Re: [9fans] proccreate() and threadcreate()?

[Sape Mullender <sape@plan9.bell-labs.com>]

> It is a nive question. Will somebody tell me the differences
> between proccreate() and threadcreate() functions? Programming
> point of view both seem to behave exactly the same!
>
> -ishwar

The Plan 9 model for parallel programming distinguishes between procs and
threads.  A proc is scheduled by the operating system.  If you have two procs,
both procs (appear to) run simultaneously.  On a multiprocessor, they really
run simultaneously.  Threads are essentially coroutines.  Each proc can have
one or more threads in it.  The threads within a proc take turns getting the
processor.  Two threads in one proc can never run simultaneously.  When a
thread gives up the processor, another thread in the same proc can run.
Threads give up the processor by communicating over channels, or by
calling yield().  They do NOT give up the processor by making read or write
system calls.  A thread that blocks on a read or write, also prevents other threads
in the same proc from running.

Typical Plan 9 programs have a single proc containing many worker threads picking
up work from shared data structures or from messages sent over channels.
These threads do not need to lock any of the shared data strcutures they use
because they are only shared by other threads in the same proc -- and those
threads will never run concurrently with the thread reading or modifying the
data structure.

I/O is done by threads in other procs, one thread per I/O proc.  The threads in
I/O procs can block on reads and writes without blocking other threads -- there's
only one in the proc.

Worker threads communicate with I/O threads using channels, which are
protected by the thread library from concurrent access problems.

Threadcreate() creates a new thread in the current proc.  Proccreate() creates
a new proc and a single new thread in that proc.

	Sape