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From: mudd-j@bear.cis.ohio-state.edu (John R. Mudd)
Newsgroups: comp.os.research
Subject: Re: THREADS, Light weight processes
Message-ID: <5287@saturn.ucsc.edu>
Date: 27 Oct 88 22:40:18 GMT
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In article <5279@saturn.ucsc.edu> rutgers!mist.math.uoregon.edu!renga@beaver.cs.washington.edu (Renganathan Sundararajan) writes:

>    What exactly is a light weight process? 
>    Is it the same as a thread?

A lightweight process, sometimes called task (Encore's UMAX) or thread (Mach),
is a smaller thread of execution that doesn't have all the capabilities of a 
UNIX process, and as such it is cheaper to create and has less overhead 
associated to it.  As UNIX processes are scheduled on a machine's processor(s),
so are tasks scheduled on a program's available UNIX processes.  A parallel 
program creates a number of UNIX processes on which the program's tasks can 
be created.

>    Where did the notion of a LWP originate and in what context? 

To do efficient (in terms of the OS) parallel processing, it isn't a wise
idea to create a lot of UNIX processes.  UNIX processes tend to have a lot
of 'stuff' in them that isn't necessary for parallel processing.

>    Is the sole purpose of a LWP to help switch contexts fast? 

Not the sole purpose, just one of the convenient purposes.  If you intend
to create a lot of tasks, you want to be able to schedule them on a process
as fast as possible.

>    If a LWP does not own/use any registers and so saving a context
>     involves only saving the PC and condition flags, what is great
>     about it? 

Speed.  See above.  

>    Do LWPs and HWPs (H = Heavy - whatever that may mean) co-exist in an OS? 

LWPs exist in a HWP, which are then scheduled by the OS.  See above.

>  Is the notion of a thread unique to MACH or do other os like V use it?

Nope.  I don't know about V, but Encore's UMAX uses the concept of 'tasks.'
>From what I've read on Sequent's DYNIX, they seem to simply run straight UNIX
processes.  I wouldn't want to say for sure, and I don't know about any other
systems.  Anybody else care to add to this?

>    Are there any machines out there that provide some kind of hardware
>    support to LWP switching?

Unknown.

>    Any clarifications/references would be appreciated.
>
>  Renganathan

I made a stab at answering your questions; if anybody has anything 
additional on this topic, I'd like to see it too.  (That means POST, not
email)
                                              John
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 John R. Mudd                     Department of Computer & Information Science
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