Andy Tanenbaum's Journal

LINUX is obsolete
I was in the U.S. for a couple of weeks, so I haven't commented much on
LINUX (not that I would have said much had I been around), but for what 
it is worth, I have a couple of comments now.

As most of you know, for me MINIX is a hobby, something that I do in the
evening when I get bored writing books and there are no major wars,
revolutions, or senate hearings being televised live on CNN.  My real
job is a professor and researcher in the area of operating systems.

As a result of my occupation, I think I know a bit about where operating
are going in the next decade or so.  Two aspects stand out:

1. MICROKERNEL VS MONOLITHIC SYSTEM
   Most older operating systems are monolithic, that is, the whole operating
   system is a single a.out file that runs in 'kernel mode.'  This binary
   contains the process management, memory management, file system and the
   rest. Examples of such systems are UNIX, MS-DOS, VMS, MVS, OS/360, 
   MULTICS, and many more.

   The alternative is a microkernel-based system, in which most of the OS
   runs as separate processes, mostly outside the kernel.  They communicate
   by message passing.  The kernel's job is to handle the message passing,
   interrupt handling, low-level process management, and possibly the I/O.
   Examples of this design are the RC4000, Amoeba, Chorus, Mach, and the
   not-yet-released Windows/NT.

   While I could go into a long story here about the relative merits of the
   two designs, suffice it to say that among the people who actually design
   operating systems, the debate is essentially over.  Microkernels have won.
   The only real argument for monolithic systems was performance, and there
   is now enough evidence showing that microkernel systems can be just as
   fast as monolithic systems (e.g., Rick Rashid has published papers comparing
   Mach 3.0 to monolithic systems) that it is now all over but the shoutin`.

   MINIX is a microkernel-based system.  The file system and memory management
   are separate processes, running outside the kernel.  The I/O drivers are
   also separate processes (in the kernel, but only because the brain-dead
   nature of the Intel CPUs makes that difficult to do otherwise).  LINUX is
   a monolithic style system.  This is a giant step back into the 1970s.
   That is like taking an existing, working C program and rewriting it in
   BASIC.  To me, writing a monolithic system in 1991 is a truly poor idea.


2. PORTABILITY
   Once upon a time there was the 4004 CPU.  When it grew up it became an
   8008.  Then it underwent plastic surgery and became the 8080.  It begat
   the 8086, which begat the 8088, which begat the 80286, which begat the
   80386, which begat the 80486, and so on unto the N-th generation.  In
   the meantime, RISC chips happened, and some of them are running at over
   100 MIPS.  Speeds of 200 MIPS and more are likely in the coming years.
   These things are not going to suddenly vanish.  What is going to happen
   is that they will gradually take over from the 80x86 line.  They will
   run old MS-DOS programs by interpreting the 80386 in software.  (I even
   wrote my own IBM PC simulator in C, which you can get by FTP from
   ftp.cs.vu.nl =  192.31.231.42 in dir minix/simulator.)  I think it is a
   gross error to design an OS for any specific architecture, since that is
   not going to be around all that long.

   MINIX was designed to be reasonably portable, and has been ported from the
   Intel line to the 680x0 (Atari, Amiga, Macintosh), SPARC, and NS32016.
   LINUX is tied fairly closely to the 80x86.  Not the way to go.

Don`t get me wrong, I am not unhappy with LINUX.  It will get all the people
who want to turn MINIX in BSD UNIX off my back.  But in all honesty, I would
suggest that people who want a **MODERN** "free" OS look around for a 
microkernel-based, portable OS, like maybe GNU or something like that.


Andy Tanenbaum (ast@cs.vu.nl)


P.S. Just as a random aside, Amoeba has a UNIX emulator (running in user
space), but it is far from complete.  If there are any people who would
like to work on that, please let me know.  To run Amoeba you need a few 386s,
one of which needs 16M, and all of which need the WD Ethernet card.