If a wait queue is defined as a "priority queue" then requests are dequeued
in blocks of 16 in order to work well with write gathering + readahead on the
server.
There are 3 levels of priority. The high priority tasks get scheduled 16 times
for each time the default level gets scheduled. The lowest level gets
scheduled once every 4 times the normal level gets scheduled.
Original patch contributed by Shantanu Goel.