I witnessed a confused conversation on Twitter today where someone was talking about how he uses the terms sockets and cores interchangeably, since everyone else does, or words to that effect. This made me think that there may be some widespread confusion about how these terms are used and what they mean in relation to SQL Server hardware selection and SQL Server licensing considerations.

The hierarchy works like this:

Physical socket on a motherboard where a physical processor fits (used for licensing)

Physical core within a physical processor (multi-core)

Logical core within a physical core (hyper-threading)

Back in prehistoric days (around 2001) all Intel and AMD processors had but a single core. If you wanted multiple threads of execution, you needed additional physical processors, since one socket = one physical processor = one physical core. Back then, the primary way to increase single-threaded performance was to increase the clock speed of the processor. Both Intel and AMD started running into problems with heat dissipation and power consumption as clock speeds approached 4.0GHz (on air cooling).

In 2002, Intel introduced the first processor with hyper-threading. Hyper-threading creates two “logical processors” within each physical processor core of an actual physical processor, that are visible to the operating system. Depending on the application, hyper-threading can improve performance by anywhere from 5-30%. The initial implementation of hyper-threading on the Pentium 4 Netburst architecture did not work as well on many server workloads (such as SQL Server), so the standard advice was to disable hyper-threading on database servers.

In 2005, AMD introduced the first dual-core processor, the Athlon 64 X2. This processor had two discrete physical cores, which provided better multi-threaded performance than hyper-threading. A single, dual-core processor would have two processor cores visible to Windows. It is important to remember that Windows (and SQL Server) cannot tell the difference between hyper-threaded logical processors and true dual-core or multi-core processors.

In late 2006, Intel introduced the first Core2 Quad, which was a processor with four physical cores (but no hyper-threading). One of these processors would have four cores visible to Windows. Since then, both AMD and Intel have been rapidly increasing the physical core counts of their processors. AMD has a processor called the Opteron 61xx “Magny-Cours” which has 12 physical cores in a single physical processor. Intel has the Xeon 75xx “Nehalem-EX”, which has eight physical cores, plus 2nd generation hyper-threading, which means that you have a total of 16 cores visible to Windows and SQL Server.

In late 2010/early 2011, AMD will release the “Bulldozer” processor, which will have 16 physical cores per physical processor, while Intel is scheduled to release the “Westmere-EX”, which will have 10 physical cores, plus 2nd generation hyper-threading, which means that you will have a total of 20 cores visible to Windows per physical processor. The 2nd generation hyper-threading in the Intel Nehalem and Westmere works much better for SQL Server OLTP workloads, so I always leave it enabled.

SQL Server licensing is only concerned with physical processor sockets, not physical cores, or logical cores. Knowing this, you should always buy processors with as many cores as possible in order to maximize your processor performance per processor license. You should also be aware that SQL Server 2008 is limited to 64 logical processors. In order to use more than 64 logical processors, you must be running SQL Server 2008 R2 on top of Windows Server 2008 R2, which will raise your limit to 256 logical processors.

Finally, you should be aware that SQL Server 2008 R2 Enterprise Edition has a license limit of eight physical processors (which would let you go up to 160 logical processors with eight Intel Westmere-EX processors). If you need more than eight physical processors, you will need to run SQL Server 2008 R2 Data Center Edition.

Remember that Windows and SQL Server cannot tell the difference between physical and logical cores. Running the query below will tell you how many logical cores are visible and how many physical CPUs you have.

-- Hardware information from SQL Server 2008 and 2008 R2 
-- (Cannot distinguish between HT and multi-core)
SELECT cpu_count AS [Logical CPU Count], hyperthread_ratio AS [Hyperthread Ratio],
cpu_count/hyperthread_ratio AS [Physical CPU Count], 
physical_memory_in_bytes/1048576 AS [Physical Memory (MB)], sqlserver_start_time
FROM sys.dm_os_sys_info;