Hyper Threading - Good or Bad?

Ello,

We have quite a few SQL Servers running on servers with hyper threaded enabled processors. We have always left hyper threading on, even on our very busy servers but i've recently begun to question this due to comments i have received.

All our servers run SQL 2000 SP3a as "support" for hyper threading was added in SP3a

So... What do you think of Hyper Threading any why?Never had the need...I'll have to read up on this....|||OK, I'll go with gimicky

www.microsoft.com/sql/howtobuy/SQLonHTT.doc

Just scale the box|||Thanks Brett.

It will be good to compare notes, my hunt for information has not found much for or against yet although when trouble shooting we are now always advised to turn it off...

This link to a newgroup article raised my eyebrow
News Group (http://groups.google.co.uk/group/microsoft.public.sqlserver.server/browse_frm/thread/99860c74db9d183/af9e69650569f1db?lnk=st&q=hyper+threading+%2B+SQL+performance&rnum=2&hl=en#af9e69650569f1db)

Quoting a MVP from the link he says

The problem with a HT CPU that the 2 simultaneous threads running on the CPU both contend for the *same* L1, L2 and L3 processor cache. During heavy processing, you may actually have the 2 clearing each other's cache. This results in each having to go to main system RAM to work.

But read the whole thing for context as he also talks about parallelism|||Yes i read that but the other post has spurred me on to get the internet shovel out and keep hunting for more information and post for peoples experiences.

:D|||I have been told that Windows Server 2003 has better support for Hyperthreading. If you can, get your SQL Servers running on 2003, and all should be well. We have had no problems on 2003, or 2000, though.