This FAQ is for informational purposes only. Check with Oracle Support before using this information. A latch is an internal Oracle mechanism used to protect data structures in the SGA from simultaneous access.
Latches are more restrictive than locks in that they are always exclusive.
Latches are never queued, but will spin or sleep until they obtain a resource, or time out. Enqueues and locks are different names for the same thing. Both support queuing and concurrency. Semaphores are an operating system facility used to control waiting. Oracle initialization or INIT.ORA parameters with an underscore in front are hidden or unsupported parameters.