64-bit Kernel Question

[John Summerfield]

Nifty Fedora Mitch wrote:
> On Mon, Mar 09, 2009 at 12:49:09PM -0700, Adam Williamson wrote:
>>> As memory requirements for 64-bit are anywhere from 50-100% greater and 
>>> the only appreciable difference is a "psychological" performance boost, what 
>>> REAL benefit is there, actually?
>> It's not psychological, it's just not noticeable in most regular
>> operations. Actually, most people wouldn't notice if you replaced their
>> CPU with one which was twice as fast (or, as the BOFH knows, half as
>> fast...), most of the time, because very few of the operations most
>> people do day-to-day are remotely CPU-bound. A few years ago I was
>> running a 2.4GHz (Pentium 4-era) Celeron as my desktop. The CPU fan gave
>> out, so the CPU throttled itself down to 800MHz and kept running. I
>> didn't notice for a fortnight.
>>
>> The most common CPU-bound operation in our world, I guess, is
>> compilation, and you would notice a definite improvement in speed there,
>> running x86-64 vs x86-32 - not huge, but noticeable. Certain database
>> and I think scientific operations that are CPU-bound also derive a
>> significant benefit. It depends on whether the code can take advantage
>> of much bigger registers, AIUI.
> 
> There are some interesting differences between 32bit and 64bit x86  boxes.
> 
>  *) calling conventions for the compiler's ABI are richer/ better in x86_64 because
> 	of the larger set of registers.  i.e. more can be passed and returned
> 	via registers and less has to be pushed/popped on and off the stack.
> 	Here the win is for 64bit.  Google "compiler register spills" for more
> 	info...

I rather think those extra registers are available to 32-bit code.

>  *) compilers matter.

More than you imagine;-)
Years ago, I had a magazine which published a comparison of computers on 
Pentium CPUs.

Later, I ran that test against two releases of GCC. The differences were 
pretty surprising.

Then there's Intel's compiler which, if I were building a super computer 
out of intellish/amd64 processors, I would use. I don't really thing gcc 
is likely to be the fastest compiler around or generates the best code.




-- 

Cheers
John

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