ReaderWriterSpinlock

Ewald Cress looks at the new ReaderWriterSpinlock in SQL Server 2016 CU2:

As a quick refresher, a traditional SQLOS spinlock is a 32-bit integer, or of course 64-bit as of 2016, with a value of either zero (lock not acquired) or the 32-bit Windows thread ID of the thread that owns it. All very simple and clean in terms of atomic acquire semantics; the only fun part is the exponential backoff tango that results from a collision.

We have also observed how the 2016 flavour of the SOS_RWLock packs a lot of state into 64 bits, allowing more complicated semantics to be implemented in an atomic compare-and-swap. What seems to be politically incorrect to acknowledge is that these semantics boil down to a simplified version of a storage engine latch, who is the unloved and uncool grandpa nowadays.

Clearly a lot can happen in the middle of 64 bits.

Definitely worth a read, as it seems that this is going to get more play in the years to come.

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