[Beowulf] Moores Law is dying

Steve Herborn herborn at usna.edu
Tue Apr 14 13:42:05 PDT 2009

Run Forrest -- Run!

Steven A. Herborn
U.S. Naval Academy
Advanced Research Computing
410-293-6480 (Desk)
757-418-0505 (Cell)

-----Original Message-----
From: beowulf-bounces at beowulf.org [mailto:beowulf-bounces at beowulf.org] On
Behalf Of Greg Lindahl
Sent: Tuesday, April 14, 2009 2:25 PM
To: beowulf at beowulf.org
Subject: Re: [Beowulf] Moores Law is dying

On Tue, Apr 14, 2009 at 10:55:22AM -0700, Jon Forrest wrote:

> I claim that there's a memory-related constant that hasn't been widely 
> recognized. This is that the amount of address space for a program's 
> text segment will never exceed 32 bits. Note that I am *not* talking 
> about the data segment.

You're right, the "large" memory model doesn't exist in x86-64, because no
one ever thought about it. (Check the gcc manpage for -mcmodel=large ...
it's not implemented, but it's spec'd out.)

> The reason for this is that it's simply too hard to write a program 
> whose instructions require even close to the
> 32 bit address space.

There are disciplines like EDA which generate large programs today.
Computer generated, of course. Companies like Intel and AMD use these
programs to design microprocessors. So yes, their architects are well aware
of this issue.

> But, it's important
> to realize that this limit exists, and unless we get much smarter, 
> isn't likely to go away.

Uhuh. You should call this "Forrest's Law".

-- greg

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