Published on

January 27, 2011

Reducing CXPACKET Wait in SQL Server

Have you ever encountered the CXPACKET wait type in SQL Server and wondered how to reduce it? In this blog post, we will discuss some suggestions to help you minimize CXPACKET wait and improve the performance of your queries.

Potential Reasons for CXPACKET Wait

There are several potential reasons why CXPACKET wait may occur:

  • Data skew: If the data is heavily skewed, the query optimizer may assign fewer threads to the query, resulting in uneven workload distribution and CXPACKET wait.
  • Resource bottlenecks: When retrieving data, if one of the threads faces IO, memory, or CPU bottleneck and has to wait for resources, it can lead to CXPACKET wait.
  • Different speed IO subsystem: In rare cases, if the retrieved data is on a different speed IO subsystem, it may contribute to CXPACKET wait.
  • Higher fragmentations: Higher fragmentations in certain areas of a table can lead to less data per page, which can result in CXPACKET wait.

While these reasons may not be the major cause of CXPACKET wait, they can contribute to the overall wait time.

Best Practices to Reduce CXPACKET Wait

Here are some best practices to help reduce CXPACKET wait:

  • Refer to the earlier article regarding MAXDOP and Cost Threshold, as these settings can have a significant impact on parallelism and CXPACKET wait.
  • De-fragmentation of indexes can increase the amount of data obtained per page, assuming a close to 100 fill-factor.
  • If your data is spread across multiple files on similar speed physical drives, it may help reduce CXPACKET wait.
  • Keep the statistics updated, as this will provide better estimates to the query optimizer when assigning threads and dividing the data among available threads. Updating statistics can significantly improve the strength of the query optimizer and positively impact parallelism.

It is important to follow these best practices to optimize your queries and minimize CXPACKET wait.

Avoiding Bad Practices

It is equally important to avoid certain bad practices that can worsen the situation:

In one of my recent consultancy projects, I encountered a situation where an “experienced” DBA attempted to reduce CXPACKET wait by increasing the worker threads. However, this approach led to several other issues. With more threads, more memory was consumed, resulting in memory pressure. Additionally, the increased number of threads caused higher “Context Switching” on the CPU scheduler, further degrading performance.

It is crucial to understand that reducing the number of threads is not the solution either, as it may create heavy stalling for parallel queries. It is recommended not to modify the thread settings when dealing with CXPACKET wait.

Conclusion

CXPACKET wait can be a common issue in SQL Server, but by following best practices and avoiding bad practices, you can effectively reduce the wait time and improve query performance. Remember to consider factors such as data skew, resource bottlenecks, IO subsystem speed, and index fragmentation when analyzing and optimizing your queries.

Disclaimer: The information presented in this blog post is based on my experience and may vary from system to system. It is recommended to test these suggestions on a development server before implementing them in a production environment.

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