Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!sharkey!indetech!rec
From: rec@indetech.UUCP (Rick Cobb)
Newsgroups: comp.os.minix
Subject: Re: fork
Summary: Encore has a neat idea (sfork())
Message-ID: <2057@ruby.UUCP>
Date: 9 Mar 89 01:04:47 GMT
References: <1989Mar3.174924.977@utzoo.uucp> <85830@felix.UUCP>
Reply-To: rec@ruby.UUCP (Rick Cobb)
Organization: Independence Technologies, Inc. Fremont, CA
Lines: 37


Encore has a research system in which fork is broken down to
a very fine level of granularity.  See ``Variable Weight Processes
with Flexible Shared Resources'' in the the
_1989 Winter USENIX Conference Proceedings_, available from the
USENIX Association, POBox 2299, Berkeley.

Their approach has two elements to it: first, you make all the process
specific stuff in the kernel (the user and proc structs, call them together
the process structure) be accessed through a pointer.  I.e., instead of

	process -> file_descriptor_array

you do

	process -> file_descript_p -> file_descriptor_array

You also stuff reference counts in the stuff that's pointed to.  
Now, you have a kernel
where a parent or child can choose *exactly* which parts of their environments
to share, and which ones not to share.

Then, what sfork() does is to just (1) create a new process pointer (providing
a new runtime context only), and (2) copy the references from the process
structure of the parent to the child.

If you want a *real* fork, you then (could) use the ``resctl'' call to
ask that you be given a copy of the parent's resource, instead of the resource
itself, or that you be given a freshly initialized structure to point to.

They do give a system call interface called fork() which does all the copying.

This facility is intended primarily for enhancing sharability of resources;
for example, if you do no copying (just an sfork()), you get something about
as light as a MACH thread.  I haven't thought about what changes or additional
features you might give to exec in this system; it seems you might be able
to get a faster fork/exec pair from it, though.