Protecting the 1,000µs heartbeat in high-fidelity Hardware-in-the-Loop simulators.
In automotive engineering, a Hardware-in-the-Loop (HIL) system is the ultimate gatekeeper. It is a high-fidelity simulator that "tricks" an ECU (Electronic Control Unit) into believing it is operating a real vehicle.
This isn't just about speed; it's about Hard Real-Time determinism.
Inside that heartbeat, tasks are strictly confined to 250 microsecond windows. In this world, "late" is a fatal system crash.
Our self-developed Linux HIL system operates on a rigid 1ms cycle, mechanically divided into four 250µs stages. If one process hangs, the entire system grinds to a halt.
Suddenly, we hit a wall at 50 signals. I discovered a hidden, lethal dependency—a Cross-Cycle Trap.
Stage 4 was tasked with packing data. As signals increased, it scraped the edge of its 250µs limit.
The transmission process was prioritized at the start of the next cycle. When TCP blocked, it paralyzed the entire heartbeat.
Replaced synchronous TCP with POSIX MQ. Stage 4 now "fires and forgets"—dropping data into a kernel buffer instantly, well within the 250µs deadline.
Evicted transmission from the Hard RT scheduler. Moving it to a standard process allowed it to consume data in "slack time" without threatening temporal determinism.
"We optimized the backend until it outpaced the human-facing visualization hardware."
Real-time optimization isn't just about 'faster' code; it's about isolating blocking IO from the critical path.
No amount of code-level micro-optimization could fix the coupling. The solution required a fundamental architecture shift.
By moving non-RT tasks to non-RT processes, we gained freedom without sacrificing hard determinism.