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From: ggs@ulysses.UUCP (Griff Smith)
Newsgroups: net.lang
Subject: Re: Slow Fortran I/O
Message-ID: <895@ulysses.UUCP>
Date: Sat, 7-Jul-84 23:25:52 EDT
Article-I.D.: ulysses.895
Posted: Sat Jul  7 23:25:52 1984
Date-Received: Sun, 8-Jul-84 01:36:03 EDT
References: <9625@lanl-a.UUCP> <5884@mcvax.UUCP>, <2078@rlgvax.UUCP>, <729@abnjh.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 34

I don't know how VMS processes formats, but the Fortran-10 system
(TOPS-10/TOPS-20) did run-time compilation as of 1981 and still does
for all I know.  Formats were not compiled by the Fortran compiler,
they were compiled by the run-time system.  The address of the format
was used to search through a table of compiled formats.  An attribute
field in the call to the run-time system indicated whether the format
was a constant (safe to re-use the compiled format) or an array
(re-compile every time).

Surprisingly, the UNIX(TM) Fortran run-time system almost does the same
thing.  Unless I am badly misreading the code, a format is compiled
into an internal data structure before it is used to direct data
translation.  The compiled format could be re-used if someone would
find a cheap way to determine that the original hasn't changed since
the last compilation.

Based on some experience I have had writing I/O routines in tight
assembly language, a carefully written general-purpose number
conversion will take about 60/MIPS microseconds per character (MIPS =
speed of processor).  This includes the usual paranoia, but leaves no
room for fancy features such as variable format fields.  The UNIX stdio
package on my VAX is within 50 percent of this value; the UNIX Fortran
system misses it by a factor of four.  The best Fortran systems that I
have seen have missed it by no worse that 50 percent.  There are ways
to beat the guideline value by a factor of 6, but the techniques used
don't adapt easily to a general-purpose I/O system.

Trademarks: UNIX - AT&T Bell Laboratories; VAX, VMS - Digital Equipment Corp.
-- 

Griff Smith	AT&T Bell Laboratories, Murray Hill
Phone:		(201) 582-7736
Internet:	ggs@ulysses.uucp
UUCP:		ulysses!ggs