public class Realtime

Entry point to real-time threads: periodic tasks that run at a fixed operating-system priority, are released on an absolute clock, and are not paused by the garbage collector.

Realtime rt = Realtime.getInstance();
rt.lockMemory();                          // no page faults in a period
rt.setMemoryLimit(64L * 1024 * 1024);     // fail fast instead of thrashing
RealtimeThread motor = rt.start("motor",
        new RealtimeParameters().setPeriodNanos(1000000L).setPriority(80).setCpu(3),
        new MotorLoop());
...
Log.p(motor.getStatistics().toString());

How the collector is kept away

A real-time task allocates in setup() and never in cycle(). The build rejects an allocation that cycle() reaches, and at run time an allocation it could not see ends the steady state with a RealtimeAllocationError – so the thread never needs the collector to make room. The collector in turn never stops it for a whole cycle: collections stay concurrent while a real-time thread exists, its stack is scanned while it sleeps between periods, and it is released as soon as that one stack has been read. Normal threads run below it, the collector’s included, so a busy collector slows them, not it.

What this is and is not

On Linux with a PREEMPT_RT kernel this is firm real time: release latency in tens of microseconds, measured, not proven. It is not certified hard real time and makes no claim under any safety standard. Measure the loop on the device, under load, with RealtimeThread.getStatistics().

Platforms

• Linux (the embedded and desktop Linux targets, the backend on Linux): every feature. Real-time scheduling needs CAP_SYS_NICE or an rtprio limit.
• Other translated targets (iOS, macOS, Windows, the backend elsewhere): the collector isolation and the allocation check, with normal-priority threads and a relative sleep.
• The simulator, Android, JavaScript: a best-effort Java thread with the same API, so the application runs and can be tested anywhere; there is no isolation from the collector and no allocation check at run time.

For port implementors

A port subclasses this class and overrides the protected primitives, which act on the calling thread. The defaults are the best-effort Java implementation.