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Hello,

On Oct 11, 2006, at 16:01, Joseph A. Dacuma wrote:

> Is it wise to enable HT? I came across various sites where they say 
> Hyper
> Threading makes a system unstable.

Not that I played much with HT Xeons, especially not recent ones, but 
it highly depends on what you're doing. For instance, we had Oracle 9i 
and some monstrous Java app on a 2.something GHz Xeon a couple years 
ago and benchmarking showed that with HT enabled the whole thing ran 
about 10% slower than without HT. This specific app mind you. Others 
have reported a slight speedup with HT so your mileage will vary.
For what I can tell - if you have two threads running bad code with 
plenty of pipeline stalls on a HT CPU it's likely to help since most of 
the core would be idle anyway and HT masks that to some degree - when 
one pipeline stalls the other one will happily continue working. If you 
have two threads that run highly optimized code which uses pretty much 
everything the CPU has and hardly ever stalls then they're likely to 
get into each other's way. HT would also help masking latency issues - 
while one pipeline waits for the cache to fetch something from RAM 
there's a chance that the other one will not.
Don't expect huge performance differences though. If you have the time 
benchmark your app with and without HT and see what happens.

have fun
Michael
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