What could be causing make to hang when compiling on multiple cores?

I don't have an answer to this precise issue, but I can try to give you a hint of what may be happening: Missing dependencies in Makefiles.

Example:

target: a.bytecode b.bytecode
    link a.bytecode b.bytecode -o target

a.bytecode: a.source
    compile a.source -o a.bytecode

b.bytecode: b.source
    compile b.source a.bytecode -o a.bytecode

If you call make target everything will compile correctly. Compilation of a.source is performed (arbitrarily, but deterministically) first. Then compilation of b.source is performed.

But if you make -j2 target both compile commands will be run in parallel. And you'll actually notice that your Makefile's dependencies are broken. The second compile assumes a.bytecode is already compiled, but it does not appear in dependencies. So an error is likely to happen. The correct dependency line for b.bytecode should be:

b.bytecode: b.source a.bytecode

To come back to your problem, if you are not lucky it's possible that a command hang in a 100% CPU loop, because of a missing dependency. That's probably what is happening here, the missing dependency couldn't be revealed by a sequential build, but it has been revealed by your parallel build.

I don't know how long you've had the machine, but my first recommendation would be to try a memory test and verify that the memory is functioning properly. I know it often isn't the memory that is the problem, but if it is, it is best to eliminate it as a cause first before trying to trace down other probably issues.

I realize this is a really old question, but it still pops up at the top of search results, so here is my solution:

GNU make has a jobserver mechanism to ensure make and its recursive children do not consume more than the specified number of cores: http://make.mad-scientist.net/papers/jobserver-implementation/

It relies on a pipe shared by all processes. Each process that wants to fork additional children has to first consume tokens from the pipe, then relinquish them when done. If a child process does not return the tokens it consumed, the top-level make while hang forever waiting for them to be returned.

https://bugzilla.redhat.com/show_bug.cgi?id=654822

I encountered this error when building binutils with GNU make on my Solaris box, where "sed" is not GNU sed. Fiddling with PATH to make sed==gsed take priority over the system sed fixed the issue. I don't know why sed was consuming tokens from the pipe, though.