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Multi-core processing: the realtime worker pool

The multi-core setting (Plugin Settings, on by default, machine-wide) spreads the chain stage's independent work across a pool of realtime helper threads. Two sections fork:

  • Stereo lanes. The two chain lanes are fully independent: separate engines, separate scratch buffers, no shared state inside the chain stage. The audio thread hands one lane to a worker, processes the other itself, and joins. With two heavy NAM lanes the chain stage approaches the cost of one.
  • NAM oversampling phases. An oversampled NamEngine holds N independent instances of the same model over disjoint per-phase buffers (see oversampling.md). The lane processing the block forks the N phase jobs across the pool instead of looping them, so an 8x model costs about one native-rate model per core instead of eight on one. This fork nests inside a lane fork when both apply, and unlike the lane fork it doesn't need stereo: a mono chain's oversampled NAM blocks fork too.

Implementation is plugin/include/RtWorkerPool.h (header-only). The contract is pinned by test/src/worker_pool_tests.cpp (the scheduling primitive: every job exactly once, nesting, degradation, churn) and test/src/multicore_tests.cpp (the audio: parallel output is bit-identical to serial across topologies, host rates, oversampling factors and mid-stream toggles, and the pool survives host lifecycle churn).

Design rules

The pool is an extension of the audio callback, not a general scheduler:

  • Workers only ever run strictly inside the audio thread's chainMutex critical section, so jobs take no locks and the message-thread mutation story is unchanged.
  • A job is a plain function pointer plus a context struct on the forking thread's stack (alive until forkJoin() returns). Forking allocates nothing.
  • forkJoin(fn, ctxs, count) runs every job exactly once and returns only when all are complete. The caller always executes job 0 itself, so completion never depends on worker liveness.
  • Nesting is supported one level deep (a lane job forking its NAM block's phases), and stays deadlock-free by construction: a joiner never parks, it claims and runs its own group's unclaimed jobs inline and only spins on jobs another thread is actively executing, so every waiter makes progress itself.

Job protocol

All lock-free, one atomic state per slot in a fixed registry (slots are pool memory, never freed; contexts on caller stacks are only dereferenced while the owning forkJoin is still on that stack):

Free --publisher CAS--> Building --> Armed --worker CAS--> Claimed --> Done
                                       |                                |
                                       +--join CAS -> run inline -> Free <-+ (join recycles)

The Armed-to-Claimed transition is a compare-exchange raced between the workers and the publisher's join loop: whoever wins runs the job. That race is the safety valve. If every worker is descheduled or busy, the joiner steals its jobs back and runs them inline, so the callback degrades to exactly the serial cost instead of stalling. A full registry (deep concurrent nesting) makes forkJoin run the overflow inline immediately: less parallel, never blocked. This is why the toggle is pure scheduling: every schedule runs the same code on the same buffers, and the output is bit-identical regardless.

Scheduling and wakeups

The threads run at realtime priority (time-constraint on macOS, MMCSS "Pro Audio" on Windows); the pool sizes itself to one worker per spare hardware core. Linux refuses realtime scheduling to processes without rtprio privileges, so there the workers fall back to normal highest-priority threads: still parallel, and the join steal absorbs any scheduling misses. On macOS workers also join the audio device's workgroup (os_workgroup) when the host provides one; without that, Apple Silicon parks them on efficiency cores and joins miss deadlines. Workgroup tokens are thread-affine, so each worker re-joins from its own loop whenever the device changes.

Wakeups are precise, not broadcast: parked workers advertise themselves in a bitmask and forkJoin signals only as many of them as it published jobs. Broadcasting to the full pool costs more scheduler time than the forked work saves (a dozen realtime threads wake to find the one job already claimed), enough to cancel the lane fork's speedup outright on a 14-core machine. The park/publish race is closed with a seq_cst fence pairing (worker: set bit, then re-check for armed jobs; publisher: arm jobs, then read the mask), and the 1 ms park timeout plus the join steal bound a doubly-missed wakeup to one serial-cost block.

One easy-to-miss detail: FTZ/DAZ denormal flags are per-thread CPU state. Each worker sets ScopedNoDenormals in its own loop; without it, NAM decay tails hit denormal range at roughly 100x cost on workers, and the phase fork would break bit-identity with the serial loop, which runs under the audio thread's flags.

Failure containment

Exceptions can't cross threads, so a NAM phase job traps its own and the joining thread rethrows once every phase has completed (NamEngine::process). The block's existing realtime failure path (disable it, flag it, log from the message thread) then works exactly as it does for a serial throw.

Branch mode

When a chain is branched, the lane-level parallel split is the branch lane versus the trunk's post-tap remainder rather than left versus right. Mono mode never forks lanes; its oversampled NAM blocks still fork their phases.