Hyperthreading hurts system performance

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http://blogs.msdn.com/slavao/archive/2005/11/12/492119.aspx


Our customers observed very interesting behaviour on high-end HT-enabled hardware. They noticed that in some cases when high load is applied SQL Server CPU usage increases significantly but SQL Server performance degrades.' Another developer, this time from Citrix, was just as blunt. 'It's ironic. Intel had sold hyperthreading as something that gave performance gains to heavily threaded software. SQL Server is very thread-intensive, but it suffers. In fact, I've never seen performance improvement on server software with hyperthreading enabled. We recommend customers disable it.
 
It is related to bandwidth.

Multiple threads share same memory contrller and even the same L2 cache.

S, the only situation where HT is going to be benefetial is where you have a fewthreads staved for I/O and a smaller number of COU intensive threads. If most of the threads are CPU intensive it might have an adverse impact.

And it is a security issue as well. Two threads share the same cache and if they do not belong to the same process then this is a security nightmare.

http://news.zdnet.co.uk/hardware/chips/0,3...39198583,00.htm
 
ok....the security issue with threads sharing the same cache is based on unfounded fears. The performance problem isn't due to hyperthreading. The problem is due to bottlenecks created in the System Bus.
 

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