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Over this past year an unusually large number of my students have looked at me strangely (option 1, options 2) when I state that latches are not locks. There are distinct differences between the two in both purpose, design, and application. Quickly contrasting these two serialization control methods is what this blog entry is all about.
Oracle sometimes needs to be in control...
At some point Oracle must ensure certain actions occur one at a time, that is, serially. Oracle serialization control has two broad applications. First there is relational object (e.g., the employee table) control, in which enqueues are used. Second, there is memory structure (e.g., cache buffer chains) control in which latches and mutexes are used.
Locks employ enqueues to ensure relational structure access control.
Oracle uses locks implemented through enqueues to ensure relational objects are changed in an orderly fashion. When an Oracle process needs to lock a relational structure, either explicitly or implicitly, the lock request is placed into a queue. That's where we get the word "in"-queue. When the lock is given, the request is dequeued. (Kind of like a stack's push and pop functions.) Precise ordering is maintained to ensure a deadlock does not occur. Many years ago as an Oracle application developer I distinctly remember being repeatedly told how to properly lock and unlock a series of Oracle tables. In other words, Oracle's locking mechanism was very well defined and dependable. I had to make sure I understood this to avoid encouraging an application deadlock. I like to say that compared to latches and mutexes, enqueues are very boring and mature. You know exactly what's going to happen and in what order. Absolutely no adventure whatsoever!
A massive requirement difference...
Memory structure list serialization control is an entirely different beast and an exciting one at that! Oracle must closely maintain tight control over memory structure list access, otherwise deep and extreme corruption would occur. Memory structure lists are fundamentally different than relational structures as well as buffers (and a buffer's associated buffer header). For example, the cache buffer chains, the least recently used lists, and the write lists are linked lists made up of buffer headers. Buffer headers do not contains rows, they contain pointers to other buffer headers, their associated data block and buffer, and control information. Buffer headers are not relational structures and they are not Oracle buffers. As a result, Oracle needed a way to control access to these and other memory lists.
Latches/Mutexes use a spin and sleep algorithm to ensure memory structure list serialization control.
In stark contrast to Oracle locks, latches and mutexes do not use an orderly acquisition and release strategy. Latches and mutexes employ a spin (i.e., repeated attempts to acquire the latch/mutex) and sleep (i.e., backing off a bit after failing to acquire the latch/mutex) methodology. The session successfully acquiring the latch/mutex got lucky when it just happened to make a request attempt (by checking a specific memory address) after the latch/mutex became available yet before a competing session attempted the acquisition. So it's not an order thing, it's a probability thing: eventually the session will likely acquire the latch/mutex, but there is no guarantee. This is what makes latching and mutex acquisition so interesting and exciting compared to boring enqueues!
If you have been wondering about the differences between an Oracle lock and latches/mutexes, I hope this entry has helped clear things up a bit. If you want to dig into serialization in more detail, just go to my web-site and search for latch. And of course I dig into this topic in my Oracle Performance Firefighting course and book.
Thanks for reading!
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