[ale] 64bit or 32bit

[Greg Freemyer]

On Thu, Jan 21, 2010 at 3:46 PM, Solomon Peachy <pizza at shaftnet.org> wrote:
> On Thu, Jan 21, 2010 at 02:45:17PM -0500, scott boss wrote:
>> does 64bit perform better?  Only if the complete stack (application,
>> os, drivers, hardware) can support it.  If any one item in the stack
>> doesn't use the 64bit-ness then 32bit at that point is good for you
>> across the board.
>
> Depends on what you mean by "perform" -- strictly speaking, there's no
> inherent benefit to going 64-bit beyond greater memory addressability,
> assuming everything else stays the same, but in the x86 case, the 64-bit
> mode doubles the numbers of registers which usually makes quite a
> difference.
>
> So if you're doing any sort of number crunching or anything else
> computationally intensive (including compiling!) then you're generally
> better off running 64-bit, even when you have <4G RAM.

We also find sustained disk i/o to be faster with a 64-bit kernel.

ie. dd if=/dev/sda of=/dev/sdb bs=4k

I assume there are 64-bit buses that are half utilized with a 32-bit
kernel.  I'm not sure that makes since most of the data should be
moving on the PCI bus via DMA, but we have done dd benchmarks that
show 64-bit slightly faster.

fyi: We actually run dd to move 100's of GB of data routinely, so it
is a good benchmark for us.

Greg