SMP detection

[Jordi Carrillo]

2006/8/30, backyard <backyard1454-bsd at yahoo.com>:
>
>
>
> --- Jordi Carrillo <jordilin at gmail.com> wrote:
>
> > I've read that SMP should be disabled for
> > performance issues (I did not know
> > that before installing freebsd). I have a P4 3GHz
> > with hyperthreading
> > technology. I have the SMP-GENERIC kernel and it
> > only launches one cpu. So,
> > I've decided to disable SMP from BIOS. Is that ok?,
> > knowing that I have a
> > Smp enabled kernel? or should I install one without
> > smp? If so, is there a
> > way to install one already precompiled?
> > Thanks in advance
> >
> > --
> > http://jordilin.wordpress.com
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>
> if the system runs with one cpu now and you don't
> enable smp with HT with the sysctl variable then you
> should be ok. If your not doing SMP then recompiling
> the kernel for single processor mode will make things
> run a little quicker because the SMP code won't come
> into play.
>
> with HT disabling in FreeBSD is more for the security
> issues about a potential exploit whereby one process
> in one pipe can access the priveledged information of
> a process in another pipe because the two cores share
> one processor cache and thus one cache table. To my
> knowledge this hasn't been exploited yet.
>
> If you just install the generic kernel you it should
> be only the uniprocessor one. I would just do a:
>
> cd /usr/src && make buildworld && make
> KERNCONF=GENERIC buildkernel && make KERNCONF=GENERIC
> installkernel
>
> as opposed to a binary version assuming you haven't
> updated yet you won't have to install world but I
> believe it must have the build in the source tree to
> build a kernel. On your P4 though the difference
> between SMP and uniproc may not be worth the trouble
> because I don't think much of a gain would be made. on
> a P1 a much different story...
>
> if you aren't concerned with bad users or hackers
> hitting the box I would just enable HT with the sysctl
> variable. This will not make things run slower at all,
> just (in theory) less secure, which is why the
> veriable was created in the first place as I recall.
> If you are concerned I would wait until you update
> your system and then just build a GENERIC/CUSTOM
> kernel without the SMP option set.
>
>
> -brian
>


I will disable smp from bios. If I have a smp kernel, I suppose there will
be no problem after all. Would that be ok?
The problem with having SMP enabled is that the smp kernel only detects one
cpu and the system monitor only features one cpu as well as gkrellm (in
Linux it shows two cpus). When compiling the system monitor shows the cpu at
a maximum of 50%, so what's going on with the other 50%?
writing machdep.hlt_logical_cpus to 2 in loader.conf does not solve
anything.
-- 
http://jordilin.wordpress.com