PREEMPTIVE_XE_DISPATCHER

Understanding the PREEMPTIVE_XE_DISPATCHER Wait Type in SQL Server

When diving into SQL Server performance tuning, one area that often draws attention is wait statistics. These provide insights into what your SQL Server is waiting on, helping to pinpoint performance bottlenecks. One of the lesser-known wait types you might encounter is PREEMPTIVE_XE_DISPATCHER.

In this blog post, we’ll explore what the PREEMPTIVE_XE_DISPATCHER wait type means, when you might encounter it, and how to address potential performance issues related to it.

What Is PREEMPTIVE_XE_DISPATCHER?

To understand this wait type, it helps to break it down:

  • PREEMPTIVE: SQL Server generally runs in a cooperative multitasking mode, meaning it yields processor time voluntarily. However, in some cases, SQL Server operates under preemptive scheduling, where it does not cooperate with the OS but rather forcefully takes CPU time when needed. The “PREEMPTIVE” prefix on a wait type usually indicates that SQL Server is performing work outside of its normal control in the Windows operating system.
  • XE_DISPATCHER: This refers to Extended Events (XE), a lightweight performance monitoring system introduced in SQL Server 2008. Extended Events allow you to track various server and database actions in real-time. XE_DISPATCHER refers to the component responsible for dispatching and managing those events.

So, putting it together, the PREEMPTIVE_XE_DISPATCHER wait type occurs when SQL Server is waiting for a preemptive task related to the Extended Events system. Specifically, it indicates that SQL Server is working on something outside its normal realm, in this case, related to the Extended Events dispatcher.

When Does PREEMPTIVE_XE_DISPATCHER Occur?

You’ll see the PREEMPTIVE_XE_DISPATCHER wait type in scenarios where SQL Server is interacting with the Extended Events infrastructure and is forced to wait while the OS performs certain tasks.

Here are a few common scenarios where this might appear:

  • High Extended Events Load: If you are running many Extended Events sessions, or those sessions are capturing large volumes of data, SQL Server may have to wait on the dispatcher to handle this load.
  • Operating System Interactions: Since the XE dispatcher involves OS-level operations, interactions with the system can occasionally cause SQL Server to wait.
  • Disk or IO Constraints: If the data generated by Extended Events needs to be written to disk, slow storage performance or disk contention might increase these waits.

How to Diagnose PREEMPTIVE_XE_DISPATCHER

While PREEMPTIVE_XE_DISPATCHER is usually not a common source of major delays, a sudden increase in its occurrence could signal an underlying problem with your Extended Events setup. Here’s how you can investigate it:

  1. Check Extended Events Sessions: Review the number and complexity of your Extended Events sessions. If you’re collecting a large number of events across multiple sessions, try scaling back or optimizing what you are capturing. Are all the events necessary? Can the session be modified to reduce the volume of data?
  2. Monitor System Resource Usage: Since this wait type indicates preemptive work being done outside SQL Server’s normal scheduling, it’s crucial to ensure that system resources like CPU and IO are not overwhelmed. Investigate CPU usage, especially if the waits coincide with heavy disk IO or CPU contention.
  3. Disk Performance: If you are capturing and writing event data to disk, make sure your disk system is performing optimally. Any latency or slow reads/writes to disk can exacerbate wait times. Check for disk bottlenecks using tools like Performance Monitor or SQL Server’s built-in DMVs.
  4. Monitor Wait Statistics: You can check wait statistics using the following query to see how much time is being spent on PREEMPTIVE_XE_DISPATCHER:sqlCopy codeSELECT wait_type, SUM(wait_time_ms) AS total_wait_time_ms FROM sys.dm_os_wait_stats WHERE wait_type = 'PREEMPTIVE_XE_DISPATCHER' GROUP BY wait_type; This query shows the cumulative wait time for this specific wait type. If it’s significantly high, there may be a deeper issue with your Extended Events infrastructure.
  5. Review Event Session Targets: Targets in Extended Events sessions specify where data is stored, such as ring buffers or files. Review whether the targets are appropriate for your use case. For instance, file-based targets can be slower, particularly on disks with high latency, and may contribute to this wait type.

How to Mitigate PREEMPTIVE_XE_DISPATCHER Waits

Once you’ve identified that PREEMPTIVE_XE_DISPATCHER waits are a problem, there are a few steps you can take to mitigate them:

  1. Tune or Reduce Extended Events: The first step is to reduce the workload imposed by Extended Events. Disable unnecessary sessions, reduce the number of events collected, or adjust the filters to limit the data being captured.
  2. Use Better Storage: If disk IO is a bottleneck, consider moving your Extended Events session targets to faster storage. SSDs or storage with lower latency can significantly reduce wait times.
  3. Optimize System Resources: Address CPU or memory contention. Ensure that SQL Server has sufficient resources to handle Extended Events and other tasks without causing preemptive scheduling waits.
  4. Consider Alternative Tools: In some cases, if Extended Events are causing too much overhead, you might consider reducing their use for continuous monitoring and instead leveraging tools like Database Health Monitor for performance tracking. Database Health Monitor provides a less intrusive way to monitor SQL Server performance with a built-in wait analysis feature.

Conclusion

The PREEMPTIVE_XE_DISPATCHER wait type, though not commonly a top offender in most environments, can indicate bottlenecks or inefficiencies with your Extended Events setup. By monitoring wait statistics, optimizing your Extended Events sessions, and ensuring system resources are sufficient, you can reduce these waits and improve overall SQL Server performance.

And remember, monitoring tools like Database Health Monitor can help you identify these waits early on, giving you the insight you need to keep your SQL Server running smoothly. If you find yourself dealing with complex performance issues and need expert help, Stedman Solutions offers SQL Server Managed Services that can proactively monitor, tune, and resolve these kinds of challenges. Let us take care of your SQL Server so you can focus on what matters most.

For more details on monitoring and performance tuning, visit DatabaseHealth.com.

Relating to extended events.

Introduced in SQL Server 2008.

Related links:

  • http://databasehealth.com/server-overview/waits-by-type/preemptive-waits/