SQL Server support in a hyper-threaded environment

This article discusses SQL Server support in a hyper-threaded environment.

The core design of SQL Server is fully functional in a hyper-threaded environment. However, if you are using a hyper-threaded environment, we recommend that you do the following:

• Run Microsoft SQL Server 2000 Service Pack 3 (SP3) or a later service pack.
• Install the latest security update.

The Microsoft Windows operating system makes the logical hyper-thread CPUs appear as physical CPUs. Because SQL Server is already highly scalable, the additional CPUs give SQL Server the ability to use more processors.

For additional information about Microsoft Windows support of hyper-threading, visit the following Microsoft Web site:
You must be aware of the following things when you are considering a hyper-threaded deployment:

• Licensing
• Processor mapping
• Performance
• Intel's CPU counting utility

More information about each item follows.

Licensing

When hyper-threading is enabled, the Basic Input Output System (BIOS) uses a logical to physical CPU ratio. Current implementations are using 2:1 ratios. This means that there are two (2) logical CPUs for each physical CPU. These ratios may change in the future; however, Microsoft SQL Server 2000 Service Pack 3 (SP3) and later versions have additional licensing code that adjusts the licensing restrictions to handle the ratio. For example, if the SQL Server product code that you have installed permits a 4 CPU license at a 2:1 ratio, the SQL Server 2000 SP3 build and later builds adjust and let you use 8 CPUs.

When you install SQL Server, you will use the physical CPU count and you let SQL Server handle the ratio conversion. In contrast, when you set the affinity mask option, you will use the logical CPU values because SQL Server is using all the CPUs as if they were physical processors.

For more licensing and supported configuration details, visit the following Microsoft Web site:

Processor mapping

By default, SQL Server creates a logical User Mode Scheduling (UMS) scheduler for each processor according to the ordinal processor order. In hyper-threaded enabled environments, Microsoft Windows makes all the processors available to the SQL Server process as if they are true physical processors. A sample of logical and of physical processor mapping is shown in the following table.

Processor mapping table 1Here is another processor mapping sample.

Processor mapping table 2Either mapping is acceptable to SQL Server. However, be careful not to create a situation where SQL Server is unexpectedly limited to a physical CPU when it is expected to use multiple physical CPUs.

You can use the SQL Server affinity mask option to change expected physical processor usage. To do this, see Processor mapping table 1. If the affinity mask is 00010001 and it is using processor 0 and processor 4, the logical processors that are in use are mapped to the sample physical processor, #0. This results in single physical processor use that supports both of the logical processors instead of two physical processors. This situation will probably decrease performance.

You can use a combination of the affinity mask option and the IO Affinity Mask startup parameter to change the default behavior. For more information about the affinity mask option or the IO Affinity Mask startup parameter, see SQL Server Books Online.

For more information, click the following article number to view the article in the Microsoft Knowledge Base: Warning Use caution when you establish the affinity mask. You can easily configure the affinity mask incorrectly by using the same physical processor to support two logical processors when you intended to use separate physical processors.

Performance

The performance of hyper-threaded environments varies. Conservative testing has shown 10 to 20 percent gains for SQL Server workloads, but the application patterns have a significant affect. You might find that some applications do not receive an increase in performance by taking advantage of hyper-threading. If the physical processors are already saturated, using logical processors can actually reduce the workload achieved.

For example, applications that cause high levels of contention can cause decreased performance in a hyper-threaded environment. We recommend that you test your application thoroughly to make sure that a hyper-threaded environment provides you the performance gain that you want versus the purchase of equivalent physical CPUs. Hyper-threading can be very helpful but hyper-threading cannot replace the full power of an additional physical CPU.

The third-party products that this article discusses are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, about the performance or reliability of these products.

The Intel Web site has some good information and tutorials about hyper-threading (Intel NetBurst microarchitecture). For more information about systems that support hyper-threading, visit the following Intel Web sites: