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Redox is close, but they have pipe-like, rather than call-like, interprocess communications semantics. That means another layer of overhead. More important, it breaks the tight integration between scheduling and interprocess calls required to make a message-passing OS work fast under load.

Here's the QNX architecture document on this, which discusses how message passing and CPU scheduling integrate.[1] Microkernel designers need to read this very carefully. A good test is to run a message-passing benchmark on an idle system, then run it again with a CPU-bound process of equal priority in round-robin mode also running. If the message passing-task starves, or the CPU-bound task starves, message passing was misdesigned. If, on a multiprocessor, a simple message pass causes a CPU switch, message passing was done wrong.

If message passing and scheduling do not play very well together, a service-oriented architecture (sorry, "microservices" architecture) will be sluggish. This is where most microkernels fail.

[1] http://www.qnx.com/developers/docs/6.4.1/neutrino/sys_arch/i...