General MPP Timing issues

• Make sure you understand exactly what the timer function returns
  • CPU time or wallclock?

  • On timesharing MPP (others using same PEs as you), is the timer running when you're sliced out?

  • Summing over all PEs or reporting for 1 PE only?

  • If CPU time, is it including or ignoring communication time?

• Use the most accurate (highest-resolution) timer function you can get.
  • Using one-second-accurate timer to time a block of code that takes 100 microseconds is a bad idea.

• Calibrate the timer function, to account for overhead of invoking the function when computing timings.
  • Generally, 3 steps:
    • Call once to allow timer function to set up internal tables, etc.

    • Call 2nd time

    • Call 3rd time, subtract value from 2nd time.
      • This is the calibration time--the overhead for getting in and out of the timer call.

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