Wait Type CLR_RWLOCK_READER

Wait statistics, in the context of SQL Server, refer to the amount of time that a query spends waiting to access data in the database. When a client application requests data from the database, the request is placed in a queue and the client application must wait for its turn to access the data. The time that the query spends waiting is called a "wait" and is tracked by SQL Server. This information can be used to identify potential performance bottlenecks and optimize the performance of the database. Wait statistics are commonly used by database administrators to diagnose and troubleshoot performance issues in SQL Server.

SQL Server’s wait types offer insights into how resources are being used and where tasks are delayed. The CLR_RWLOCK_READER wait type is related to the Common Language Runtime (CLR), which enables SQL Server to execute .NET code. This wait type occurs when a thread is waiting to acquire a reader lock on a resource managed by CLR. Understanding this wait type is critical for troubleshooting CLR-related performance issues.

What Is the CLR_RWLOCK_READER Wait Type?

The CLR_RWLOCK_READER wait type appears when a SQL Server thread executing CLR-based code needs to read a resource that is locked for writing or is being accessed by other readers. CLR uses reader-writer locks to manage access to shared resources, ensuring that multiple readers can work simultaneously but only one writer can modify the resource at a time.

When Does CLR_RWLOCK_READER Appear?

This wait type typically shows up in the following scenarios:

• CLR Code Execution – When SQL Server is running user-defined CLR functions, stored procedures, or triggers that rely on shared resources.
• High Concurrency – When multiple CLR threads attempt to read the same resource simultaneously or are blocked by a writer thread.
• Inefficient CLR Code – Poorly optimized .NET code that frequently locks resources can lead to contention among threads.
• Long-Running Writes – Writer threads holding locks for extended periods can block readers, increasing wait times.

Why CLR_RWLOCK_READER Waits Matter

While some CLR_RWLOCK_READER waits are expected in environments using CLR integration, excessive waits may indicate inefficiencies or resource contention. Common causes include:

• Contention Between Readers and Writers – High contention for shared resources can lead to delays as threads wait for locks to be released.
• Long-Running Write Operations – Writers holding locks prevent readers from accessing the resource, increasing wait times.
• Resource Bottlenecks – Limited CPU or memory can exacerbate delays in managing locks and executing CLR tasks.
• Poorly Optimized Code – Inefficient CLR routines that frequently lock resources or perform lengthy operations within locks.

How to Address CLR_RWLOCK_READER Waits

If this wait type is affecting SQL Server performance, consider the following strategies:

• Optimize CLR Code – Review the .NET code for inefficiencies, especially around lock usage, and optimize it to minimize contention.
• Reduce Lock Durations – Ensure that CLR routines hold locks for the shortest time necessary to avoid blocking other threads.
• Balance Readers and Writers – Investigate workloads to ensure that write operations do not disproportionately block readers.
• Allocate Adequate Resources – Ensure the server has sufficient CPU and memory to handle CLR workloads efficiently.
• Monitor Resource Usage – Use tools like dynamic management views to identify high-concurrency areas and adjust workloads accordingly.

Monitoring CLR_RWLOCK_READER Waits

To effectively track and analyze this wait type, use the Historic Waits Monitoring feature in Database Health Monitor. This tool provides a detailed view of wait statistics over time, helping you pinpoint areas of contention and take proactive measures to resolve them. Regular monitoring ensures optimal performance of CLR-based processes.

Stedman Solutions specializes in SQL Server performance tuning and troubleshooting. If you’re dealing with CLR_RWLOCK_READER waits or other SQL Server performance challenges, our Database Health Monitor can provide critical insights, and our managed services deliver expert support to keep your SQL Server environment running smoothly. Contact us today to learn how we can help improve your SQL Server performance and reliability.

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Applies to

Related Waits

See Also

• More About CLR_RWLOCK_READER