80386 support in -current

[Peter Jeremy]

On Mon, Jan 26, 2004 at 07:37:48AM +1000, Andy Farkas wrote:
>Peter Jeremy wrote:
>
>> Interesting.  Does anyone on this list actually use -CURRENT on a 386?
>...
>>
>> Is it time to bite the bullet and fully desupport the 80386?  It looks
>> like threads don't work and it's likely that other bitrot will set in
>> in the absence of active testing.
>
>Yes. Bring on the axes!
>
>This came up almost a year ago (late Feb 2003). Here are some exerps from
>a few emails I kept on the subject:

I remember one thread on this subject but don't recall if it was that
one.  As I recall, the agreed outcome went something like drop 80386
support from GENERIC but retain it primarily for embedded applications.

>%%%
>Poul-Henning Kamp <phk at FreeBSD.ORG> wrote:
>
>My main concern would be if the chips have the necessary "umphf"
>to actually do a real-world job once they're done running all the
>overhead of 5.0-R.  The lack of cmpxchg8 makes the locking horribly
>expensive.
>%%%

Note that cmpxchg8 doesn't exist in 486 either and is only necessary
to support 8-byte atomic operations - I may be wrong but it should be
possible to handle i386 locking with only 4-byte operations.  Of
course, the 80386 doesn't have cmpxchg either, which _does_ make
locking horrible (requires sti/cli which doesn't work in the multi-
master case).

>This last point is the clincher. The chip does NOT have enough "umphf". I
>actually managed to boot a -current (from back then) on a 80386SX and it
>was torturously slow. An ls(1) on my empty home directory took 15 seconds.
>My VAX is faster.

This is a bug in FreeBSD 5.x - the performance in general has degraded
since 4.x.  Performance degradation is often more obvious in lower end
machines.

Peter