Latch Contention - SQL Server

Latch contention is a common issue in SQL Server, and each version has introduced various enhancements to improve latch management and reduce contention. Here are some examples of enhancements in each version:

                                        SQL Server 2017:

Latch-Free IO Completion Latch: This latch is used to improve performance when reading data from disk. It reduces contention on latches by allowing multiple threads to perform IO operations without contention. For example, when reading data from a table, multiple threads can read data without contention, which can improve performance.

                                        SQL Server 2019:

AdaptiveLatch: This new latch class is designed to reduce contention on heavily contended data structures. The latch dynamically adjusts its behavior based on the workload, reducing contention when it detects high concurrency. For example, when multiple threads try to access the same data page, the AdaptiveLatch can adjust the behavior to reduce contention and improve performance.

Spinlock Improvements: SQL Server 2019 includes improvements to the database engine to reduce contention on spinlocks, which are another type of synchronization mechanism used by SQL Server. For example, when multiple threads try to acquire a spinlock, SQL Server can use a queue-based approach to reduce contention and improve performance.

                                        SQL Server 2022:

Scalable Shared Access (SSA) data structure: This new data structure is latch-free and can significantly reduce contention on latches, enabling better scalability for memory-optimized tables in high-concurrency scenarios. For example, when multiple threads try to access a memory-optimized table, the SSA data structure can reduce contention and improve performance.

Latch Manager Enhancements: SQL Server 2022 includes further enhancements to the latch manager to reduce contention on heavily contended data structures. For example, when multiple threads try to access a system table, the latch manager can adjust its behavior to reduce contention and improve performance.

Each version of SQL Server has introduced enhancements to improve latch management and reduce contention, and these improvements can have a significant impact on performance and scalability in high-concurrency scenarios.


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