Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!usc!elroy!judy!stevo
From: stevo@judy.Jpl.Nasa.Gov (Steve Groom)
Newsgroups: news.software.b
Subject: Re: Enhanced dbz for SunOS 4.0
Message-ID: <17099@elroy.Jpl.Nasa.Gov>
Date: 29 Jun 89 20:25:09 GMT
References: <17059@elroy.Jpl.Nasa.Gov> <1989Jun28.180430.2743@utzoo.uucp>
Sender: news@elroy.Jpl.Nasa.Gov
Reply-To: stevo@elroy.Jpl.Nasa.Gov (Steve Groom)
Organization: Image Analysis Systems Grp, JPL
Lines: 29

In article <1989Jun28.180430.2743@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes:
>In article <17059@elroy.Jpl.Nasa.Gov> david@elroy.Jpl.Nasa.Gov (David Robinson) writes:
>>Below is a SunOS 4.0 specific enhancement to dbz that mmap()'s
>>the history file and .pag file into main memory and accesses it
>>directly instead of through lseek() and read()...
>
>One of our early C News users has modified dbz to do roughly the same
>thing portably, by just keeping everything in memory and writing it out
>at the end.  I want to take a hard look at his code before promising
>anything, but it may be part of the next patch.  On machines with plenty
>of address space and physical memory, apparently it really makes things
>fly.

Keeping the whole file in memory can be better than seeking around the
file and doing selective reads, but you have to have the memory for
it.  Part of the beauty of mmap'ing is that you don't need lots of
physical memory for it to be a win.  One of the reasons David and I got
into this in the first place was that we don't have a lot of physical
memory.  With mmap'ing, you just set up a memory area to correspond to
a file and let the kernel take care of the I/O.  When physical memory runs
short, instead of paging out (= more I/O) parts of an in-core file, the
kernel can just invalidate mapped pages (= no I/O) and re-read them
later.  There is also an I/O speed gain with mmap'ing, which appears to
be the result of the elimination of copying data from kernel buffers
into user space.

Steve Groom, Jet Propulsion Laboratory, Pasadena, CA 91109
Internet: stevo@elroy.jpl.nasa.gov   UUCP: {ames,cit-vax}!elroy!stevo
Disclaimer: (thick German accent) "I know noothingg! Noothingg!"