Re: SMP detection
- From: Skylar Thompson <skylar@xxxxxxxxxxxxxx>
- Date: Wed, 30 Aug 2006 14:30:22 -0700
Jordi Carrillo wrote:
2006/8/30, backyard <backyard1454-bsd@xxxxxxxxx>:I believe FreeBSD uses the other logical CPU to handle hardware
--- Jordi Carrillo <jordilin@xxxxxxxxx> wrote:
I've read that SMP should be disabled forhttp://lists.freebsd.org/mailman/listinfo/freebsd-questions
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.
interrupts, which can still help performance. You can check dmesg to see
how it's actually handling it.
--
-- Skylar Thompson (skylar@xxxxxxxxxxxxxx)
-- http://www.cs.earlham.edu/~skylar/
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