The in-flight batch signal is POA's main backpressure signal: the dispatcher blocks when the dispatched-but-unpopped in-flight batch count or their total bytes of ops exceed a threshold (e.g., 1 GB of in-flight ops). Since a batch is only popped from the inflight FIFO once all of its workers complete, a single slow worker naturally holds its batches in the FIFO queue. Tracking bytes alongside the count also bounds the total memory held by dispatched-but-unapplied ops. A per-worker queued-bytes cap (SERVER-134753) complements this as a stopgap against a single worker concentrating memory while the total stays under the global cap.

This ticket should include:

• A threshold server parameter (IDL) for the max in-flight batches, with the dispatcher blocking when the FIFO length exceeds it.
• A threshold server parameter (IDL) for the max in-flight bytes, incremented by the batch's total op size at dispatch and released when the advancer pops the batch, with the dispatcher blocking when the total exceeds it.