.h/ctime.h functions

cycle counting (rdtsc cycle counter)

• source code
• precise
• cpu frequency required
  • get the cpu frequency in MHz with

   cat /proc/cpuinfo | grep -i "cpu MHZ" -m 1 | awk '{print $4}'

example 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 high resolution

MPI_WTime()

• May offer a high resolution than 1 second.
  • MPI_WTick() - query to determine resolution.

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

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

time.h/ctime.h functions

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