-flto=jobserver does not use all CPU cores

["R. Diez via gcc-help" <gcc-help@xxxxxxxxxxx>]

Hi all:

I have an AMD FX-4300 Quad-Core Processor with no hyperthreading.

If I run GCC with -flto, only core is used, as expected.

If I run GCC with -flto=4, all 4 cores are used, as expected.

If I run GCC with -flto=jobserver, only 2 cores are apparently used, even if the top-level makefile runs with -j 40. The increased linking 
time also corroborates this.

I am using a self-built GCC 8.3.0 cross-compiler for ARM Cortex-M4F on Ubuntu 18.04.4 LTS.

I am also using a self-built GNU Make 4.2.1. Everything else is building in parallel with all cores.

Is this problem with "-flto=jobserver" already known? I couldn't find anything on this.

How can I debug what is going on? Is there a way to trace jobserver-related information?

Thanks in advance,
  rdiez