What is "dual-channel DDR"?

[William Park]

On Wed, Mar 16, 2005 at 08:59:49AM -0500, Lennart Sorensen wrote:
> On Tue, Mar 15, 2005 at 08:49:53PM -0500, William Park wrote:
> > What is dual-channel DDR?  Is it also called "DDR2"?
> > 
> > I'm interested in Abit AV8 (AMD64, S939)
> >     <http://www.abit-usa.com/products/mb/products.php?categories=1&model=175>
> > which takes dual-channel DDR.  But, I don't know what that means.  My
> > experience ends with SDRAM and Pentium 3.
> 
> Dual channel means you take two sticks of memory and run them in
> parallel so you have twice the bandwidth available.  This works by
> having two memory controllers running the memory as two seperate chunks
> of memory.  Access to memory is then split evenly across the two chunks
> (usually works best if you have each chunk identical size so you can
> alternate between the two chunks for every address).
> 
> So assuming you have 64bit memory (most is today) you would have bytes
> layed out like this:
> 
> Channel1 Channel2
> 01234567 89ABCDEF
> 
> So if you go to read 128bytes you get the throughput of both channels at
> the same time.  Most benchmarks I have seen indicate a 5 to 10% boost in
> speed on AMD systems, and usually a bit more than that on P4 (the P4 is
> more bandwidth hungry because of a very large cacheline size, which
> makes it read a big chunk every time it accesses memory).
> 
> DDR2 is simply a new version of DDR designed to allow higher clock
> speeds and lower latencies.  Some boards support both (but NOT at the
> same time).

Thanks Lennart and Jason,

I'm trying to spec a server for thin-clients in purely "office"
environment.  Because of the saving on the client-side, I have room to
indulge on the server-side.  At the moment, I'm eyeing
    
    1. Abit AV8 (AMD64, dual-channel 4GB max)
    2. Tyan Tiger K8W (dual-Opteron, single-channel registered 8GB max)

If anyone has more than 4GB, was there an occasion when you really used
that much?  :-)

-- 
William Park <opengeometry-FFYn/CNdgSA at public.gmane.org>, Toronto, Canada
Slackware Linux -- because it works.
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