sqlindian – On Locks and Deadlocks

Random thoughts on performance and concurrency

Deadlocks Involving Lock Partitions

with one comment

Lock Partitioning is a feature in SQL Server – which is automatically enabled for machines with more than 16 logical processors – aimed at reducing lock contention. To quote from the MSDN link on the topic,

For large computer systems, locks on frequently referenced objects can become a performance bottleneck as acquiring and releasing locks place contention on internal locking resources. Lock partitioning enhances locking performance by splitting a single lock resource into multiple lock resources. This feature is only available for systems with 16 or more CPUs, and is automatically enabled and cannot be disabled. Only object locks can be partitioned.

When lock portioning is enabled NL, SCH-S, IS, IU, and IX lock modes are acquired on a single partition. All other lock modes must be acquired on all partitions.

One key thing to notice is that, when lock partitioning is enabled, Objects locks are acquired on lock partitions,not the object itself. For eg. If you run the below query on a server where lock partitioning is not enabled, you will get the below result. (irrelevant results omitted)

USE AdventureWorks
GO

SET TRANSACTION ISOLATION LEVEL REPEATABLE READ
BEGIN TRAN

SELECT * FROM Person.[Address]
WHERE AddressId = 1

SELECT * FROM Person.ContactType WITH(TABLOCKX)

SELECT OBJECT_NAME(resource_associated_entity_id) as objectName,resource_description, resource_lock_partition,request_mode  FROM sys.dm_tran_locks
WHERE resource_type = ‘OBJECT’

ROLLBACK TRAN

ObjectName resource_lock_partition request_mode
Address 0 IS
ContactType 0 X

If you run the same script on a machine on which lock partitioning is enabled, you will get the following results.

ObjectName resource_lock_partition request_mode
Address 5 IS
ContactType 15 X
ContactType 14 X
ContactType 13 X
ContactType 12 X
ContactType 11 X
ContactType 10 X
ContactType 9 X
ContactType 8 X
ContactType 7 X
ContactType 6 X
ContactType 5 X
ContactType 4 X
ContactType 3 X
ContactType 2 X
ContactType 1 X
ContactType 0 X

You can see that the IS lock is acquired only on one partition, whereas the X lock is acquired on all partitions. The actual number of partitions depend upon the number of available logical processors.

As the locking is done at the lock partition level, naturally these locks can participate in a deadlock and the resources listed under the resource list will be the lock partitions.

A deadlock can be identified as a lock portioning related deadlock from the below attributes.

1. The wait resource is “Object”

2. The wait resource identifier will be in the format “7:1797581442:12” where the last two digit indicates the lock partition.

3. In the resource list the value for sub resource is “FULL”

4. The resource list will be in the format “lockPartition=12”

5. If you execute the statement involved in the deadlock in a query and inspect resource_lock_partition column in sys.dm_tran_locks, you will see a non-zero value.

The following demo will illustrate a deadlock on lock partitions.

Setup

SQL server automatically enables lock partitioning on machines with more than 16 logical processors. If you don’t have a machine with 16 cores, you can use the (undocumented and unsupported) startup parameter –Pn.

I’ve used –P16 to simulate 16 processors.

When lock partitioning is enabled, you will see the following message in the SQL server log.

Lock partitioning is enabled.  This is an informational message only. No user action is required.

Once you verify that lock partitioning is enabled, run the below script to create the table for our test.

IF DB_ID(‘DeadLockTest’) IS NULL
CREATE Database DeadLockTest
GO

USE DeadLockTest
GO

CREATE TABLE dbo.lpdl(id int identity(1,1) primary key clustered, filler char(100) default ‘abc’)
GO
SET NOCOUNT ON
INSERT INTO dbo.lpdl DEFAULT VALUES
GO 100

Now open a new session (session 1) and run the below query.

BEGIN TRAN

UPDATE dbo.lpdl
SET filler = ‘xyz’
WHERE id = 1

WAITFOR DELAY ’00:00:05′

SELECT * FROM dbo.lpdl WITH(TABLOCKX)

ROLLBACK TRAN

Open another session (Session 2) and run the below query.

BEGIN TRAN

UPDATE dbo.lpdl
SET filler = ‘xyz’
WHERE id = 1

WAITFOR DELAY ’00:00:05′

SELECT * FROM dbo.lpdl WITH(TABLOCKX)

ROLLBACK TRAN

One of the sessions will fail with the deadlock error. You have probably noticed that we are forcing a conversion deadlock here by first acquiring a compatible IX lock and the trying to acquire a non-compatible X lock on the same table.

Analyze

To analyze this deadlock we will go ahead and create our deadlock table.

Process Resource Mode State Command
process4ee8508 OBJECT: 26:242099903:2 X WAIT SELECT
process4ee8508 OBJECT: 26:242099903:0 X GRANT SELECT
process7c4988 OBJECT: 26:242099903:0 X WAIT SELECT
process7c4988 OBJECT: 26:242099903:2 IX GRANT UPDATE

The lock resource reported in the deadlock trace will be in the format OBJECT:DBID:OBJECTID:LOCKPARTITION. AS we can see here  process process7c4988 is holding an IX lock on partition 2. Since IX lock is acquired only on the local partition, we can assume that partition 2 is local to  process7c4988. This process now requires an X lock on partition 0. When a process requires any locks other than locks that can be acquired on the local partition, it must access the locks in a sequential order starting from partition 0. But since process process4ee8508 is already holding an X lock on partition 0, process process7c4988 is now blocked.

Meanwhile process4ee8508 is holding an X lock (converted its own IX lock on the partition to X lock) on partition 0 and it needs to acquire the X lock on the remaining 15 partitions in that order. When it reaches partition 2, it will be blocked since the other thread is holding an incompatible lock and a deadlock ensues.

Regardless of whether lock portioning was enabled or not, the above script will cause a deadlock.  But there are extremely rare scenarios where the lock partitioning feature will cause a deadlock whereas the normal locking behavior will only cause blocking. I will talk about that in the next post.

Fix

The key to avoid this kind of deadlock is to avoid object level locks. These kind of deadlocks are pretty rare. I’ve only found one reference of deadlock involving lock partitions in MSDN forums.

This post is part of the series SQL Server deadlocks and live locks – Common Patterns. If you have questions, please post a comment here or reach me at @sqlindian

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Written by sqlindian

July 7, 2012 at 1:16 AM

One Response

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  1. I found these types of deadlocks on my server because there is some indexes with both ALLOW_PAGE_LOCKS and ALLOW_ROW_LOCKS is OFF. This was causing in first moment a one INSERT acquire IX lock on the object plus X LOCK on the new row for the clustered index, which don’t have any limitations for locking level. But when the session go to update the index with limitation, the session ask for a X lock in the table level, converting your IX Lock to a X lock, but in all partitions.

    In this moment, other sessions also execute same insert and lock other partitions with IX lock, that cause block on sessions attempting get X lock on all table.

    Rodrigo Ribeiro Gomes

    July 23, 2013 at 2:22 PM


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