Follow-up to a discussion on the dev list: http://search-hadoop.com/m/jOovV1uAJBP
The CoprocessorHost ReentrantReadWriteLock is imposing some overhead on data read/write operations, even when no coprocessors are loaded. Currently execution of RegionCoprocessorHost pre/postXXX() methods are guarded by acquiring the coprocessor read lock. This is used to prevent coprocessor registration from modifying the coprocessor collection while upcall hooks are in progress.
On further discussion, and looking at the locking in HRegion, it should be sufficient to just use a CopyOnWriteArrayList for the coprocessor collection. We can then remove the coprocessor lock and eliminate the associated overhead without having to special case the "no loaded coprocessors" condition.