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Re: SUMMARY: pentium optimized debian discussion



On 30-Jul-1998, Guy Maor <maor@ece.utexas.edu> wrote:
> Joey Hess <joey@kitenet.net> writes:
> 
> > Pentium optmized binaries are about 10% larger than i386 binaries (the
> > compiler probably inserts NOPs for pipelining purposes). If debian switched
> > to pentium optimized binaires, it would increase the installed size of
> > debian a lot,
> 
> Why "a lot"?  Let's say we compile only 10% of the distribution that
> matters and that binaries account for about half of a package's
> installed size.  That's an 0.5% increase in distribution size?

I agree that an increase in distribution size isn't a big issue.
However, memory footprint is, and larger executables have a larger
memory footprint (use more memory, more cache, more swap, etc).

> 
> > Pentium optimized binaries will run slower on 386's than normal i386 binaries.
> 
> Because of the 10% size presumably?  They can't run more than 10%
> slower.  And probably much less as NOPs take minimal time to execute.

The 10% size could easily mean that code that once fitted in the cache
now doesn't.  In addition, the code that does fit in the cache may now
be executing a bunch of NOPs instead of doing useful work.  NOPs don't
run much faster than other instructions -- they take one cycle to
execute.

It's not just NOPs, compilers for pipelined CPUs can put extra instructions
in because of the pipelining (e.g. compute this value before branching
because one branch needs it, and it won't take any extra time).

Also the extra code size may mean that we swap 10% more often (or that
we swap, when once we didn't).

So less work, more page faults, more swapping.  Without measurements
that show there is little effect on older machines, I don't think it
makes sense.

> 
> >From the estimates so far, I don't think it makes any sense to add the
> complication of a whole new architecture.  Instead just recompile the
> few packages that are computation intensive.

As long as the new architecture maps back to i386 for 95% of the
packages, I see little harm.  Pentium optimization seems to be the
flavour of the month.  For the PR value alone it may be worth it.

-- 
       Tyson Dowd           # "Bill Gates is a white persian cat and a monocle
                            # away from becoming another James Bond villan."
     trd@cs.mu.oz.au        # "No Mr Bond, I expect you to upgrade."
http://www.cs.mu.oz.au/~trd #                -- Dennis Miller and Terri Branch


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