machdep.hlt_logical_cpus being ignored

Mikhail Goriachev mikhailg at webanoide.org
Mon May 22 18:48:17 PDT 2006


Jon Falconer wrote:
> Greetings All,
> 
> I have installed FreeBSD 6.1 Release on a Dell 2850. I is has two Xeon
> CPUs. Seeing the comment in the SMP man page about hyperthreading being a
> problem in some situations, I added "machdep.hlt_logical_cpus=1" in
> /boot/loader.conf file. Upon rebooting, the system still finds four CPUs
> and starts all of them. I would have expected it to start only the two
> physical CPUs. Am I missunderstanding what machdep.hlt_logical_cpus is
> supposed to do? Should I even be concerned about the hyperthreading
> problem. This system will be an email server for a small college campus
> with about 2000 active accounts.
> 
> Also, the man page for SMP say that the machdep.hlt_cpus can be used to
> halt a specific CPU. I have not set this value, but sysctl shows it has a
> value of 10, even before I set the machdep.hlt_logical_cpus to 1. Why
> would this be the default value? I would think this would mean that one of
> the system CPUs is halted. But dmesg output claims it launched four CPUs.
> 
> Can someone fill me in on what this all means?


Hi,

This issue has been addressed in:

http://security.freebsd.org/advisories/FreeBSD-SA-05:09.htt.asc

You could also read this:

http://www.daemonology.net/hyperthreading-considered-harmful/

I hope this helps.

Cheers,
Mikhail.


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Mikhail Goriachev
Webanoide

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