Never reason from the results of a sampling profiler

In the quest for software optimization, a trusty companion is the sampling profiler, a tool available in most programming languages. These profilers work unobtrusively, taking snapshots of the program’s state and recording the currently executing function or instruction.

While profilers sound like a silver bullet for identifying performance bottlenecks, their usefulness has limitations. They excel at highlighting bottlenecks, but often lack the nuance to pinpoint the root cause. In simpler cases, a profiler’s report might be enough, but relying solely on this data can lead one astray. Misinterpretations of profiling results are a common pitfall I’ve observed amongst programmers.

Imagine a store manager facing customer complaints about long lines. A frustrated customer like myself might be stuck due to a malfunctioning cash register. However, if the manager, instead of fixing the register, simply photographs the queue to identify the “culprit,” I, the innocent bystander, might be wrongly blamed for the delay. Profilers can be just as misleading, highlighting symptoms without revealing the underlying cause.

You are taking a few simple screenshots of a complex system. Of course, you can take more screenshots, and make your screenshots richer, but then you start affecting the system, and falling prey to a software-equivalent Heinsenberg uncertainy principle: you can either know the state of your program very precisely at all times, but then you know little about its speed, or you can measure quite precisely the speed, but with little idea of the intermediate speeds.

Do use sampling profilers. I find them useful. Just do not reason about your problems from them. They merely offer a starting point.

Further reading: Sampling profilers can mislead, and mastering any one tool (e.g., perf or VTune or uPerf) won’t magically confer analysis expertise.

Daniel Lemire, "Never reason from the results of a sampling profiler," in Daniel Lemire's blog, May 30, 2024.

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Daniel Lemire

A computer science professor at the University of Quebec (TELUQ).

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