Shared-memory mutexes

The database server uses mutexes to coordinate threads as they attempt to modify data in shared memory. Every modifiable shared-memory resource is associated with a mutex.

Before a thread can modify a shared-memory resource, it must first acquire the mutex associated with that resource. After the thread acquires the mutex, it can modify the resource. When the modification is complete, the thread releases the mutex.

If a thread tries to obtain a mutex and finds that it is held by another thread, the incoming thread must wait for the mutex to be released.

For example, two threads can attempt to access the same slot in the chunk table, but only one can acquire the mutex associated with the chunk table. Only the thread that holds the mutex can write its entry in the chunk table. The second thread must wait for the mutex to be released and then acquire it.

For information about monitoring mutexes (which are also called latches), see Monitor the shared-memory profile and latches.

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