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In-Memory OLTP Garbage Collection

SQL Server 2014

A data row is considered stale if it was deleted by a transaction that is no longer active. A stale row is eligible for garbage collection. The following are characteristics of garbage collection in In-Memory OLTP:

  • Non-blocking. Garbage collection is distributed over time with minimal impact on the workload.

  • Cooperative. User transactions participate in garbage collection with main garbage-collection thread.

  • Efficient. User transactions delink stale rows in the access path (the index) being used. This reduces the work required when the row is finally removed.

  • Responsive. Memory pressure leads to aggressive garbage collection.

  • Scalable. After commit, user transactions do part of the work of garbage collection. The more transaction activity, the more the transactions delink stale rows.

Garbage collection is controlled by the main garbage collection thread. The main garbage collection thread runs every minute, or when the number of committed transactions exceeds an internal threshold. The task of the garbage collector is to:

After a user transaction commits, it identifies all queued items associated with the scheduler it ran on and then releases the memory. If the garbage collection queue on the scheduler is empty, it searches for any non-empty queue in the current NUMA node. If there is low transactional activity and there is memory pressure, the main garbage-collection thread can access garbage collect rows from any queue. If there is no transactional activity after (for example) deleting a large number of rows and there is no memory pressure, the deleted rows will not be garbage collected until the transactional activity resumes or there is memory pressure.

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