Thoughts on limiting the parallel build load, suggestion for a new "-j auto" option

[R. Diez]

Hi all:

I recently came across a build test script that launched "make -j" with no 
limits, which consumed all available RAM and ground my Linux system to a halt. 
I had to power the computer off, there was nothing else I could do.

After this experience, I believe that the default limitless behaviour of -j is 
too dangerous. Even if your PC does not end up dying, GNU Make may 
inadvertently consume too many resources. An explicit "-j infinite" would be a 
better option, if anybody really needs something like that.

I recently came across option -l , which limits the amount of parallel tasks 
based on the system's average load. However, this flag does not seem safe 
enough, as, according to Wikipedia, not all Unix systems include in the average 
load those processes currently waiting for disk I/O. Besides, the maximum 
average load one would almost certainly want to use depends on the number of 
CPUs, so the calling script (or user) has to find out how many CPUs there are. 
How you find out may also depend on the operating system underneath, so 
everybody gets a little extra work every time.

I am writing a build test framework myself, and I have been trying to 
coordinate all sub-makes from a main makefile. The top-level script decides how 
many parallel processes are allowed for the entire build and relies on 
MAKEFLAGS in order to let all sub-makes talk to each other so as to limit the 
overall load. Because different makefiles may require different GNU Make 
options, I am filtering out all others and leaving just the parallel build 
flags in place, like this:

  export MAKEFLAGS="$(filter --jobserver-fds=%,$(MAKEFLAGS)) $(filter 
-j,$(MAKEFLAGS))" && $(MAKE) ...etc...

By the way, option --jobserver-fds is not documented, but I think it should be. 
As you can see, the user may need to filter it out manually after all.

The trouble with this MAKEFLAGS technique is that I often come across some 
third-party script which insists on calculating and setting its own -j value, 
rendering my coordination efforts useless. When this happens, I get warnings 
like this:

  warning: -jN forced in submake: disabling jobserver mode

Needless to say, most heuristics to calculate the -j value are as lame as mine 
(see below). When writing build scripts, nobody seems to have much time left 
for finding out how to retrieve the relevant system information
in bash/perl/whatever in a portable way and then calculate a good -j value out 
of it.

I have been thinking about the best way to overcome such parallel woes, and I 
wanted to share this thought with you all. How about adding to GNU Make a new 
-j parameter like this:

  make -j auto

The behaviour of "-j auto" could be as follows:

1) If MAKEFLAGS specifies -j and --jobserver-fds , then use those settings (no 
warning required).

2) Otherwise, calculate the maximum number of parallel tasks with some trivial 
heuristic based on the number of CPUs and/or the system load. I'm using <CPU 
count> + 1 at the moment, but I'm sure there are better guesses.

I could think of several alternative heuristics:

  make -j auto-system-load      # Use -l <CPU count + 0.5>
  make -j auto-processor-count  # Use -j <CPU count + 1>

I guess most people would then end up using some "-j auto" variant, in order to 
avoid overloading or underloading the system without having to implement their 
own heuristics. That way, a top-level script will be much more likely to 
succeed at setting a global limit when launching third-party sub-makes in 
parallel.

Please copy me on the answers, as I'm not on this list.

Thanks,
  R. Diez