Another very common question around the cheese post and watering dish.  What does it all mean?

Hyper-Threading

“If I enable Hyper-Threading on a dual-processor server, I’m really getting a 4–way, right?”

Not exactly.  Even though 4 processors show up in Task Manager, it doesn’t necessarily mean you have four proc’s.  Instead, you have 2 physical processors and 2 virtual processors.  Intel's Hyper-Threading Technology allows a single physical processor to execute multiple threads (instruction streams) simultaneously, potentially providing greater throughput and improved performance.  But 2 hyper-threaded enabled processors are not the same as 4 physical.

Yaniv Pessach, said it best in a June 2005 article in MSDN Magazine:

“Hyper-threading can result in better utilization of CPU resources and therefore better performance. The flip side is that those CPU resources are indeed shared, so when both streams of instructions require the same shared resource, performance can suffer.”

This means that different applications will behave differently depending on how they were written. 
(This is yet another example why customers come to the Microsoft Services Labs for testing). 

Multi-Core

And then comes Multi-Core processors.

Unlike hyper-threading, the multi-core design puts multiple processor cores on the same die. 

As Intel puts it:

“Like other hardware-enhanced threaded capabilities advanced at Intel, multi-core capability reflects a shift to parallel processing - a concept originally conceived in the supercomputing world. For example, Hyper-Threading (HT) Technology, introduced by Intel in 2002 enables processors to execute tasks in parallel by weaving together multiple "threads" in a single-core processor. But whereas HT Technology is limited to a single core's using existing execution resources more efficiently to better enable threading, multi-core capability provides two or more complete sets of execution resources to increase overall compute throughput.”
http://www.microsoft.com/whdc/winhec/partners/intel05_multicore.mspx

Here’s a cool video from Intel that does a great job in explaining the differences:

http://intel.com/technology/computing/dual-core/demo/popup/demo.htm

Hope this helps clear up the fog on some very interesting technology.

-Marcus

Related Article:
Performance Benchmarks for Computers Running Exchange Server 2003

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