What is involved in switching from i386 to AMD64?

[Bill Moran]

Oliver Iberien <odilist at sonic.net> wrote:
>
> On Sunday 31 December 2006 11:05, Erik Trulsson wrote:
> > On Sun, Dec 31, 2006 at 10:40:37AM -0800, Oliver Iberien wrote:
> > > Hi,
> > >
> > > What actually needs to be done when switching machine architectures? I am
> > > facing a motherboard/CPU upgrade following the demise of an Athlon+
> > > setup. (I am running my FreeBSD drive in a P3 while trying to figure this
> > > out.)
> > >
> > > Replacing it with an oldish P4 would be easiest but they are rapidly
> > > becoming extinct. After much Googling I can't quite figure out what
> > > happens if I were to put the FreeBSD i386 HD into a 64-bit system.
> > > Nothing? Disaster? Do I totally have the wrong end of the stick? I'd be
> > > grateful for any advice.
> >
> > Since the AMD64 architecture is completely backwards-compatible with the
> > older i386 architecture, the i386 version of FreeBSD should work just fine
> > in the new system.
> 
> Thanks very much for this. I have the impression from scanning the 
> freebsd-amd64 archives that it is difficult to convert an existing system 
> from i386 to AMD64, and probably not worth it. Do people have any opinions on 
> this?

Depends on your need.  I've never upgraded from i386 to amd64, so I can't
say what the process is like.

If you're running a desktop system, I recommend against running amd64.  A
lot of desktop code burns down, falls over, then sinks into the swamp when
run on amd64.

If you're running a server, not much problem.  Most serious server programs
have been running on 64-bit systems since before amd64 existed.

One workaround is that you can run i386 programs on an amd64 kernel, if you
compile with a special kernel option.  I have no actual experience with this
to comment on how well it works.

Except for some very specific workloads, CPUs tend to perform roughly
equally when run in i386 mode vs. amd64 mode.

The only majorly compelling argument for amd64 at this time is that it
makes life much easier if you have more than 3.5G of RAM in your system.
In i386 mode, you either need to compile your kernel with PAE (which is
an ugly, poorly supported hack in my opinion) or give up the RAM over
3.5G.

-Bill