amd64 webpage, 12GB memory? 64-bit or not? SMP or not?

Oliver Fromme olli at
Tue Mar 28 09:15:15 UTC 2006

Jeremy C. Reed <reed at> wrote:
 > In my case, the system was installed by a knowledgable network 
 > administrator who happened to install the common (and frequently 
 > available[1]) "i386" disk on a 64-bit system. He was new to FreeBSD, and 
 > since his system was "Intel" not "AMD" and since the CD booted and 
 > installed fine, it made sense to him. If possible, some output in dmesg 
 > that said something like "AMD64 should be used for this system" and "This 
 > system appears to have multiprocessors so consider using options SMP" 
 > could be useful for even a novice.

I'm afraid I have to disagree.  Such "hints" could be very
misleading, _especially_ for a novice.

Wether amd64 should be used on a particular system is a
decision which should be made by the admin (or whoever
installs the system), depending on various things.

It is _not_ as simple as "this is amd64 hardware, so you
should run FreeBSD/amd64".  That would be plain wrong.
Running amd64 has advantages as well as disadvantages.
There are a lot of things to take into account.  There
may be good reasons to run FreeBSD/i386 on amd64 boxes.

Some people even might find a sentence like the above one
frustrating or even offending, because they would _like_
to run amd64, but they can't because they need a certain
piece of software that doesn't run on amd64.

I think it would be okay if the i386 kernel printed a line
that says something like "this processor supports running
FreeBSD/amd64 (64bit mode), please refer to <URL> for
cons and pros."

It's similar with the SMP issue.  First of all, it's
not trivial to detect whether enabling SMP would be an
advantage or not.

Detecting SMP support of the hardware is one thing.
Detecting if actually two or more processors are present
is another thing (or two cores, or hyperthreading support,
which are all different things with different advantages
and disadvantages).  If only one processor is present,
enabling SMP can make performance worse because of the
additional locking overhead.

Also take into account that some high-end hardware such
as Sun servers support enabling and disabling processors
while the OS is running (even if FreeBSD doesn't support
that yet, but it shouldn't be ruled out for the future),
and even hot-swapping of CPU boards.  So while there might
be just one processor present or enabled when the system
boots, more of them could be enabled later on.  Another
thing to keep in mind is that some drivers or kernel
options might not be compatible with SMP.  For example,
the polling option used to be incompatible with SMP (I
think that has been fixed).  For a particular situation
it could have made sense not to enable SMP because polling
might be more beneficial.

Just my two cents.

Best regards

Oliver Fromme,  secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing
Dienstleistungen mit Schwerpunkt FreeBSD:
Any opinions expressed in this message may be personal to the author
and may not necessarily reflect the opinions of secnetix in any way.

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