lightweight pooling Server Configuration Option
Applies to: SQL Server
Use the lightweight pooling option (also called "fiber mode") to provide a means of reducing the system overhead associated with the excessive context switching sometimes seen in symmetric multiprocessing (SMP) environments. When excessive context switching is present, lightweight pooling can provide better throughput by performing the context switching inline, thus helping to reduce user/kernel ring transitions.
Fiber mode, which is based on Windows fibers, is intended for situations in which the context switching of worker threads are the critical bottleneck in performance. Because this is rare, fiber mode rarely enhances performance or scalability on the typical system. Improved context switching in Microsoft Windows Server has also reduced the need for fiber mode. We do not recommend that you use fiber mode scheduling for routine operation. That's because fiber mode can decrease performance by inhibiting the regular benefits of context switching, and because SQL Server components that use Thread Local Storage (TLS) or thread-owned objects, such as Windows mutexes, cannot function correctly in fiber mode.
Setting lightweight pooling to 1 causes SQL Server to switch to fiber mode scheduling. The default value for this option is 0.
The lightweight pooling option is an advanced option. If you are using the sp_configure system stored procedure to change the setting, you can change lightweight pooling only when show advanced options is set to 1. The setting takes effect after the server is restarted.
Lightweight pooling is not supported for Microsoft Windows 2000 and Microsoft Windows XP. Windows Server 2003 provides full support for lightweight pooling.
Common language runtime (CLR) execution is not supported under lightweight pooling. Disable one of two options: "clr enabled" or "lightweight pooling". Features that rely upon CLR and that do not work properly in fiber mode include the hierarchy data type, replication, and Policy-Based Management.
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