Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!samsung!munnari.oz.au!sirius.ucs.adelaide.edu.au!augean!sibyl!ian
From: ian@sibyl.eleceng.ua.OZ (Ian Dall)
Newsgroups: comp.arch
Subject: Re: vfork (was Re: Paging page tables)
Message-ID: <774@sibyl.eleceng.ua.OZ>
Date: 14 Jul 90 06:15:41 GMT
References: <920@dgis.dtic.dla.mil> <5830@titcce.cc.titech.ac.jp> <5DL4SPD@xds13.ferranti.com> <5855@titcce.cc.titech.ac.jp>
Reply-To: ian@sibyl.OZ (Ian Dall)
Organization: Engineering, Uni of Adelaide, Australia
Lines: 39

In article <5855@titcce.cc.titech.ac.jp> mohta@necom830.cc.titech.ac.jp (Masataka Ohta) writes:
>>> 1) An utterly broken implementation where some important system
>>> process (such as inetd, ypbind or sendmail) may killed if there
>>> is not enough swap space.
>
>>Alternatively, put the program in a wait state until swap space is available.
>>Deadlocks are possible, but unlikely. Indefinite deferment is more likely,

>Once swap space shortage occurs, it will tend to occur continually
>until some large process exits. So, if all such processes put in
>wait states (which is very likely to occur, because active process
>often requires new pages) the situation is deadlock.

Masataka seems to live in a binary world were there is only the BSD
and the SysV implimentations. These are no the only possibilities! I
like the SysV method of not allocating swap space until it is
necessary. It allows the total virtual memory used to (almost) equal
the sum of the swap space and the physical memory which seems to me
desirable. It is true that it is undesirable for the system to into
deadlock due to lack of swap space.  However, there are ways around
it. A simple solution is to have a high water mark for free swap space
and when that high water mark is exceeded cause any process, except
those with certain effective uids, to block if it does anything which
increases its memory requirements (including COW). To the user the
system will appear to have deadlocked, but not to the super user who
can at least run ps and kill. Of course if you are in that situation
often you need more swap space or more physical memory. (With the BSD
scheme you need both). Of course some systems (such as VMS) have per
user quotas on swap space. Whether that is good depends on your
environment I suppose. (I personally disliked VMS quota for everything,
but that might just have been because it was never big enough!)

Others might think up more elegant solutions. The point is you
shouldn't throw out the baby with the bath water. Copy on write
is an elegant idea. Don't stomp on it just to keep vfork!

-- 
Ian Dall     life (n). A sexually transmitted disease which afflicts
                       some people more severely than others.