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Re: 64-bit Kernel Question



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.




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Cheers
John

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