Performance Analysis is Cost Analysis

Ryan James Spencer

Benchmarks and profiler in hand, you’re ready to start taking stabs to improve your program’s performance. Looking around, there are some large numbers that seems obvious to attack, but this is the start of a broken perception about performance analysis. Bottlenecks are worth evaluating, but they aren’t the only thing you should be evaluating. The title of this article may seem obvious at first glance, but the truth is that performance analysis is not about focusing on one view on the measurements you’ve collected.

tl;dr Next time you are profiling, benchmarking, or laying out plans for what you’re about to build, think about all of the costs. By thinking holistically you’ll better understanding the value of every expenditure, meaning you can both build an intuition for the benefit of certain classes of costs as well as tangibly improving overall program, and non-program, characteristics. The value of some costs will be obvious, but if the value of a cost isn’t apparent, it may still be valuable in an indirect manner; usually this means an upfront cost that leads to improvements later on or a “tradeoff” that improves a quality that may seem unrelated. For example, amortized growing of dynamically resizable arrays is done on purpose to help improve overall performance on the whole, despite the effort put in at each stage of allocation and copying. Or, we may decide it’s not worth optimising anything in the program at all because we need to work on a new feature that is much more valuable to customers than a 5ns improvement to a rarely used endpoint. And when in doubt, if your program is hurting others or damaging the world around it, consider if the software should even exist rather than helping it accomplish its job faster (hat tip to Itamar Turner-Trauring on this point).

Costs are everywhere; they are in the pooled costs we see pile up as well as the costs that spread about a program. These latter costs constitute the diffuse profile. Turning your attention on evaluating all costs means changing your attitude from being frugal in the local sense to being economic in the global sense. In other words, you might save a lot on a big item purchase, but you might equally save as much over the course of a year with smaller, consistent savings over time. Having both as savings is the real aim of performance analysis as cost analysis.

In this cost analysis view of the world, payments should come with returns. Sometimes the return on investment is definite, but other times it is not, in which case it is deemed either an acceptable or careless risk depending on the variables at play. We can define the former as direct and the latter as indirect returns.

If we write code that brings random allocations in the heap into a single contiguous block of memory, we are paying for an indirect return on investment such that every attempt to read the memory afterwards is now much faster than it would have been.

Alternatively, a direct return for a cost paid out might be the actual core operation we need done; consider the difference of running several instructions to calculate a population count (number of ones) of an integer or calling a processor specific POPCNT instruction.

Keep in mind that not all costs are about the characteristics of your program. There are the costs of a team of engineer’s salaries or the readability of code. What is wasteful for our system’s context can be considered an acceptable loss if we consider what it pays for elsewhere.

Projecting what costs are going to be potentially encountered is just as important as reviewing costs. This can take two forms: either considering the budget(s) of what we’re about to spend, or projecting what we assume we might spend. One gives us a threshold for which to judge future expenditures while the other gives us a a baseline for where we might might know if we are drifting too far where we thought we would wind up. The distinction is subtle but important. With a budget you want to stay as close as possible to the line, while with a projection you are not trying to aim for anything, really, but you know how far off your estimate was when you wind up with something tangible.