3 ms·

Why bother? The comment is a waste of time. Performance profile it and go from there. If you're doing anything other than using a performance profiling tool on

Why bother? The comment is a waste of time. Performance profile it and go from there. If you're doing anything other than using a performance profiling tool on real code with vaguely realistic data you're completely and utterly wasting your time taking stabs in the dark. Leaving comments that will become obsolete and misleading as the code evolves seems just as bad as pre-optimizing code. I've been working on a project at the moment where exactly what you've suggested was done, but they'd used todo comments. Last winter they'd had to up to an azure large SQL instance and it'd still max out DTU every now and again (DTU being MS's db performance measure on azure). Client was hoping I might fix a couple more problems as the volume is expected to triple or more this winter. In 6 days of simply running a local profiler I found and fixed 3 major performance problems, none in any way related to the pointless comments claiming X method needed optimising, none of which were even 1/10000th of the program's execution time. One of those problems was even caused by a previous "optimisation". I confirmed the problem spots by checking the web logs and avg page response times to confirm production was having the same problems. The site now potters along on a small SQL instance never going above 5% DTU, the worst pages are all being served 5 times faster. Done the same thing to a site with slow rendering (js problems). Chrome's profiler is good enough. If you don't know how to use your language's profiler, learn, now.

Klathmon 10y ago
Don't get me wrong, I do use the profiler a ton, and it's the first thing i reach for when i start noticing issues, but having that little comment about why i did it that way lets me relax a bit. I'd never mark it as a TODO or something that would clutter a tool up with crap, but having it there could help. A comment out of something I did recently where it's a loop that iterates over an array that's expected to be a few hundred long, and each iteration loops over the array again: // This is O(n^2) out of pure laziness. Might cause issues if the array gets really big. It's not hurting anyone, took like 10 seconds to write, and if it ever becomes a problem, the first thing i'll see is that in the function docblock which lets me (or someone else) know that there is no technical reason it needed to be O(n^2).
- tlarkworthy 10y ago
  Profilers help identify hot spots, but don't really explain the dynamic behaviour of why something is a hot spot. So comments saying that something is O(n^2) when it could be O(n) is very helpful when you see the inner function hogging 90% of resources. Profilers don't get complexity classes.
  - mattmanser 10y ago
    Not really, comments like this are often wrong or misleading. After all, the programmer hasn't actually tested anything, they're just making assumptions. Otherwise they'd have actually fixed it. It's just pointless noise. A profiler will tell you that a particular function was run 10,000 times costing 0.5ms per run. You're not going to have a problem tracking down the problem, the comment won't help.
- macspoofing 10y ago
  >// This is O(n^2) out of pure laziness. Might cause issues if the array gets really big It just feels lazy. You're passing the buck to another dev who may be working on the code, weeks, months or years down the line. What are they supposed to do about it? Randomly rewrite working code and risk introducing a bug? If you think the input might reasonably scale to a size that will cause an issue then do some preemptive defensive coding and fix it. Otherwise don't. Stand by your decision. TODOs and FIXMEs tend to linger and get ignored. If I'm looking for performance problems, I'm not scanning for source code TODOs. I load a profiler and I'll see which methods take too much time.
  - Klathmon 10y ago
    >It just feels lazy. You're passing the buck to another dev who may be working on the code, weeks, months or years down the line. What are they supposed to do about it? Randomly rewrite working code and risk introducing a bug? Yes. If it's a performance problem weeks, months, or years down the road, the developer then is supposed to rewrite it, possibly introducing bugs, but also solving the performance problems that manifested in the past weeks, months, or years. >If you think the input might reasonably scale to a size that will cause an issue then do some preemptive defensive coding and fix it. Otherwise don't. I don't know about you, but I'm not omniscient. I can make some pretty good guesses, but I can't predict the future. Optimizing the code now to be super efficient when I'm pretty sure it won't be necessary would be a massive waste of time. But on the off chance it is necessary, I'd like to give a hint to myself (or another dev) about why I did what I did, and give some assurances that it's a place that can be pretty easily optimized if needed. >If I'm looking for performance problems, I'm not scanning for source code TODOs. I load a profiler and I'll see which methods take too much time. Again, I'm not leaving TODOs I'm leaving comments explaining the code a little. But I completely agree with that whole statement, and it's exactly what I do as well. But when I get to a method that is taking too much time, I can now see a small comment saying it's running in O(n^2) and there is no technical reason why it needs to. Now I can spend less time checking if it needs to run that slowly, and can get right into fixing it. The handful of bytes taken up by that comment, and the few seconds it took me to write it isn't going to make anything worse for anyone, and it might make life easier for someone in the future, so i'll do it.

