Re: SMP detection

backyard píše v čt 31. 08. 2006 v 07:45 -0700:
--- Michal Mertl <mime@xxxxxxxxxxxx> wrote:

Skylar Thompson wrote:
Jordi Carrillo wrote:
2006/8/30, backyard
<backyard1454-bsd@xxxxxxxxx>:



--- Jordi Carrillo <jordilin@xxxxxxxxx> 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

_______________________________________________
freebsd-questions@xxxxxxxxxxx mailing list



http://lists.freebsd.org/mailman/listinfo/freebsd-questions
To unsubscribe, send any mail to
"freebsd-questions-unsubscribe@xxxxxxxxxxx"


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.

I believe FreeBSD uses the other logical CPU to
handle hardware
interrupts, which can still help perormance. You
can check dmesg to see
how it's actually handling it.

No! Kernel threads (e.g. handling interrupts) aren't
that much different
to normal processes.

Logical CPUs on a single HTT capable CPU share most
of the CPU logic,
especially all the external stuff (handling
interrupts). Scheduling
handling of interrupts on the "secondary/logical"
core wouldn't
probably help performance at all (if that is at all
possible).

When FreeBSD sees logical CPUs it means HTT is
either enabled in BIOS or
that disabling HTT in BIOS does not hide the CPUs to
FreeBSD (bug in
BIOS/FreeBSD).

Until you enable scheduler to schedule tasks to HTT
cores (with
machdep.hyperthreading_allowed=1 in loader.conf)
(disabled by default
due to mentioned security/performance reasons)
machine won't utilize the
logical HTT CPUs. Therefore total CPU utilization
won't be more than
50%, because there are the (unused) logical CPUs
which don't get
scheduled tasks.


are you sure about this???

Almost sure but can't check at the moment. I believe there were problems
when all HTT CPUs weren't launched sometimes (when HTT wasn't disabled
with BIOS), so the logical CPU cores are started and fully visible but
only run the idle kernel thread (are 100% idle).

I would have figured the scheduler wouldn't see the
other core at all without this option set and so it
wouldn't be used in calculating load at all. 50% on a
compile is fairly normal from my experience. I don't
have too much experience with HT as I always opt for
true SMP so I can't speak with authority on the
matter.

but if

top

isn't showing CPU 1 or 0 next to a process then it
isn't computing the load on multiple cores... Also if

dmesg |grep cpu

doesn't show application cpu1 (and on through all your
cores)... launched then the system isn't looking at
the HT core at all.


-brian


_______________________________________________
freebsd-questions@xxxxxxxxxxx mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-questions
To unsubscribe, send any mail to "freebsd-questions-unsubscribe@xxxxxxxxxxx"

Relevant Pages

  • Re: SMP detection
    ... I have the SMP-GENERIC kernel ... I've decided to disable SMP from BIOS. ... way to install one already precompiled? ... cpu and the system monitor only features one ...
    (freebsd-questions)
  • different oops on xeon with smp kernel
    ... then SMP kernels oops very early at boot at different places. ... CPU: Trace cache: 12K uops, ... Unable to handle kernel paging request at virtual address 8528244c ...
    (Linux-Kernel)
  • Re: 2 x quad-core system is slower that 2 x dual core on FreeBSD
    ... Can you try a generic SMP kernel (called "SMP" in 6.x; ... relationship between how many CPUs are running and what the sys load is. ... Now I'm running yesterday's FreeBSD 7.0-BETA3 amd64 with GENERIC kernel. ... I.e. it seems to work good as a simple server with not so powerfull CPU. ...
    (freebsd-stable)
  • Re: SMP detection
    ... I have the SMP-GENERIC kernel and it ... only launches one cpu. ... I've decided to disable SMP from BIOS. ... way to install one already precompiled? ...
    (freebsd-questions)
  • Re: SMP detection
    ... Smp enabled kernel? ... cpu and the system monitor only features one cpu ... now that multi-cored chips exist I do not know. ...
    (freebsd-questions)