There is a class of bugs lurking out there that I class as performance bugs. A performance bug is when the code computes the correct result, but runs slower than it should due to a programming mistake.

The nefarious thing about performance bugs is that the user may never know they are there - the program appears to work correctly, carrying out the correct operations, showing the right thing on the screen or printing the right text. It just does it a bit more slowly than it should have. It takes an experienced programmer, with a reasonably accurate mental model of the problem and the correct solution, to know how fast the operation should have been performed, and hence if the program is running slower than it should be.

I started documenting a few of the performance bugs I came across a few months ago, for example (on some platforms) the insert method of std::map is roughly 7 times slower than it should be, std::map::count() is about twice as slow as it should be, std::map::find() is 15% slower than it should be, aligned malloc is a lot slower than it should be in VS2015.

### Types of performance bugs

There are a few types of performance bugs I come across quite often:

* Doing the same work repeatedly and redundantly - It's pretty common to mistakenly perform the same operation multiple times when a single time would have sufficed, for example zeroing the same memory multiple times. This is suprisingly common when you start looking out for it.

* Performing unnecessary work - For example, in Qt, if you call dataChanged() on a single item in a data model, it updates the view for just that item. However if you update 2 items, it updates the entire view rectangle, even if those items are right by each other (e.g. two columns of the same row). See the bug report.

* Choosing a poor algorithm, for example an $$O(n^2)$$ algorithm by accident. See Accidently Quadratic.

### Defining a performance bug

I don't have a precise and bulletproof definition for a performance bug, but it would be something like:
a performance bug is one where the code is slower than the simple, textbook solution of the problem, due to performing redundant or unnecessary work, or choosing the wrong algorithm, or a logic error.

In some cases there will be a fine line between a performance bug, and plain old unoptimised code.

### How to find performance bugs

* Use a profiler.

* Try single-stepping through your program in a debugger, making a note of what work is being done. Is work being repeated? John Carmack suggests this method:

"An exercise that I try to do every once in a while is to “step a frame” in the game, starting at some major point like common->Frame(), game->Frame(), or renderer->EndFrame(), and step into every function to try and walk the complete code coverage."
There's also an interesting quote from Carmack about a near performance bug on that same page:
"[...] when I was working on the Doom 3 BFG edition release, the exactly predicted off-by-one-frame-of-latency input sampling happened, and very nearly shipped. That was a cold-sweat moment for me: after all of my harping about latency and responsiveness, I almost shipped a title with a completely unnecessary frame of latency."

* Benchmark implementations of the same functions against each other. If one of the implementations is a lot slower, it may indicate a performance bug in the implementation.

If you do find a perf bug in third party code, make sure to make a bug report to the maintainer(s)! This way software gets faster for everyone.

### Conclusion

Performance bugs are out there, lurking and unseen, like the dark matter that (possibly) pervades our universe. How many performance bugs are there in the wild? It's anyone's guess.

What has been your experience with performance bugs?

Discuss on Hacker News or Reddit.