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From: bron@bronze.wpd.sgi.com (Bron Campbell Nelson)
Newsgroups: comp.sys.sgi,comp.lang.fortran
Subject: Re: SGI comments?
Summary: file access
Keywords: SGI info, FORTRAN
Message-ID: <88688@sgi.sgi.com>
Date: 4 Mar 91 20:17:21 GMT
References: <958@vax.cs.athabascau.ca> <88675@sgi.sgi.com>
Sender: guest@sgi.sgi.com
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 33

In article <88675@sgi.sgi.com>, bron@bronze.wpd.sgi.com (Bron Campbell Nelson) writes:
> In article <958@vax.cs.athabascau.ca>, willis@cs.athabascau.ca (Tony Willis) writes:
> > Most of my FORTRAN I/O comes from reading/writing
> > direct access files more or less like
> > 
> > 	DO 300 I = 1, 1024
> > 		WRITE(UNIT=12,REC=I)(DATA(J,I),J=1,1024)
> > 300	CONTINUE
> > 
> > Could I replace this sort of thing by calls to C and
> > speed up the I/O, particularly in multi-user mode? (Come on
> > you SGI guys in Mountain View - help make a sale!!)
> > 
> 
> The use of direct access file complicates interfacing to C a bit.
> You'd need to know the format of records on the disk, which sadly
> I do not.  I'll ask my co-worker who knows this stuff to comment.
> 

I talked to the man with the answers, and he tells me that direct
access files are in fact laid out in the simple straight forward
manner: each record has RECORD-LENGTH bytes in it, and record number
one begins at byte zero in the file.  If I understand the above loop
correctly, we could interface directly to the "write" system call
and replace the loop with:
	istatus = write(12, data, 1024*1024*recordLength)
which should speed up this I/O operation by a LOT.


--
Bron Campbell Nelson
bron@sgi.com  or possibly  ..!ames!sgi!bron
These statements are my own, not those of Silicon Graphics.