GoKawiilBlog post details how GNU Make's -j jobserver behaves across recursive make calls
A developer investigated how recursive invocations of GNU Make handle the -j parallelism flag across versions 3.81 through 4.3. Without a '+' prefix or use of $(MAKE), recent GNU Make versions warn that the jobserver is unavailable and fall back to -j1, losing parallelism and not necessarily invoking the same make binary. The post includes test Makefiles used to compare behavior when different -j values are set on the command line versus inside recursive recipes.