Locking, Blocking, Versions: Concurrency for Maximum Performance. Kalen Delaney, Moderated By: Daniel Janik

Transcription

1 Locking, Blocking, Versions: Concurrency for Maximum Performance Kalen Delaney, Moderated By: Daniel Janik

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5 BIO Background: MS in Computer Science from UC Berkeley Working exclusively with SQL Server for almost 29 years Author: SQL Server In-Memory OLTP Internals (RedGate, 2014) Primary Author: SQL Server 2012 Internals (O Reilly, 2013) Author: SQL Server Concurrency (RedGate, 2012) Primary Author: SQL Server 2008 Internals (MS Press 2009) Author: Inside SQL Server 2005: The Storage Engine (MS Press, 2006)

6 Aspects of Locking Type of Lock Duration of Lock Granularity of Lock

7 Types of Locks Shared Lock (S) Acquired when data is read Compatible with other S locks Exclusive Lock (X) Acquired when data is modified Not compatible with any other locks

8 Update Locks A Hybrid between Shared and Exclusive Update (U) locks taken during search phase UPDATE DELETE Update lock is promoted/converted to X when performing modification U locks are incompatible with U and X locks Used as a deadlock avoidance mechanism

9 Lock Duration Duration is Dependent on Owner SHARED_TRANSACTION_WORKSPACE (Resource = DATABASE) Held as long as connection is using DB context Prohibit some status changes EXCLUSIVE_TRANSACTION_WORKSPACE (Resource = DATABASE) Acquired during DROP, RESTORE Acquired during status changes Transaction Locks Shared locks held until done reading Exclusive locks held until end of transaction Cursor Locks Scroll locks held until next FETCH Session Locks (Resource = DATABASE) Only used with Application Locks

10 Granularity of Locks: Resources Row: RID or Key Page Table Partition Extent Database

11 Multi-Granular Locking To Lock a Fine Level, SQL Server Places Intent Locks at Higher Levels T1: Acquires IX lock on table Table Page Page Page T1: Acquires IX lock on page T1: Locks row in X mode Row Row Row Another Transaction, T2, cannot obtain X lock on page or table

12 Locking Metadata Type of lock: request_mode Duration of lock: request_owner_type Resource locked: resource_type, resource_database_id, resource_description, resource_associated_entity_id

13 Lock Metadata Demo

14 Locking Granularity and Escalation Default is row lock After optimization, SQL Server may decide to lock pages or the whole table If too much memory is used for locks If optimizer has decided to do a scan Row/key/page locks can be escalated during execution Tradeoff is resources used vs. concurrency By default, escalation is to a table; can be overridden to escalate to partition, or to never escalate ALTER TABLE <table_name> SET ( LOCK_ESCALATION = {TABLE AUTO DISABLE }

15 Mode Requested Lock Compatibility Matrix Lock Mode Already Granted IS S U IX X SchS SchM BU IS Yes Yes Yes Yes No Yes No No S Yes Yes Yes No No Yes No No U Yes Yes No No No Yes No No IX Yes No No Yes No Yes No No X No No No No No Yes No No SchS Yes Yes Yes Yes Yes Yes No Yes SchM No No No No No No No No BU No No No No No Yes No Yes Blocking occurs when one process requests a lock on the same resource held by another process in an incompatible mode

16 Special Locks Sch-S Acquired when query is being compiled Sch-M Acquired during ALTER TABLE, when table structure is modified BU Acquired during Bulk Insert (or BCP) Must be requested explicitly Allows parallel loading

17 Low Priority Waits Low Priority Waits for Schema Locks New in SQL Server 2014 Online Index Rebuild and Partition Switching Other processes will not stack up behind After <n> minutes, low priority waiter can: Give up Kill the blocker Revert to regular wait status

18 Low Priority Waits Demo

19 SQLPASS Precon: Locking, Blocking and Row Versions Isolation Levels More Metadata Snapshot Isolation s Row Versions Troubleshooting and Controlling Locking/Blocking Optimistic Concurrency in In-Memory OLTP

20 Thank You Learn more from Kalen Delaney

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