Path: utzoo!attcan!uunet!munnari!otc!metro!basser!usage!csdgwy!ewa
From: ewa@csdgwy.csd.unsw.oz
Newsgroups: comp.os.vms
Subject: Re: MPW_HILIMIT --- and page caching
Message-ID: <548@csdgwy.csd.unsw.oz>
Date: 15 Jul 88 11:57:29 GMT
References: <880708120821.27603e9e@LBL.Gov>
Organization: University of New South Wales
Lines: 40

In article <880708120821.27603e9e@LBL.Gov>, nagy%warner.hepnet@LBL.GOV (Frank J. Nagy, VAX Wizard & Guru) writes:
>> According to what I've been told at one VAX/VMS performance seminar, it
>> does not make sense to define a large MPW_HILIMIT. The reason is that whenever
>> a process with pages in the modified list terminates, the whole list is
>> written to disk, so its size gets down to zero. That's probably why it can
>> never grow really big.
>      
> This is 95+% wrong!  When an image exits, most of its modified pages are
> flushed to the free list since they have NOWHERE to go on the disk.  The
> only modified pages written back to disk are those from files mapped as
> writable sections.  In .EXE files, the code is read-only (99%) and so
> become RO pages which are flushed to the free list on termination.
> Initialized variables are usually in Copy-On-Reference sections and
> unitialized variables (and ones inited to 0) in Demand-Zero sections;
> in both these cases the page in memory DOES NOT get written back to the
> .EXE file but has the system paging files as their backing store.  When
> the image exits it makes no sense (and so VMS does not do it) to write
> these pages to the paging file.
> 
> = Frank J. Nagy   "VAX Guru & Wizard"
> = Fermilab Research Division EED/Controls

I was not referring to IMAGE exit, but to PROCESS termination, therefore
the above argument is irrelevant. Also I used the term DISK in general sense,
meaning page file, swap file or private file; all on disk, aren't they ...

Anyway, true or not (I would be reluctant to put a percentage on it),
it would be interesting to find another explanation for the modified page 
list on a busy system to go down to zero.

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