P2 CPU "class," but 386/486 ISA "core" are commonplace -- WAS: i586/686 glibc/kernel

[Bryan Smith]

This isn't about making a "lite" distro.
Fedora "Core" _should_ continue to ship with the _same_ performance,
memory and storage requirements.

I'm talking about not assuming that a 100MHz+ system has a Pentium ISA
(instruction set architecture) compatible CPU or higher.  Most people
seem to assume this, when there are a _lot_ of systems that don't.
A _lot_ of 100MHz+ CPUs.

We're talking a "system-on-a-chip" that eats up maybe 1-2Ws, or maybe
hundreds of milliwatts.  But they run at 133MHz or even higher, have
32-bit or even 64-bit memory bus interfaces, support even 128Mbit SDRAM
technology (upto 512MB of SDRAM) and have peripherials up the wazzu on
the chip itself (PCI masters, ATA100, PCCard/PCMCIA, USB, etc...).

But the ISA is often only a 486 or lower!  Even when the product is sold
as 5th-Gen (Pentium) or 6th-Gen (Pentium II) "class" chip!

Now AMD has licensed National Semi's Geode products.  The Geode are the
Cyrix M2 core, which are Pentium ISA.  These are quickly commonplace.

But AMD was is still selling its E86/SCLAN series too (and was its
_only_ product for awhile), which kicks some serious butt at 133MHz,
with lots of on-board peripherials.

Same deal with STMicro and others.  These are _huge_ in Europe, as well
as non-US markets, and a lot of imports into the US as well.  STMicro is
notorious for marketing them as "Pentium/Pentium-II" class because they
run at 100MHz, have 64-bit memory busses, PCI, ATA, USB, etc...
support.  But underneath, the _majority_ of STMicro products are i486
ISA.

These systems are all over the place.  In black boxes atop of your TV
that have 128MB+ of RAM, 10-20GB disks, etc...  Serious power.

That's why I'm all for making the distro i686 optimized, but i386 ISA
compatible.  With x86-64 being adopting this year, there is little
reason to start worrying about "performance."  x86-64 distros will be
optimized to the max, because x86-64 is a whole new architecture.

But for plain Jane x86, there is little sense to break i386 ISA
compatibility.  Yes, _assume_ the "performance/memory/disk" equivalent
of a Pentium Pro/II or higher.  But leave the instruction set
compatibility (ISA) at i386 -- please, for Fedora adoption sake.


-- 
Bryan J. Smith, E.I. -- b.j.smith(a)ieee.org

[Per Bjornsson]

Bryan J. Smith wrote:
<snipping discussion re: relatively fast old-ISA chips>
>  But the ISA is often only a 486 or lower!  Even when the product is
> sold
>  as 5th-Gen (Pentium) or 6th-Gen (Pentium II) "class" chip!
>  
>  Now AMD has licensed National Semi's Geode products.  The Geode are the
>  Cyrix M2 core, which are Pentium ISA.  These are quickly commonplace. 
Pentium = 586 obviously.

>  But AMD was is still selling its E86/SCLAN series too (and was its
>  _only_ product for awhile), which kicks some serious butt at 133MHz,
>  with lots of on-board peripherials. 
Also a 586:
"The ÉlanSC520 microcontroller combines a 32-bit, low-voltage Am5x86 CPU 
with a complete set of integrated peripherals..."

(quoted from 
http://www.amd.com/us-en/ConnectivitySolutions/ProductInformation/0,,50_2...
)

>  Same deal with STMicro and others.  These are _huge_ in Europe, as
> well
>  as non-US markets, and a lot of imports into the US as well.  STMicro is
>  notorious for marketing them as "Pentium/Pentium-II" class because they
>  run at 100MHz, have 64-bit memory busses, PCI, ATA, USB, etc...
>  support.  But underneath, the _majority_ of STMicro products are i486
>  ISA. 
Yup, looks like there are a lot of 486s there, looking at this page:
http://www.st.com/stonline/products/selector/107.htm

>  These systems are all over the place.  In black boxes atop of your
> TV
>  that have 128MB+ of RAM, 10-20GB disks, etc...  Serious power.
>  
>  That's why I'm all for making the distro i686 optimized, but i386 ISA
>  compatible.  
But... All of the examples you quoted were 486+. Now consider what Jakub 
   Jelinek wrote a little while back:
http://www.redhat.com/archives/fedora-devel-list/2004-May/msg00617.html

"I'd like to make NPTL the default threading library for FC3,
as a step in phasing out LinuxThreads."
...

