OT: To Hyperthread or NOT to Hyperthread

Dhawal Doshy dhawal at netmagicsolutions.com
Thu Jun 8 17:35:15 IST 2006

Craig Retief wrote:
> HI All,
> A question maybe 4 the gurus…
> When running a Linux server with MailScanner, SpamAssassin, Sendmail, 
> “the milters”, razor, pyzor, dcc, bdc, clamav, etc, etc, etc. Would it 
> be better to have the servers hyperthreading enabled or disabled?
> I have read articles where the general recommended consensus is that it 
> should be disabled. Has anyone actually experienced this scenario or 
> have some comments relating?

*Disclaimer*: i am NOT (and probably never will be) a linux guru.

Hyperthreading is somewhat useful if your server utilization is less 
than 50%..

It works by appearing to your OS that you have 2 logical CPUs per 
physical CPU. It is also important that the applications on your server 
understand and take advantage of SMP.

If your server is running at more than 50% utilization.. hyperthreading 
is going to be bad for you.. if you are not sure, turn it off.

Once you understand how hyperthreading works.. it all makes sense. Also 
at the end of the day understand that there is no such thing as a free 
lunch. The CPU manufacturers are using this as a marketing gimmick with 
hardly any real world increase in performance. No one is giving you 2 
CPUs for the price of one ;-).

With a similar setup as yours, i use hyperthreading since my CPU usage 
is never more than 50% (mostly RAM and I/O) with BDC being the app using 
35% of it. I am taking my chances though and intend on turning it off 
someday.. if only i could get out of my basement..

- dhawal

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