Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!pt.cs.cmu.edu!andrew.cmu.edu!zs01+
From: zs01+@andrew.cmu.edu (Zalman Stern)
Newsgroups: comp.realtime
Subject: Re: Lightweight Tasks
Message-ID: <EYnoUGm00Vs8M9wZ0z@andrew.cmu.edu>
Date: 27 Jul 89 17:36:18 GMT
References: <2153@gmu90x.UUCP> <129300004@p.cs.uiuc.edu>,
	<1244@atanasoff.cs.iastate.edu>
Organization: Information Technology Center, Carnegie Mellon, Pittsburgh, PA
Lines: 31
In-Reply-To: <1244@atanasoff.cs.iastate.edu>

I have to disagree with Jim Gillies' description of lightweight tasks. I
have always heard the term used to describe an entity which has a stack and
a register set (including the program counter). A lightweight task does not
have its own address space or other context (i.e. signal masks, uid,
file descriptor table etc...) Light weight tasks are often called threads
(e.g. MACH, Concert MultiThread architecture, Modula 3), or LWPs
for Light Weight Processes. This definition is consistent with the usage
of the terms in MACH, various Xerox projects, work from DEC Systems
Research Center including modula-3, Concert MultiThread architecture which
originated at DEC Western Research Lab (I think), Synthesis (from
Columbia), and various products from companies like Apollo and Sun.

My understanding is that lightweight tasking predates UNIX' popularity by
quite a bit.  Especially if you consider coroutine schemes to be LWP
systems.  (The internals of coroutines and non-premptive LWPs are just
about identical.) In any event, it is not a valid comparison to RISC vs.
VAX in that lightweight processes solve different problems than UNIX
processes. Most systems that support LWPs also have heavy weight processes
or "address spaces" to allow one to partition subsystems so that they
cannot access each other's memory. (This last comment does not apply to
systems which don't have virtual memory.)

Finally, there are implementations of LWP done using UNIX processes and
shared memory to emulate multiple threads of control within a single
address space (i.e. Xerox PCR). With a decent UNIX implementation (some of
which do exist), this solution has acceptable performance.

Sincerely,
Zalman Stern
Internet: zs01+@andrew.cmu.edu     Usenet: I'm soooo confused...
Information Technology Center, Carnegie Mellon, Pittsburgh, PA 15213-3890