Klathmon 10y ago

Don't get me wrong, I do use the profiler a ton, and it's the first thing i reach for when i start noticing issues, but having that little comment about why i did it that way lets me relax a bit. I'd never mark it as a TODO or something that would clutter a tool up with crap, but having it there could help. A comment out of something I did recently where it's a loop that iterates over an array that's expected to be a few hundred long, and each iteration loops over the array again: // This is O(n^2) out of pure laziness. Might cause issues if the array gets really big. It's not hurting anyone, took like 10 seconds to write, and if it ever becomes a problem, the first thing i'll see is that in the function docblock which lets me (or someone else) know that there is no technical reason it needed to be O(n^2).

tlarkworthy 10y ago
Profilers help identify hot spots, but don't really explain the dynamic behaviour of why something is a hot spot. So comments saying that something is O(n^2) when it could be O(n) is very helpful when you see the inner function hogging 90% of resources. Profilers don't get complexity classes.
- mattmanser 10y ago
  Not really, comments like this are often wrong or misleading. After all, the programmer hasn't actually tested anything, they're just making assumptions. Otherwise they'd have actually fixed it. It's just pointless noise. A profiler will tell you that a particular function was run 10,000 times costing 0.5ms per run. You're not going to have a problem tracking down the problem, the comment won't help.
macspoofing 10y ago
>// This is O(n^2) out of pure laziness. Might cause issues if the array gets really big It just feels lazy. You're passing the buck to another dev who may be working on the code, weeks, months or years down the line. What are they supposed to do about it? Randomly rewrite working code and risk introducing a bug? If you think the input might reasonably scale to a size that will cause an issue then do some preemptive defensive coding and fix it. Otherwise don't. Stand by your decision. TODOs and FIXMEs tend to linger and get ignored. If I'm looking for performance problems, I'm not scanning for source code TODOs. I load a profiler and I'll see which methods take too much time.
- Klathmon 10y ago
  >It just feels lazy. You're passing the buck to another dev who may be working on the code, weeks, months or years down the line. What are they supposed to do about it? Randomly rewrite working code and risk introducing a bug? Yes. If it's a performance problem weeks, months, or years down the road, the developer then is supposed to rewrite it, possibly introducing bugs, but also solving the performance problems that manifested in the past weeks, months, or years. >If you think the input might reasonably scale to a size that will cause an issue then do some preemptive defensive coding and fix it. Otherwise don't. I don't know about you, but I'm not omniscient. I can make some pretty good guesses, but I can't predict the future. Optimizing the code now to be super efficient when I'm pretty sure it won't be necessary would be a massive waste of time. But on the off chance it is necessary, I'd like to give a hint to myself (or another dev) about why I did what I did, and give some assurances that it's a place that can be pretty easily optimized if needed. >If I'm looking for performance problems, I'm not scanning for source code TODOs. I load a profiler and I'll see which methods take too much time. Again, I'm not leaving TODOs I'm leaving comments explaining the code a little. But I completely agree with that whole statement, and it's exactly what I do as well. But when I get to a method that is taking too much time, I can now see a small comment saying it's running in O(n^2) and there is no technical reason why it needs to. Now I can spend less time checking if it needs to run that slowly, and can get right into fixing it. The handful of bytes taken up by that comment, and the few seconds it took me to write it isn't going to make anything worse for anyone, and it might make life easier for someone in the future, so i'll do it.