D582 - The guest clock is logical by default

Status: decided Date: 2026-09-08

Every clock a guest reads comes from clocks::since_start_nanos, which by default advances one microsecond per reading and by the requested duration on a sleep; the wall clock is a fixed synthetic epoch plus that elapsed time. ORBISTOUN_CLOCK=host uses real time instead.

The logical time is kept per thread (D735).

Why: a clock read from the host makes two runs of one build diverge, and a measurement that cannot be repeated is not one. A clock that repeats need not stand still: a fixed step per reading keeps timing plausible, ends a millisecond spin in a thousand readings, and lets a guest that waits for time to pass proceed. One source keeps process time, the tick counter and the monotonic clock consistent and switched by one setting.

Rejected:

• Host time by default: runs stop being comparable.
• A pinned clock: stops any guest that waits for time to pass.
• A clock per library: conversions between them drift, and one setting cannot govern them all.