Real-time is some of the most misused jargon in modern history.

In general, most JIT or VM can't even claim guaranteed latency. People that mix these concepts betray their ignorance while seeming intelligent.

FreeRTOS is small and feasible.

VxWorks if your budget is unconstrained.

LinuxRT kernel (used in LinuxCNC) with external context clocking, and or FPGA memory DMA overlap module (zynq SoC etc.)

Real-time is a specialized underpaid area, and most people have too abstract of an understanding of hardware to tackle metastability problems. =3