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From: HELLER@cs.umass.EDU.UUCP
Newsgroups: comp.os.vms
Subject: Re: VAX C compilation speeds on 750
Message-ID: <8710230018.AA15920@ucbvax.Berkeley.EDU>
Date: Wed, 21-Oct-87 09:04:00 EST
Article-I.D.: ucbvax.8710230018.AA15920
Posted: Wed Oct 21 09:04:00 1987
Date-Received: Sun, 25-Oct-87 07:52:21 EST
Sender: kupfer@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 43

There are some other factors the are different between the VAX/VMS C
compiler and most MS-DOS C compilers -

First of all, MS-DOS C compilers probably limit the maximum size of a single
module - afterall, there is a 64K segment-size limitation imposed by the
hardware. This means these compilers can be setup to buffer a module
completely in RAM (which is not paged!). There is no limit on module size on
a VAX (no segments!), and it is also a virtual-memory system. This means
paging. Thus it is posible for a MS-DOS C compiler to buffer things to fast
RAM and save lots of time (including such hacks as pre-allocated
intermediate code space). You can't do this on a VAX and still have a full
production compiler.

Also, you had the /DEBUG option on. This of course means more work for the
compiler, including lots more disk I/O (all of those debug records to spit
out). I suspect that the VAX C compiler has to work just as hard (i.e. take
just as long, if not longer) generating debug records as it does if it is
optimizing. A better test is /noopt/nodebug.

Also:  were you measuring elapesd *real* time or *cpu* time.  Since the VAX
is paging (and has various system detached processes around), you might be
measuring more than you think.  Was the VAX on a network?  Was DECNET
running?  While I won't doubt that the VAX 750 is not a super fast system
and newwer PCs are probably faster (at least at some things) it is posible
that your timings are skewwed by additional factors.

Oh one other thing:  I don't think a SUN 3 is that much faster than a VAX
750, particularly for heavily memory-bound processing.  We (the VISIONS
group) ran some benchmarks and the SUN we had on loan was just about equal
in speed as the VAX 750's we have (things like image processing operations
on a 256x256 image) - SUN's benchmarks are somewhat deceptive - they are
accurate for small/tiny programs (yes the SUN cpu is faster than a VAX 750),
but the VAX pages faster (at least under VMS).  

		Robert Heller
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