What is the standard microkernel?

Hello.

Recently I saw a C++ code which implements the recursive descent parser.
The code uses own stack variable and own loop.  It actually says "the
function call is the overhead."

I knew Apache web server says "fork() is the overhead."  And I thought
that the goals of this kind of things must be the dedicated machine, the
dedicated OS, or the dedicated language.

I came up with an idea that "How about the Apache directly on microkernel."
I checked microkernels and I see (the IPC of) Mach was a failure. I also
checked Linux on L4 shows only a few percent performance degradation.

I think this is a good chance and direction for microkernels.  So what's
the GNU's plan?