Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!lamaster
From: lamaster@ames.arc.nasa.gov (Hugh LaMaster)
Newsgroups: comp.arch
Subject: Re: VM needed for rapid startup
Message-ID: <10094@ames.arc.nasa.gov>
Date: 9 Jun 88 22:32:57 GMT
References: <19730@beta.UUCP> <4332@killer.UUCP> <802@elxsi.UUCP> <10078@ames.arc.nasa.gov> <20128@beta.UUCP>
Reply-To: lamaster@ames.arc.nasa.gov.UUCP (Hugh LaMaster)
Organization: NASA Ames Research Center, Moffett Field, Calif.
Lines: 51

In article <20128@beta.UUCP> jlg@beta.UUCP (Jim Giles) writes:

>I've been able to find out form the hardware people, the Cyber VM accounts
>for a one clock delay in the speed of scalar memory references vs. what

There is also an address adder in the path in the Cray which adds a cycle.
You have to generate the global address somehow.  Admittedly, the adder
takes up a lot less room than a VM associative register array.

>The issue that NO ONE has yet responded to is this: a code which has
>predictable data usage patterns can pre-read (and post-write) data
>asynchronously - sometimes the code can run to completion without ever
>waiting for I/O to complete.  No VM system I've ever seen can do this.

Actually, VSOS, the "old" Cyber 205 O/S, has user callable advise routines
which do exactly this.  They allow the user to tell the system in advance
what pages will be needed, and which ones will not be needed.
With careful programming, (exactly the same care as needed on a non-VM
system to do the same job), it is possible to stream from disk 
at the full disk data rate.

I am not aware of what CDC/ETA has done with respect to this on the new
Unix system for this architecture now available on the ETA machines.
If this feature is not available, it would be a significant loss.

Another nice feature which many systems have these days is the ability
to map files onto sections of VM.  When this is combined with another
VM feature, "VM readahead" on sections of VM, you can get the same
performance from VM mapped data files that you got from reading ahead
sequential files.

******************

A separate issue with VM systems is more of a psychological one.  I have
heard it argued that VM makes people lazy.  While it is just as easy to
write a lousy program on a non-VM system which performs poorly (a Cray doing
random disk file accesses is just as slow as a Cyber 205 doing random paging)
there is a gray area in the middle where it is easier to get, and therefore
to accept, 50% of the maximum performance of a job.  On a non-VM system,
it is often almost as easy to get maximum I/O performance, once you have
gone to trouble to restructure your job.  So you tend to get more programs
on a VM system which are doing just well enough to get by, but could be a
lot better with some more effort.  Of course, you could turn this around
by assuming that your development time/costs were probably lower on the
case where you got 50%.  Probably hard to quantify...


-- 
  Hugh LaMaster, m/s 233-9,  UUCP {topaz,lll-crg,ucbvax}!
  NASA Ames Research Center                ames!lamaster
  Moffett Field, CA 94035    ARPA lamaster@ames.arpa
  Phone:  (415)694-6117      ARPA lamaster@ames.arc.nasa.gov