In Hard Real-Time Automotive Simulation, "latency" isn't just an inconvenience. It means the engine stalls, the test fails, and the hardware disconnects.
My role was to engineer the Linux System Layer for a Hardware-in-the-Loop (HIL) simulator. This system tricks a real car ECU into thinking it's driving on a road by generating thousands of electrical signals per second.
The system runs on a strict 1ms cycle. If we miss a deadline, the "illusion" breaks. The ECU detects the lag and shuts down for safety.
We hit a wall. The system crashed whenever we monitored more than 50 signals. Why? I traced the issue to a fatal design flaw: Temporal Coupling.
The Linux Scheduler divides each 1ms into 4 phases (250us each). The system forced a dangerous relay race between Phase 4 of one cycle and Phase 1 of the next.
I profiled the kernel calls and realized something absurd: We were blocking a Hard Real-Time thread (microsecond precision) to wait for a TCP Socket (millisecond unpredictability).
The data receiver was a Windows Dashboard for human operators. Humans can't see updates faster than 60Hz (16ms). Why were we forcing the Network Stack to run at 1000Hz inside the Real-Time boundary?
Blocks Critical Path!
I implemented POSIX Message Queues. The Real-Time process now simply dumps data into a shared memory buffer (taking mere microseconds). A separate, lower-priority process wakes up later to handle the slow network transmission.
// The Transformation:
// 1. Replaced TCP Socket with Non-Blocking MQ
mq_open("/hil_data_pipe", O_WRONLY | O_CREAT | O_NONBLOCK, 0644, &attr);
// 2. Real-Time Loop (Phase 4) is now instant:
mq_send(mq_handle, data_ptr, size, 0);
// ^ Costs ~5us instead of ~200us+
// 3. Phase 1 is now EMPTY and available for control logic.The Linux backend can actually handle far more than 200 signals now. The new system limit is actually defined by the SciChart rendering engine on the Windows UI, which lags visually above 200 lines.
The bottleneck has successfully shifted out of the critical Real-Time Core.