[Beowulf] Moores Law is dying

Jon Forrest jlforrest at berkeley.edu
Tue Apr 14 13:55:22 EDT 2009


Joe Landman wrote:

> We know of (and have worked with) many applications that have required 
> tremendous memory footprint.  One that required hundreds of GB of ram in 
> the late 90s might use a bit more today.

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.

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. Such a program would be too complex
to understand, assuming it's written by humans. Maybe
such a program could be generated by a program, but
I'm not talking about this.

I once added up the text segment of every executable
and shared library on a Linux system. I probably counted
some files more than once. Even so, the total text size
of all these files was less than 2GB.

I'm not proposing doing anything about this, such
as coming out with an architecture that uses
32-bit text pointers and 64-bit data pointers.
That would add needless complexity. But, it's important
to realize that this limit exists, and unless
we get much smarter, isn't likely to go away.

Cordially,
-- 
Jon Forrest
Research Computing Support
College of Chemistry
173 Tan Hall
University of California Berkeley
Berkeley, CA
94720-1460
510-643-1032
jlforrest at berkeley.edu
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