[gentoo-amd64] Re: Re: Giving up 64 platform

[Duncan]

Brandon Edens posted <20060425055400.GA8124@×××××.lan>, excerpted below,
on Tue, 25 Apr 2006 01:54:00 -0400:

> Having 16 (well actually only 12 according to the SBCL hackers) general
> purpose registers (AMD64) versus x86's 8 GPRs is reason enough for me to
> run AMD64.
>
> I believe the major performance bottle-neck in modern computers is
> memory access latencies and the like. Not just RAM btw, I'm talking
> about the memory hierarchy: register -> cache -> RAM -> hard-drive
> (ordered by speed of access: left being fastest, right being slowest)
>
> Processor calculation speeds have improved at a faster rate than memory
> access speeds. Your processor is essentially just sitting there idling
> as it waits for instructions to be fed to it (starved). Having more
> registers means you have a larger scratch-pad to perform calculations
> on.

This is the big reason AMD64 isn't simply a 64-bit address space over a
32-bit one.  Were it just that, it would actually be /worse/ than x86 for
most desktop stuff, as the 64-bit address space and integers simply aren't
needed by most software at this point, and doubling the size of parts  of
your code means less fits in memory, and it takes twice as long to move
them around.

The extra registers are the big thing that turns this AMD64 into an
advantage in nearly all cases, over comparible x86.


That's also why I tend to favor -Os and certain other similar
optimizations (I add various register re-use and hot/cold function
reordering flags as well, as the register optimizations will be very good
for amd64, even at the cost of code size, and hot/cold function reordering
regroups functions such that the frequently used ones stay in cache
better, even at the expense of overall code size). Smaller code, given
memory latencies, should be faster code, even if it's less efficient at
the CPU itself, because that much more of the code fits in the next closer
level of cache, that would otherwise be in far slower memory or cache
futher from the CPU.

That said, performance is one thing, human frustration is an entirely
different thing.  Because the (desktop/workstatiion) computer spends most
of its time waiting for human input, an efficient human/computer interface
is a far more important factor than all the others.  Of course, one
person's richly efficient interface is another's bloat, making it
virtually impossible to benchmark.

The reason I suggested going back to regular x86 Gentoo in this case is
that it's quite apparent there's a lot of friction in that computer/human
interface at present, and for him, x86 may just work best, at the moment.
While it makes us geek types cringe at the supposed resource waste, is he
actually wasting more /not/ using AMD64 because he's too frustrated with
it, or smoothly using x86, which he knows and is comfortable with? There's
nothing "wrong" wth using it in x86 mode, and if that's what suits is
style best, then more power to him (literally, more power over the
computer)!

The thing is, it's his decision, not your or mine.  He's the one living
with the consequences.  If he's more comfortable on x86, and all amd64 is
giving him is frustration, then x86 is the way he should go.  If he
believes he can pull it out with the help of a chroot or the like, then
let him try, but don't force it on him, and don't make him so despairing
he quits Gentoo in frustration, when he's already said he's very
comfortable with x86 Gentoo.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman in
http://www.linuxdevcenter.com/pub/a/linux/2004/12/22/rms_interview.html


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