Re: SMP detection



--- Jordi Carrillo <jordilin@xxxxxxxxx> wrote:

2006/8/31, backyard <backyard1454-bsd@xxxxxxxxx>:

--- 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

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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.

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???
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


When you do a "top" in the column marked as "C"
should put a 1 or 0 in each
process depending on the cpu the process is being
executed. Well, top does
so, only if you write the line
machdep.hyperthreading_allowed=1 in your
loader.conf. So, anyone having a pentium with
hyperthreading, when you
install freebsd it will install the SMP-GENERIC
kernel (my case) and then
you must enable it by writing
machdep.hyperthreading_allowed=1. If you don't
then the other cpu is not taken into account (at
least when you do a top
only appear 0s in every process, and no 1s). Is my
explanation correct?


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sounds like it to me. except by what you mean by
"taken in to account" top is only reading internal
scheduler information so it will display 1 and 0 or 3
4 5 6 however many cores/cpus you have that make
things up. I just don't want to mislead or
misunderstand the

machdep.hyperthreading_allowed=1

variable tells an SMP Kernel to allow the scheduler to
send task to the second core. Thus allowing SMP
execution of processes and interrupts. I know I am
likely abriviating what acutally internally occurs.

this does not affect top other then top will read the
schedule tasks and if processes are on the second core
it will report C=1.

But your a lucky dog it always seems I have to rebuild
the installed kernel to enable SMP with my systems.
Very frustrating, but forces me to immediately update
the tree. The point is the above variable is in the
BSD world and must be set if you want HT to work.
Linux does things their own way, which is why I'm
nuking my Gentoo partition this weekend; way too much
chaos for me.

-brian
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