The "worker_aio_requests" directive. The default value is 32 AIO simultaneous requests per worker. Previously they were hardcoded to 1024, and it was too large, since Linux allocated them early on io_setup(), but not on request itself. So with default value of /proc/sys/fs/aio-max-nr equal to 65536 only 64 worker processes could be run simultaneously. 32 AIO requests are enough for modern disks even if server runs only 1 worker.