Thursday, 18 April 2013

Iterating faster

Today I spent a few minutes writing a Makefile. I think they may turn out to be some of the most effective minutes I've ever spent.

At work we build using SCons. When I change a single source file, rebuilding takes between 50 and 65 seconds. That's more than long enough to lose my focus and get distracted, so to stay productive I have to work in larger batches. I code in longer bursts and recompile less often, so that the ratio of productive time to down time stays up; unfortunately that also means fewer opportunities to test my changes.

That's not ideal, but it's manageable much of the time. Not at the moment though. Right now I'm developing an algorithm where I need to be able to quickly try out different approaches for each of the steps. In this situation I really want to see the effect of each change in isolation, so I can't batch up my coding. All of a sudden those 50 second build times are a real problem.

I spent a little while looking into ways to speed SCons up and managed to get the rebuild time down to around 45 seconds. Still nowhere near fast enough.

Fortunately I'm working in a very self-contained part of our code-base, so I could limit the amount of files I had to consider. Re-running SCons with verbose output turned on gave me the exact command being used to compile each of them, so I saved those to a shell script. Rebuild time with this shell script was down to about 15 seconds: a huge improvement, but still not quite quick enough. This script was recompiling every file each time I ran it though - surely I could do something a bit smarter than that...?

Enter make. It knows how to check for changed files & I've used it enough that I can put together a Makefile pretty quickly. Factoring out the commands from the shell script into rules and adding enough dependency information to be useful, I had it set up in a couple of minutes. The result: most rebuilds are now around 3 seconds. Perfect.

So now, not only am I saving almost a whole minute of wasted time with each rebuild, I've all but removed the chance to get distracted and also opened the door on a more effective way of working. I can make small changes and rebuild to see the effect of each in isolation. I can develop my algorithm a lot faster now, because I can iterate faster; and that's why I think they'll be some of the most effective minutes I've ever spent.

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