This article discusses SQL Server support in a
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
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:
- Processor mapping
- Intel's CPU counting utility
More information about each item follows.
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.
licensing and supported configuration details, visit the following Microsoft
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
Processor mapping table 1
Collapse this tableExpand this table
Here is another processor mapping sample.
mapping table 2
Collapse this tableExpand this table
Either 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.
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
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:
Understanding how to set the SQL
Server I/O affinity option
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
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.
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:
Max degree of parallelism (MAXDOP)
A hyper-threaded CPU introduces common CPU cache invalidation issues that physical multiprocessor implementations do not experience. The application workload can affect the performance gains and the common CPU cache behavior. We recommend that you test the application workload with several variations of the MAXDOP
setting. Performance gains may be achieved by using a MAXDOP
setting that is no more than the number of physical processors that are being used. If you are using NUMA, the MAXDOP
setting should be set to no more than the number of CPUs in each NUMA node. For example, if the system is configured for two physical processors and four logical processors, the MAXDOP
setting should be 2. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
Recommendations and Guidelines for 'max degree of parallelism' configuration option