Re: [9fans] Re: Threads: Sewing badges of honor onto a Kernel

[Linus Torvalds <torvalds@osdl.org>]

On Fri, 27 Feb 2004, Dave Lukes wrote:
>
> I'm not the one who wants to put 200Mb on the stack (see below):
> who's crazy, again?

Neither am I.

I'm saying that you CANNOT "message pass" a hash table. You pass its
address, and that has NOTHING to do with message passing. You use a
function call that passes the address to a data structure around.

> > Because message passing is idiotic, when the real hardware just passes
> > pointers around?
>
> Ohh, yea, and pointers scale really well to NUMAs, right?

Hey, when did you last write an operating system that worked on NUMA
machines?

Trust me. Passing pointers around scales a _hell_ of a lot better than
copying data around as messages.

> > Hint: you can't message-pass a hash table that describes 200 megabytes
> > worth of filesystem names.
>
> No shit, Sherlock!  Thanks for the enlightenment!
> Next question: are you _seriously_ suggesting having 200Mb on a stack?

No.

I'm suggesting passing the pointer around, and not messing with messages
at all. You're the one who complained about me using pointers:

  "your examples all use shared memory as a cheap substitute
   for message passing: why?"

and I'm telling you that pointers are NOT a "cheap substitute for message
passing". Pointers are fundamentally MORE POWERFUL than message passing
is, and anybody who calls them a "cheap substitute" is a moron.

That was my point: a pointer can point to hundreds of megabytes of complex
data structures with lots of interdependencies and interesting locking
rules. THAT is the real world. A message it is NOT.

A message is a way to pass data by value. It has its place too, of course,
especially in networks, but comparing it to a pointer is just misguided.

Th eonly people who confuse pointers and messages are the microkernel
people who noticed that real messages are too expensive to use, so they
started calling pointers "messages", and play other semantic games.

			Linus