How to evaluate a run

Last updated on 2026-10-27 | Edit this page

Overview

Questions

  • What do you do after a run finishes, and what do you do with that answer?
  • Which line of time -p is the time you waited?

Objectives

  • Name the three steps in the life cycle of an HPC application.
  • Time one short command with time -p and read real.

The life cycle

An HPC application is software written to use a supercomputer or a cluster on a hard problem. It does three kinds of work: intensive computation, communication between parallel processes, and input and output on a fast file system.

The life cycle is a loop with three steps.

  1. Run the application.
  2. Evaluate the performance.
  3. Make changes to the application.

Then you run it again. A change you have not timed is a guess.

What “evaluate” can mean

The deck lists several ways to evaluate a run:

  • Tools such as Score-P, Scalasca, Nsight Systems, and AMD uProf.
  • The execution time.
  • The memory footprint.
  • The network load.
  • A comparison with a similar application or model.
  • A comparison on different hardware.
  • A guess you have not measured.

This episode uses the second one. You time one short command and write down a single number. Later episodes compare that kind of number. They do not replace this first step.

time -p

time -p is the POSIX form. It prints three lines on standard error.

  • real is the elapsed time, the seconds you waited. That is the wall-clock time.
  • user and sys are CPU time. Added together they can be larger than real when several cores run at once. They can be near zero when the process is waiting.
Challenge

One short timed run

Run:

BASH 

time -p sleep 1

Which line is the wall-clock time, and about how large is it?

real is the wall-clock time. For sleep 1 it is about 1 second. user and sys are near zero because the process spent the second waiting, not computing. Write the real line down. That is the whole measurement this episode asks for: one short timed run, one number.

Key Points
  • The life cycle is run, evaluate, change, and run again.
  • time -p prints real, user, and sys.
  • real is the wall-clock seconds you waited.
  • Start with one short timed run before you reach for a profiler.