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From: mohta@necom830.cc.titech.ac.jp (Masataka Ohta)
Newsgroups: comp.arch
Subject: Re: vfork() and swap space allocation
Message-ID: <5892@titcce.cc.titech.ac.jp>
Date: 19 Jul 90 07:22:35 GMT
References: <5813@titcce.cc.titech.ac.jp> <2699E08D.117A@tct.uucp> <5844@titcce.cc.titech.ac.jp> <269DBEFB.583C@tct.uucp>
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Organization: Tokyo Institute of Technology
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In article <269DBEFB.583C@tct.uucp>
	chip@tct.uucp (Chip Salzenberg) writes:

>My favorite Unix is SCO Xenix/386 2.3.  I haven't experimented with
>its behavior under memory and swap space exhaustion.

Then, what can you say about the subject?

>I'm sure,
>however, that a SCO person will know how it works and can followup
>with that information.

So far, no.

>The Unix philosophy of resource usage is: "Pay as you go." Files are
>not pre-allocated, but dynamically extended.  The data segment is not
>pre-allocated, but grows and shrinks on demand.  Given this
>philosophy, the only consistent treatment of swap space is to allocate
>it when it is needed, and not before.

The more important philosophy of resource usage of UNIX is: "Don't
pay at all if not necessary." For example, no blocks will be allocated
in holes of a file (due to seek). Vfork is another example.

>>With vfork, such a situation never occur (except for stack segment),
>>becasue fork is denied.

>Note the "except" clause.  Mr. Masataka himself here points out that
>vfork() is an incomplete solution, since a kernel with vfork() must be
>prepared to deal gracefully with swap space exhaustion due to stack
>modification.

Vfork is a complete and elegant solution for the forking problem.

Problem of stack segment is the other problem which should be solved
separately. Moreover, a solution exists. I will explain it if you are
interested in.

>So vfork() does not eliminate the swap space contention
>inherent in new process creation.

Vfork totally eliminate the swap space contention in new process
creation to exec.

						Masataka Ohta