What is "dual-channel DDR"?

[Christopher Browne]

On Wed, 16 Mar 2005 12:21:35 -0500, Lennart Sorensen
<lsorense-1wCw9BSqJbv44Nm34jS7GywD8/FfD2ys at public.gmane.org> wrote:
> On Wed, Mar 16, 2005 at 10:42:00AM -0500, Andrew Hammond wrote:
> > We run database machines on Celestica A8449 quad optreons w/ 16GB of
> > memory. If you want to do Opterons with > 4GB, you need a 64bit distro.
> > SuSE is the best, although debian isn't bad and is constantly improving.
> 
> No, you only need 64bit if you want more than 2 or 3GB per process.  PAE
> works just fine up to 64GB.  You will gain a bit of performance in some
> cases going 64bit, although for some tasks you also use more ram since
> pointers are 8 instead of 4 bytes each.

PAE requires that I/O goes into "bounce buffers" that must be copied
to/from user process memory.

That's an extra set of memory copies that have a pretty appreciable cost...
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