Makefile profiling

Question

So I have this Makefile based build system that my users feel is working too slowly. For the sake of this question lets define performance as the time it takes make to figure out what it should actually do.

I can see some avenues for optimization --

• Reducing the number of times Makefile is parsed and the DAG recalculated due to including a Makefile fragment.
• Reducing the number of going to an external Makefile with make -C
• Reducing variable expansions
• etc.

-- however I want to know first where are my bottlenecks. Since optimization without profiling is a waste of life, I want to ask: How to profile a Makefile?

Assume that the system I inherited is fairly well designed, i.e. it already implements the most common tricks of the trade: (mostly) non recursive make, ccache, precompiled headers, auto generated header dependencies etc).

... and just to preempt some of the possible answer. I know that there might be faster and better build systems then GNU make - (Personally, I am eagerly waiting to see what the CMake folks will come up with regards to the Ninja system) - but unfortunately swapping build system is not in the cards.

Answer 1

Try running make with the -d flag. This will output debugging information outlining exactly what make is doing and in what order, as well as why it is doing it. I use that output to find things that I would otherwise miss, such as unintentional recursive make calls or things that are out of order and forcing certain files or parts of the source tree to be processed multiple times. My guess is that you'll be surprised at how much make is doing behind the scenes that you weren't aware of.

One warning though. The -d option causes make to produce an obscene amount of output. You will definitely want to redirect output to a file. You terminal will thank you later.

If you are the source hacker type, you could build a custom version of make that put a high-resolution timestamp on each line that was printed due to the -d flag. This would essentially create a timeline showing you exactly how much time make wasted on each step.