Synchronization

The synchronization unit implements neighbor synchronization using BISP, the booking-based synchronization protocol from Distributed-HISQ. A sync target instruction adds a synchronization event at the current time point. The target must name a directly connected controller.

Booking and completion

When the TCU triggers the event, SyncUnit sends a signal to the target and starts a countdown equal to the configured outgoing link delay. The timer continues while this countdown runs.

At the countdown deadline, the timer pauses if the peer’s signal has not arrived. Synchronization releases its pause when the signal arrives; an empty queue after wait 0 can keep the TCU paused. A signal received before the deadline is retained until the countdown completes. Each signal satisfies one synchronization request.

For booking ticks B0 and B1 and directional delays L0 and L1, both controllers satisfy their conditions at max(B0 + L0, B1 + L1). Schedule each synchronized operation at its local booking cycle plus its outgoing delay in cycles. Physical output delays apply after event triggering.

The protocol follows Distributed-HISQ, Section 4.1.

Connections and state

Core owns a SyncUnit. Simulator owns the shared SyncNetwork. The network stores signals with fixed arrival ticks in bounded directional queues. SyncUnit retains one active target, countdown deadline and received flag. It updates these before the TCU transition on each TCU edge. Subsequent sync events occupy the bounded TCU synchronization queue until their time points trigger. Starting another synchronization before the active one completes raises UnsupportedSynchronization.

Pausing the TCU does not pause CPU execution, physical time, active pulses, acquisition or backend evolution. Queues can still accept future time points and measurement results while the timer is paused.

Reset and errors

Session reset clears requests, signal queues and pause state across all cores. An unconnected target raises UnsupportedSynchronization. A full TCU synchronization queue delays admission; exceeding a signal queue’s capacity raises Capacity. A missing peer request prevents completion and eventually raises Watchdog.

Regional synchronization through routers is unsupported.

Implementation and tests

Source: sync.cpp, sync.hpp, core.cpp and tcu.cpp.

CTest: sync.neighbor, systemc.distributed.

The tests cover early and late peer signals, directional delays, repeated requests, bounded queues, reset, unconnected targets and unmatched requests. Integration checks preserve output timestamps when process order is reversed.

See configuration and the two-controller simulation example.