Monday, December 27, 2004

The Philosophy of Profilers

When you are writing a profiler, you can choose from two ways of collecting data: probing and sampling.

A probe profiler collects data by inserting probes or hooks into the application so the profiler runtime is called whenever the program executes that hook. It has the following benefits:
  • The inserted probes will always be called when the application executes.
  • Having a complete picture of the run
  • Parent-child relationships between functions is guaranteed to be correct and the profiler can report perfect call trees necessary for you to easily find the call paths that took the longest time.

It has the following drawbacks:

  • The instrumentation scheme can be cumbersome to use
  • Since it's rewriting at the binary level, there's plenty of areas to introduce potential errors.
  • Probes also take up space, resulting in some code bloat and slower performance.
  • Slowdown makes almost impossible to run the instrumented binary on a production system

Sampling profilers take a snapshot of what's executing in the application at predefined intervals. VS Debugger is an example. It has the following benefits:

  • Having far less overhead than probe profilers.
  • Standing a much greater chance of using them on a production system without grinding the server to a halt.

It has the following drawbacks:

  • It's entirely possible that all the samples taken of your application could show no code at all.
  • Traditional sampling profilers that only grab the currently executing instruction for each thread make it much more difficult to determine the parent-child relationship between methods, so determining the worst performing code path is much more difficult.

Notes above are taken from John Robbins's article:

Make Your Apps Fly with the New Enterprise Performance Tool

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