QNX!!!

It's a really cool microkernel.  See www.qnx.com

What is a microkernel?

It's a very small kernel that can do almost nothing.  The main things a microkernel can do are
 

thread services -- Neutrino provides the POSIX thread-creation primitives.

signal services -- Neutrino provides the POSIX signal primitives.

message-passing services -- Neutrino handles the routing of all messages between all threads throughout the entire system. 

synchronization services -- Neutrino provides the POSIX thread synchronization primitives.

scheduling services -- Neutrino schedules threads for execution using the various POSIX realtime scheduling algorithms.

timer services -- Neutrino provides the rich set of POSIX timer services.

process management services -- the Neutrino microkernel and the process manager together form a unit (called procnto). The process manager portion is responsible for managing processes, memory, and the pathname space.

• Filesystem stuff
• Device driver stuff
• Networking stuff
• Create any new processes
• Paging and VM stuff

One sends a message to the server that can do these things.

Send-Recv Paradym

One easy thing is to make a message passing class look like a system call.  You say

fd = open( ....)

send( ...)
recv(fd);

• Beaking a service need not crash the machine.  But if everthing works except the file system, what do you really have?
• It's easy to write new kernel code, since it's not really in the kernel.  You can use the debuggers.  You never need to reboot!
• Can try new things.  Try a new file system just by running a new file system server
• Can leave things out.  If your CD-player needs a ISO9660 fs but no TCP stack, then run one and not the other.
• Very easy to SMP.  Trivial.  Can even distributivly process easy.  Very cool!

• It's slow!  But QNX advertises a very fast context switching time.  But it cannot be as fast as a macrokernel, unless it uses better algorithms.
• It doesn't really work.  When a critical subsystem dies, you dead.
• Mach gave it a bad rap.  (Hint:  MacOS X is based on Mach)

Mach has a "look up the service and get a port number then write to the port number" thingie.
QNX has a "open up a file, but the file is really a process and might even be a directory of processes" thingie.

What's This FileTable Things

It's a table of mount points.  When you open a file, it finds the appropriate mount point, and redirects your request to that process.  That process can allow, deny, or whatever.  It can even export a directory instead of a file.

What to do with ls /net/http/*/index.html

What's the Right Process?

Suppose you get a request for /foo/bar, and you table says
/foo                 process 12
/foo/bar           process 100

Do you want to search the entire table on every open?  Do you want to return the first match, even if' it's dumb?  (Tell Xenu story).