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Time exchange rate

johndcook.com

30 pointsjaimebuelta11 comments

And what about spending time to optimize a end user's time?

Exactly. If we are to stick to such calculations, then it's worth years of programmer's time to save seconds of end-user time for any remotely successful piece of software.

I wish more programmers and companies would think of that. Everything they do scales milionfold - that includes potential time savings for users as well as wasting coal on doing stupid things like parsing literally millions lines of XML for basic configuration (OSGi, I'm looking at you! Also, news for some, processor cycles cost electricity, which is not unlimited or free.). But I know - incentives, money, optimization, capitalism, Moloch!

Yep, or even your own time if you run the program multiple times. E.g., if your code is running on a thousand machines, that CPU-hour savings that cost $0.05 now costs $50.00; past a certain point it does pay to throw engineers at insane micro-optimizations. Or nonlinear payoffs from CPU time improvements -- e.g. meeting a realtime deadline or else the system is useless. The "CPU-hour vs. engineer-hour" comparison works only when the engineer him/herself runs their own program, exactly once, without other constraints.

> The "CPU-hour vs. engineer-hour" comparison works only when the engineer him/herself runs their own program, exactly once, without other constraints.

Or when the system is designed such that you can scale hardware to decrease run time. Sometimes the choice is between throwing programmer time at a performance problem or throwing hardware at it, and often the economics work out in favor of the latter.

Sure, but I think this is just reiterating the original point that hardware is cheap and programmers are expensive. If you need to meet a performance goal and hardware is cheap but programmers are expensive, then throw hardware at it. The point I was (trying to) make is that when you have N instances of the problem to solve (where N is large), often the cost of "scaling the hardware" is prohibitive; optimizing performance by 1% might yield operational cost savings that justify the programmer's salary, simply because the 1% is multiplied by N. This is orthogonal to parallelism/scalability.

I'll grant that this case may be rare (i.e. most people are optimizing for initial results, not optimizing a large existing machine) but it does happen!

Of course it does happen, it's not a rare case, it's the usual case. Any remotely successful product will have N = millions of users. The only reason this doesn't enter the calculation is that you don't get the money you saved by not making your users do unnecessary hardware upgrades.

Ah! So there's the key point: do your users run your software or do you, on your servers? If the former, yes, it's an externalized cost and may not enter your economic calculations. If the latter, you care a lot :-)

That depends entirely on how much the client is paying. Usually clients don't value their time very much.

Yes! I can't stand this kind of thinking in code reviews, someone will say X is slow, and I say, "How slow is it?" they'll say something about O(X) and I'll say, no really exactly how much time in seconds will be saved by changing this.

Usually at this point they have no answer.

In general your O(n^4) algo is perfectly fine, and no one will ever notice your O(1) replacement. When it isn't fine it's painfully clear that it's not OK.

Cute, but there are a lot of situations where this breaks down. It assumes that a programmer's efficiency is independent of a program's efficiency.

If I have to wait hours for a job to complete that could be completed in moments with a bit of extra work, there certainly is value in working on the optimisation.

One metric I think about a lot in my work is the sum of runtime and programming time, because I often write one-off analysis programs and just care about when I get the results. It's nice because I can focus on the sweeping optimizations and not the small ones.

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