Re: [tlug] Only 32bits for Linux on UltraSparc II :(

["Stephen J. Turnbull" <stephen@example.com>]

>>>>> "Zuco" == Zuco Pietro <zucomailinglist@example.com> writes:

    Zuco> There is something I don't understand. If the benefit of use
    Zuco> 64 bits for userland applications is so poor, why spend time
    Zuco> and money, compiling an entire operating system to 64 bits
    Zuco> as Solaris?  One of the elements of the race to improve
    Zuco> performance is incrementing from 16, 32, 64, and so
    Zuco> forth. UltraSparc is useful only for large databases or
    Zuco> complex mathematical operations?

If you think of graphics or national accounting (this definition of
accounting includes Toyota and GM and Microsoft as nations :-) as
complex, yes.  For most purposes, it's clock speed and how much you
can get into RAM (somebody else covered that, though).  I don't know
anybody who has more than 65535 fingers, and for most applications
that's what you're doing: math that's about as complex as counting on
your fingers.  Think about it: how often do you use numbers bigger
than 255?  As we used to say in junior high, "more than a mouthful is
a waste!"

In fact there's a theory of the "natural distribution" of integers,
and it's basically negative exponential: the probability of
encountering the integer n is proportional to exp(-n) or something
like that.

Wide words can be used in a lot of situations.  For example, if you
know that a string is word-aligned, there are algorithms that allow
you to detect whether there are any zero bytes in the word in two
machine instructions.  So these algorithms buy nothing with 16 bit
words, a 2:1 speed up for 32 bit words, and 4:1 for 64 bit words, plus
some overhead for handling memory alignment.  That's a lot if you're
doing a lot of strlen() or memcpy().

However, these situations are fairly specialized.  In most cases,
except for the big speedup that comes if the CPU (and your wallet) can
handle enough RAM to never use swap, most advanced applications spend
most of their CPU chasing pointers!  Ie, the size of the address
doesn't matter.

    Zuco> For example the X server will not be faster if it is 64 bits
    Zuco> compiled?

With the Render extension (heavily used in Xft), the device
independent portion will get faster, I suspect, and the device
dependent portions will get faster if and only the graphics bus for
the card can handle 64 bits.

However, there's lots and lots of code that is not 64-bit safe.

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