[Beowulf] Moores Law is dying

Jon Forrest jlforrest at berkeley.edu
Tue Apr 14 16:27:52 EDT 2009


Robert G. Brown wrote:

> Are you
> suggesting that e.g. a long-running program with fully unrolled loops
> cannot exceed 4 GB in size and still be "simple"?

Unrolled loops probably add only a few percent to the text size
of a program. I admittedly don't have any data to prove this
but try to imagine a case in which a standard compiler would
do enough loop unrolling to add significant size to a program.
As I understand it, loop unrolling is only used in certain cases,
and the size of the unrolled loops themselves can't be too large,
otherwise any benefits from the unrolling evaporate.

> Are you suggesting
> that compilers will never try to unroll code at that level, even when
> enormous memory systems are commonplace?

Again, the enormous memory systems you mention consist mostly
of enormous amounts of data, not text.

> Are you suggesting that even
> when concatenated, the space of all possibly functional operational
> phonemes in computational semantics cannot fill a 4 GB dictionary?

I'm not sure what you mean by "functional operational
phonemes" but to me that means some sort of data, which
again, is not what I'm talking about.

> Another such program is "the operating system" especially a multitasking
> operating system.  There is no real bound on the number of threads an
> operating system can run,

True, but somebody still has to write the threads.

> and "the program" being run on a multitasking
> operating system is the union of all "sub" programs being run on the
> system, with or without shared libraries (sharing is expensive in
> performance, remember -- we do it to save memory because it is a scarce
> resource).

Why is sharing expensive in performance? It might take a little
overhead to setup and manage, but why is having multiple virtual
addresses map to the same physical memory expensive?

> Clearly that can and does exceed 4 GB, even routinely on a
> heavily loaded server and we'd do it a lot more often without shared
> libraries.

Really? Show me one case where this is true. Again, remember, I'm
only talking about program text.

> And there MAY be new compilers that are a lot more
> generous in their usage of space than they are now.  There may be
> new-gen RISC-y processors that use far more instructions to do things
> that are currently done with fewer ones.  Is your observation
> Intel-arch-only?

I did my test for both Alpha OSF/1 a while back, and modern
Intel x86.

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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