The other day the reddit community discarded my advice for switching from text-based to binary serialization formats. It was labeled “premature optimization”. I’ll zoom out of the particular case, and discuss why not all optimization is premature.

Everyone has heard of Donald Knuth’s phrase “[..] premature optimization is the root of all evil”. And as with every well-known phrase, this one is usually misinterpreted. And to such an extent that people think optimizing something which is not a bottleneck is bad. That being the case, many systems are unnecessarily heavy and consume a lot of resources…because there is no bottleneck.

What has Knuth meant? That it is wrong to optimize if that is done at the cost of other important variables: readability, maintainability, time. Optimizing an algorithm can make it harder to read. Optimizing a big system can make it harder to maintain. Optimizing anything can take time that should probably be spent implementing functionality or fixing bugs. In practice, this means that you should not add sneaky if-clauses and memory workarounds in your code, that you shouldn’t introduce new tools or layers in your system for the sake of some potential gains in processing speed, and you shouldn’t spend a week on gaining 5% in performance. However, most interpretations say “you shouldn’t optimize for performance until it hits you”. And that’s where my opinion differs.

If you wait for something to “hit” you, then you are potentially in trouble. You must make your system optimal before it goes into production, otherwise it may be too late (meaning – a lot of downtime, lost customers, huge bills for hardware/hosting). Furthermore, “bottlenecks” are not that obvious with big systems. If you have 20 servers, will you notice that one piece of code takes up 70% more resource than it should? What if there are 10 such pieces. There is no obvious bottleneck, but optimizing the code may save you 2-3 servers. That’s why writing optimal code is not optional and is certainly not “premature optimization”. But let me give a few examples:

you notice that in some algorithms that are supposed to be invoked thousands of times, a linked list is used where random access is required. Is it premature optimization to change it to array/array list? No – it takes very little time, and does not make the code harder to read. Yet, it may increase the speed of the application a lot (how much is ‘a lot’ doesn’t even matter in that case)

you realize that a piece of code (including db access) is executed many times, but the data doesn’t change. This rarely accounts for a big percentage of the time needed to process a request. Is it premature optimization to cache the results (provided you have a caching framework that can handle cache invalidation, cache lifetime, etc.)? No – caching the things would save tons of database requests, without making your code harder to read (with declarative caching it will be just an annotation).

you measure that if you switch from a text to a binary format for transmitting messages within internal components you can do it 50%+ faster with half the memory. The system does not have huge memory needs, and the CPU is steady below 50%. Is replacing the text format with a binary one a premature optimization? No, because it costs 1 day, you don’t loose code readability (the change is only one line of configuration) and you don’t loose the means to debug your messages (you can dump them before/after serialization, or you can switch to text-based format in development mode. (yeah, that’s my case from the previous blogpost)

So, with these kinds of things, you saved a lot of processing power and memory even though you didn’t have any problems with that. But you didn’t have the problems either because you had enough hardware to mask them or you didn’t have enough traffic/utilization to actually see them. And performance tests/profiling didn’t show a clear bottle-neck. Then you optimize “in advance”, but not prematurely.

An important note here is that I mean mainly web applications. For desktop applications the deficiencies do not multiply. If you have a piece of desktop code that makes the system consume 20% more memory, (ask Adobe) then whatever – people have a lot of memory nowadays. But if your web application consumes 20% more memory for each user on the system, and you have 1 millions users, then the absolute value if huge (although it’s still “just” 20%).

The question is – is there a fine line between premature and proper optimization? Anything that makes the code “ugly” and does not solve a big problem is premature. Anything that takes two weeks to improve performance 5% is premature. Anything that is explained with “but what if some day trillions of aliens use our software” is premature. But anything that improves performance without affecting readability is a must. And anything that improves performance by just a better configuration is a must. And anything that makes the system consume 30% less resources and takes a day to implement is a must. To summarize – if neither readability, not maintainability are damaged and the time taken is negligible – go for it.

If every optimization is labeled as “premature”, a system may fail without any visible performance bottleneck. So assess each optimization rather than automatically concluding it’s premature.