looking for build order doc in parallel build

[Yasushi SHOJI]

Hello,

I'm experimenting parallel build with GNU Make 3.81 on Debian Sid.
I've seen that "build order change" depending on -j flag. I know I'm
missing something but could not figure it out. so hope someone give me
a pointer to doc.

here is my makefile:

        all: 1.pdf 2.pdf 3.pdf
                @echo Making $@
        
        %.pdf: %.fo
                touch $@
        
        %.fo:
                touch $@

and the outputs:

        $ make -j 1
        touch 1.fo
        touch 1.pdf
        touch 2.fo
        touch 2.pdf
        touch 3.fo
        touch 3.pdf
        Making all
        rm 1.fo 2.fo 3.fo

        $ make -j 2
        touch 1.fo
        touch 2.fo
        touch 3.fo
        touch 1.pdf
        touch 2.pdf
        touch 3.pdf
        Making all
        rm 1.fo 2.fo 3.fo


see that with "-j1", the order is interleaved. but with "-j2", all fo
is created first and then pdf.  it does not seems to matter if I
increase either a number of pdf (say 1 to 10.pdf) or -j10.  except
-j1, if you enabled parallel build, make build all .fo then goes on to
.pdf.

so, what I am missing? is there implicit rule I have to take care?

TIA,
-- 
              yashi