1. Tech Ed North America 2010
8/23/2018 1:32 AM
Required Slide
SESSION CODE: DAT315
Strategies to Get Maximum Concurrency for Your Workload in Microsoft SQL Server
Sunil Agarwal
Senior Program Manager
Microsoft Corporation
© 2010 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

2. Goals Concurrency challenges with applications
Identifying and troubleshooting concurrency issues
Strategies for creating applications for higher concurrency

3. Common Concurrency Issues with Applications
Wrong transaction isolation level
Incorrect locking granularity
Missing Indexes
Accessing large dataset leading to lock escalation
Long running transactions
User interactions in active transaction
Accessing objects in reverse order across transactions
Application induced hotspots

4. Isolation Levels SQL-Standard specifies four isolation levels
SQL Server has implemented these all along using lock based implementation.
Read Uncommitted
Most concurrent, least restrictive, more anomalies are possible
SQL Server: No share locks
Read Committed
Only reads committed data. Data can change over multiple reads
SQL Server: Data access only after acquiring S-lock. Lock released when moved to next row/page
Repeatable Read
Read rows don’t change. Phantom rows can appear
SQL Server: Data access only after acquiring S-lock. Lock released at the end of the transaction
Serializable
Least concurrent, most restrictive, always ‘correct’
Isolation-levels impact the concurrency
SQL2005 introduced RCSI (Read Committed Snapshot) and SI (Snapshot Isolation) using non-blocking implementation.

5. Reader Writer Blocking
Data Page
Tran2 (Select)
Tran1 (Update)
Row-1
Row-1
S-Lock
Blocked
X-Lock

6. Read Committed Snapshot Statement-Level ‘Snapshot Isolation’
New “flavor” of read committed
Turn ON/OFF on a database
Readers see committed values as of beginning of statement
Writers do not block Readers
Readers do not block Writers
Writers do block writers
Can greatly reduce locking / deadlocking without changing applications

7. Snapshot Isolation Transaction level ‘Snapshot Isolation’
SQL Server proprietary isolation level
Turn ON/OFF on a database
Available since SQL Server 2005
Readers see committed values as of beginning of the Transaction
Writers do not block Readers
Readers do not block Writers
Writers do block writers
Can greatly reduce locking / deadlocking but requires application change
Set Transaction Isolation level Snapshot
Scenario: Cross query data view consistency

8. Locking Granularity Small lock granules, such as rows:
Maximize concurrency
Are expensive to maintain(require many lock manager calls).
Approx 100 byte overhead
Large granules, such as tables:
Expensive in terms of concurrency
Cheap to acquire (few lock manager calls)
Multi-granular lock protocols allow us to have it both ways
Decided at run-time using internal heuristic
Can be over-ridden using sp_indexoption or locking hint
Lock Escalation

9. Lock Modes Common locking modes Mode Description
S Share - used for reading
X Exclusive - typically used for writing (insert, update, delete)
U Update - used to evaluate prior to writing
IS Intent Share - share locking at finer level
IX Intent Exclusive - X locking at finer level
BU Bulk Update. Used in Bulk Load
SCH-S Schema Share
SCH-M Schema modify lock

10. Locking Hints Locking hints are used to Lock duration
Change locking behavior at statement level
Example: TABLOCK, UPDLOCK
To change isolation level at statement level
Examples: NOLOCK, READPAST, TABLOCK, READCOMMITTED, REPEATABLEREAD
Lock duration
Depends on isolation level it is acquired at.

11. Blocking Detection and Troubleshooting
Tech Ed North America 2010
8/23/2018 1:32 AM
Blocking Detection and Troubleshooting
Sunil Agarwal Senior Program Manager Microsoft
DEMO
© 2010 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

12. Lock Escalation Lock Escalation HOBT Page Row T1: IX T1: X T1: IX

13. Lock Escalation Converting finer-grain locks to coarse grain locks.
Row to Table
Page to Table.
No escalation from Row to Page
Benefits
Reduced locking overhead
Reduces Memory requirement. Each lock structure is 100 bytes
Impedes concurrency
Triggered when
Number of locks acquired on a rowset > Periodically after that
It is not based on the locks in a transaction
Memory pressure

14. Lock Escalation… Cont Controlling Lock Escalation
TF-1211 disables lock escalation for the instance.
SQL Server can run out of lock memory
TF 1224 disables 5000 limit but keeps lock escalation due to memory pressure.
Using ALTER TABLE to disable/enable lock escalation (SQL2008)
Allows lock escalation at partition level

15. Lock Escalation Detection and Troubleshooting
Tech Ed North America 2010
8/23/2018 1:32 AM
Lock Escalation Detection and Troubleshooting
Sunil Agarwal Senior Program Manager Microsoft
DEMO
© 2010 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

16. Silent Blocker: Page Latches
What is a Pagelatch?
Schema and access patterns leading to latch contention
Clustering index has a increasing primary key with high rate of inserts
Relatively small tables with a lot of read/write/delete
Indexes with low cardinality in the key with high rate of inserts
Allocation Structures (e.g. SGAM)
Observed with 16+ concurrent threads
Page Header R1
Free Offset R2 R3

17. Silent Blocker: Page Latches
What can you do?
Cluster on some other column
Partition the table on some other attribute (computed column (hash))
Add another major key column
Change the application

18. Page Latch Contention and Troubleshooting
Tech Ed North America 2010
8/23/2018 1:32 AM
Page Latch Contention and Troubleshooting
Sunil Agarwal Senior Program Manager Microsoft
DEMO
© 2010 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

19. Deadlock Example: User 1 holds X lock on page P1
User 1 requests to acquire SH lock on P2
It waits for User 2
User 2 requests to acquire SH lock on P1
It waits for User 1
Leads to an indefinite wait

20. Deadlock: Detection Background Thread (Lock Monitor)
Runs every 5 seconds (adjusted dynamically)
When deadlock detected, a victim is chosen based on
Transaction that has done minimum amount of work
Deadlock priority.
Victim transaction is rolled back and an error 1205 is generated.
Deadlock Troubleshooting
Use SQL profiler
User TF-1204 (SQL2000)
TF-1222 (SQL2005)

21. Deadlock Detection and Troubleshooting
Tech Ed North America 2010
8/23/2018 1:32 AM
Deadlock Detection and Troubleshooting
Sunil Agarwal Senior Program Manager Microsoft
DEMO
© 2010 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

22. Deadlock: Avoidance Access objects/rows in same order
Use lower isolation level
Use read-committed-snapshot or snapshot isolation
Use shorter duration transactions
Watch out for lock escalation

23. Guidelines: Concurrent Application
Choose lowest isolation needed for consistency
Use shorter transaction when possible
Avoid using larger row sets
Break the statement into smaller rowsets
Minimize large scans
Enable index usage when possible
When possible, design to spread DML across multiple pages

24. Summary Balance consistency vs. concurrency
Factors that impact concurrency
Troubleshooting Blocking
Best practices for developing concurrent applications

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Tech Ed North America 2010
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© 2010 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

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Tech Ed North America 2010
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© 2010 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

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Tech Ed North America 2010
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© 2010 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

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29. Tech Ed North America 2010 8/23/2018 1:32 AM
© 2010 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.
© 2010 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.

30. Required Slide Tech Ed North America 2010 8/23/2018 1:32 AM
© 2010 Microsoft Corporation. All rights reserved. Microsoft, Windows, Windows Vista and other product names are or may be registered trademarks and/or trademarks in the U.S. and/or other countries.
The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.