What is "dual-channel DDR"?

[Andrew Hammond]

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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.

But don't buy the Tyan Tiger K8W. The whole point of Opterons is the IO
interconnects. The K8W has only one (paired) memory bank, and it's hung
off the same processor which has all the other IO too. Terrible design.
Any of the Thunder series are far better, the Thunder K8WE is
particularly nice, although they still haven't really balanced the IO
for integrated components.

Also, the Opterons use single channel DDR memory because they have
superior memory and IO capabilities. There's no need for the hack that
is dual-channel. Instead, each CPU has two dedicated hypertransport
channels for memory.

Finally, I can strongly recommend Alliance Technologies (416-385-3255)
as an integrator / VAR for people interested in running Linux or BSD on
Opteron. We have been very pleased with both the quality of their work
and the excellent service. My only relation to them is as a very happy
customer. They're not the cheapest place to pick up hardware, but they
may be the most cost effective I've ever dealt with.

- --
Andrew Hammond    416-673-4138    ahammond-swQf4SbcV9C7WVzo/KQ3Mw at public.gmane.org
Database Administrator, Afilias Canada Corp.
CB83 2838 4B67 D40F D086 3568 81FC E7E5 27AF 4A9A


William Park wrote:
| 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?  :-)
|
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