"3) NPTL has not been ported to i386, only i486+, x86_64,
    ia64, ppc32, ppc64, s390, s390x, alpha, sh and sparcv9.
    i386 (and sparc < v9) lacks atomic instructions powerful
    enough for NPTL needs.  Well, I have tried to port NPTL
    to i386, http://sources.redhat.com/ml/libc-hacker/2004-05/msg00019.html,
    but that port is severely limited and maintaining two
    different IA32 ports would be too much work."

So clearly a very important dividing line seems to lie between 386 and 
486 support. I'm all for going wint NPTL as default for Fedora; this is 
supposed to be a forward-thinking distribution.

>  But for plain Jane x86, there is little sense to break i386 ISA
>  compatibility.  Yes, _assume_ the "performance/memory/disk" equivalent
>  of a Pentium Pro/II or higher.  But leave the instruction set
>  compatibility (ISA) at i386 -- please, for Fedora adoption sake. 
Well, as far as I can see it sure seems like letting all programs use 
NPTL-specific features by default can be a somewhat important perfomance 
win? That's a real difference that ignoring the 386 would give us, and I 
have yet to see any examples even of modern embedded processors that 
don't support the 486 ISA (admittedly, I might not have looked hard 
enough but the examples you came up with were all 486 and you seem to 
have much more insight in the embedded world than I do). I think that 
this makes 486 a realistic base target, and given that there isn't much 
of an NPTL port for 386

I summary, my main point is that 386 is not the same as 486. Jakub 
suggests in the e-mail I linked to earlier that in general there is 
little to be gained from going to --march=i486; however, given that 
there won't be an NPTL library available for 386 I think it feels more 
honest to just go with --march=i486 anyways. Not that I feel all that 
strongly about it though.

Basically I think that I'm more or less agreeing with you: Fedora 
shouldn't dump old ISAs unless there is a real, tangible gain; moving 
beyond 486 doesn't really seems to give us that. Where we differ is that 
I think that ignoring the 386 by explicitly using NPTL is a good idea 
and very much in line with the Fedora goals.

/Per

-- 
Per Bjornsson <perbj at stanford dot edu>
Ph.D. Candidate, Department of Applied Physics, Stanford University

[Alan Cox]

On Tue, Jun 01, 2004 at 09:37:43PM -0400, Bryan J. Smith wrote:
>  Now AMD has licensed National Semi's Geode products.  The Geode
> are the
>  Cyrix M2 core, which are Pentium ISA.  These are quickly commonplace. 
The Geode isnt an M2 core, and its not licensed. AMD bought Geode outright
from NS. If you look at the timings the Geode is much more a 486 with extra
instructions.

>  But AMD was is still selling its E86/SCLAN series too (and was its
>  _only_ product for awhile), which kicks some serious butt at 133MHz,
>  with lots of on-board peripherials. 
Thats 486 with buggy hw emulation.

>  support.  But underneath, the _majority_ of STMicro products are
> i486
>  ISA. 
486..

>  But for plain Jane x86, there is little sense to break i386 ISA
>  compatibility.  Yes, _assume_ the "performance/memory/disk" equivalent
>  of a Pentium Pro/II or higher.  But leave the instruction set
>  compatibility (ISA) at i386 -- please, for Fedora adoption sake. 
Every example you give is 486 or higher ISA, as are the others you
missed that I am aware of - SiS55x for example. Going 386->486 makes a 
really big difference.