Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!ucbvax!mtxinu!taniwha!paul
From: paul@taniwha.UUCP (Paul Campbell)
Newsgroups: comp.sys.mac
Subject: Re: System 7.0 Virtual Memory Question
Message-ID: <372@taniwha.UUCP>
Date: 30 May 89 17:18:21 GMT
References: <758@cf-cm.UUCP> <653@bridge2.ESD.3Com.COM> <31723@apple.Apple.COM> <8054@batcomputer.tn.cornell.edu> <2108@internal.Apple.COM>
Reply-To: paul@taniwha.UUCP (Paul Campbell)
Organization: Taniwha Systems Design, Oakland
Lines: 37

In article <2108@internal.Apple.COM> goldman@apple.com (Phil Goldman) writes:
>In article <8054@batcomputer.tn.cornell.edu> 
>mha@batcomputer.tn.cornell.edu (Mark H. Anbinder) writes:
>> Does Apple's VM at least know how to take advantage of contiguous space,
>> and will Apple recommend that users provide contiguous space?  Having a
>
>Using contiguous disk space will not speed up paging time in most cases,  
>as long as the file is not *seriously* fragmented (which is rare unless 
>the disk is very close to full).  The odds of a particular page lying in 

The need for paging files/partitions to be contiguous is an idea which
has hung around from the days of SWAPPING systems (note - many unix systems
still page and swap - when things get tight whole processes are swapped out
and taken out of contention for scheduling - this doen't make as much sense
in the Mac environment where processes must remain around to respond
to update events [BTW it was great to finally hear an Apple speaker use the
word 'process' at the developer's conference]).

It doesn't make much sense to require a paging file to be contiguous
provided individual pages are contiguous on disk (in 4k chunks). What 
might make more sense are paging policies that place pages near the
paged application's code and/or data files on disk, or near the
last block read/written to/from the disk (ie near where the heads are
now). In many ways the concept of putting the paging file inside the
file system is better than the unix way of putting it in a different
partition on the same disk (less head movement). On a two disk system
it makes more sense to put the paging file on a different disk (esp.
if you have a real async SCSI manager).

	Paul


-- 
Paul Campbell
Taniwha Systems Design			UUCP:		..!mtxinu!taniwha!paul 
Oakland CA				AppleLink:	D3213
Achtung! Ve are from ze Interface Police! Ve vant to look und feel!