Create the PSAME task queue (a MultiProducerSingleConsumerQueue, producers use non-blocking tryPush()), a single worker thread running on low-priority tickets (TaskType::Background), and PSAMEService (ReplicaSetAwareService: onStartup initializes the queue and worker; onStepUpComplete/onStepDown notify primary state). PSAMEService is instantiated in mongod_main.cpp's _initAndListen.

The queue has a single consumer, so there is one worker, not a pool.

See WRITING-11150, PSAME Core and ANNEX I.