D735 - Logical time is per thread

Status: decided Date: 2026-09-28

The logical clock (D582) is kept per guest thread. A thread's time advances by its own readings and its own waits only; another thread's readings and waits do not move it. A guest thread starts at the time its creator had when it created it, so no thread reads a time earlier than the one that made it. Every clock the guest reads still comes from clocks::since_start_nanos, so one thread's process time, tick counter and monotonic clock stay consistent. Under ORBISTOUN_CLOCK=host nothing changes.

Why: one shared counter moved by every thread made the main thread's readings depend on when the host scheduled the others. Bugdom's SDL audio thread advanced it by 21.3 ms at each buffer it queued, at host-timed moments, and two runs of one build parted after about ten seconds. With the clock per thread they agree for about forty. A comparison between runs, and a compatibility record anyone can reproduce, needs runs that repeat.

Limits: a guest whose logic waits on another thread's progress still depends on the host's scheduling. Bugdom's logo screen runs until its song has played, and the song plays on the audio thread at the host's pace, so runs part again after a transition that waits on audio. Making that repeat needs deterministic scheduling of guest threads, which this does not attempt. Clocks on two threads can disagree; a guest comparing timestamps across threads sees the difference.

Rejected:

• Only device waits stop moving the shared clock: fixes Bugdom's audio thread, and leaves any title whose worker threads read the clock or sleep diverging the same way.
• Lamport merges at every synchronisation (a woken thread taking its waker's time): consistent across threads, and repeats only where the order of synchronisations does, which the host's scheduler still decides.