LCK_M_SIU_LOW_PRIORITY Wait Type Explained

This wait type exists exclusively for ALTER TABLE and ALTER INDEX operations configured with the WAIT_AT_LOW_PRIORITY option, introduced in SQL Server 2014. The lock manager implements a priority-based scheduling system where these low-priority operations voluntarily yield CPU and lock resources to normal-priority transactions.

When an ALTER operation specifies WAIT_AT_LOW_PRIORITY, SQL Server's lock manager places the request in a separate low-priority queue. The scheduler periodically checks if higher-priority operations need the same resources. If conflicts exist, the low-priority operation enters the LCK_M_SIU_LOW_PRIORITY wait state and releases its time slice.

The SIU (Shared Intent Update) lock type combines shared access with intent to escalate to update locks, required for schema modification operations. Unlike standard lock waits, this wait type includes built-in timeout and abort mechanisms. The operation can specify MAX_DURATION to limit total wait time and ABORT_AFTER_WAIT behavior when timeouts occur.

SQL Server 2016 enhanced the priority scheduling algorithm to better handle mixed workloads. SQL Server 2019 improved the lock escalation detection for low-priority operations. The core behavior remains consistent through SQL Server 2022.

The wait occurs in these scenarios: online index rebuilds with low priority, table alterations during peak hours, and partition switching operations configured to minimize blocking. The wait duration depends on competing workload intensity and the MAX_DURATION setting.

-- Check current low priority lock waits and their duration
SELECT 
    r.session_id,
    r.wait_type,
    r.wait_time,
    r.blocking_session_id,
    t.text AS sql_text,
    r.command,
    s.login_name
FROM sys.dm_exec_requests r
INNER JOIN sys.dm_exec_sessions s ON r.session_id = s.session_id
CROSS APPLY sys.dm_exec_sql_text(r.sql_handle) t
WHERE r.wait_type = 'LCK_M_SIU_LOW_PRIORITY'
ORDER BY r.wait_time DESC;

-- Identify objects involved in low priority operations
SELECT 
    r.session_id,
    DB_NAME(r.database_id) AS database_name,
    OBJECT_NAME(p.object_id, r.database_id) AS object_name,
    p.index_id,
    i.name AS index_name,
    r.percent_complete,
    r.estimated_completion_time,
    r.command
FROM sys.dm_exec_requests r
INNER JOIN sys.dm_os_tasks t ON r.session_id = t.session_id
INNER JOIN sys.partitions p ON t.task_address = p.partition_id
LEFT JOIN sys.indexes i ON p.object_id = i.object_id AND p.index_id = i.index_id
WHERE r.wait_type = 'LCK_M_SIU_LOW_PRIORITY';

-- Monitor blocking chains affecting low priority operations
WITH BlockingHierarchy AS (
    SELECT 
        session_id,
        blocking_session_id,
        wait_type,
        wait_time,
        1 AS level
    FROM sys.dm_exec_requests 
    WHERE blocking_session_id != 0
    
    UNION ALL
    
    SELECT 
        r.session_id,
        r.blocking_session_id,
        r.wait_type,
        r.wait_time,
        bh.level + 1
    FROM sys.dm_exec_requests r
    INNER JOIN BlockingHierarchy bh ON r.session_id = bh.blocking_session_id
    WHERE bh.level < 10
)
SELECT * FROM BlockingHierarchy 
WHERE session_id IN (
    SELECT session_id FROM sys.dm_exec_requests 
    WHERE wait_type = 'LCK_M_SIU_LOW_PRIORITY'
)
ORDER BY level, session_id;

-- Check ALTER operations with WAIT_AT_LOW_PRIORITY settings
SELECT 
    r.session_id,
    r.start_time,
    r.command,
    r.percent_complete,
    SUBSTRING(t.text, r.statement_start_offset/2 + 1,
        CASE WHEN r.statement_end_offset = -1 
             THEN LEN(t.text) 
             ELSE r.statement_end_offset/2 - r.statement_start_offset/2 + 1 
        END) AS current_statement
FROM sys.dm_exec_requests r
CROSS APPLY sys.dm_exec_sql_text(r.sql_handle) t
WHERE r.wait_type = 'LCK_M_SIU_LOW_PRIORITY'
   OR UPPER(t.text) LIKE '%WAIT_AT_LOW_PRIORITY%';

Increase MAX_DURATION for long-running operations

-- Modify existing ALTER operation to allow longer wait times
-- CAUTION: Only increase if operation is critical and blocking is minimal
ALTER INDEX IX_YourIndex ON dbo.YourTable REBUILD 
WITH (
    ONLINE = ON,
    WAIT_AT_LOW_PRIORITY (
        MAX_DURATION = 30 MINUTES,  -- Increased from default
        ABORT_AFTER_WAIT = SELF     -- Operation aborts itself after timeout
    )
);
-- Expected impact: Operation waits longer before timing out, may complete successfully
-- Test in development first to validate timeout values

Kill competing high-priority sessions (use with extreme caution)

-- Identify and terminate blocking sessions if absolutely necessary
-- WARNING: This can cause application errors and data loss
DECLARE @BlockingSession INT;
SELECT @BlockingSession = blocking_session_id 
FROM sys.dm_exec_requests 
WHERE wait_type = 'LCK_M_SIU_LOW_PRIORITY'
  AND session_id = <your_session_id>;

-- Only execute after confirming the blocking session can be safely terminated
-- KILL @BlockingSession;  -- Uncomment only after verification

-- Expected impact: Immediately resolves the wait, allows operation to proceed
-- Risk: May cause rollbacks, connection errors, or data inconsistency

Switch to offline operation during maintenance window

-- Convert online operation to offline for guaranteed completion
-- Use during scheduled maintenance when blocking is acceptable
ALTER INDEX IX_YourIndex ON dbo.YourTable REBUILD 
WITH (ONLINE = OFF);

-- Alternative: Disable low priority and accept blocking
ALTER INDEX IX_YourIndex ON dbo.YourTable REBUILD 
WITH (ONLINE = ON);  -- No WAIT_AT_LOW_PRIORITY clause

-- Expected impact: Operation completes faster but may cause blocking
-- Schedule during low-activity periods to minimize user impact

Schedule ALTER operations with WAIT_AT_LOW_PRIORITY during maintenance windows or low-activity periods. Monitor sys.dm_exec_requests regularly to identify patterns of competing workloads that consistently block low-priority operations.

Configure appropriate MAX_DURATION values based on historical operation completion times. Set ABORT_AFTER_WAIT to BLOCKERS rather than SELF when the operation is more critical than competing transactions. Use BLOCKERS carefully in production environments.

Implement workload management through Resource Governor to separate maintenance operations from user workloads. Create dedicated resource pools for index maintenance with higher importance settings when low-priority operations consistently fail.

Monitor index fragmentation levels and rebuild frequency to optimize maintenance schedules. Consider partitioning large tables to reduce ALTER operation scope and duration. Use incremental statistics updates instead of full rebuilds when appropriate.

Establish baseline measurements for typical ALTER operation durations without low priority settings. Set monitoring alerts when LCK_M_SIU_LOW_PRIORITY waits exceed normal thresholds, indicating increased workload conflicts or system resource pressure.