Dual channel DDR and the E7500 chipset

[Mark Hahn]

> 1) Can a single CPU installed alone in a dual Xeon motherboard with
>    dual channel DDR 200Mhz memory have access to the full 3.2Gb/s
>    memory bandwidth?  I.e. does the chipset restrict each CPU to a
>    single channel of the DDR memory, or some such?

as far as I can tell, the DDR is 128b wide, not dual independent channels.

though you'll obviously never see 3.2 GB/s if for no other reason than 
the fact that the ram protocol has overhead.  stream scores without any
heroic efforts (just gcc, no fancy prefetching, etc) are about half that
(which is quite good.)

> 2) Anyone know of any comparisons/have any experience in the relative
>    memory bandwidth performance between a single P4 CPU motherboard
>    with PC800 RAMBUS, and a dual Xeon board with only one CPU
>    installed and dual channel DDR 200?

dual PC800, such as an i850 board with two rimms, tends to deliver 
around 1.6 GB/s by Stream.

> 3) Certain dual boards appear to support dual channel DDR at 266 Mhz.

perhaps the vendor is simply saying that you can plug normal pc2100
dimms into the board.

>    Given that the Intel E7500 chipset only supplies a total memory
>    bandwidth  of 3.2 Gb/s, isn't this extra memory speed wasted?

if you plug pc2100 into an e7500, the ram simply runs "underclocked".
there's an upcoming revision of the chipset which supports PC2100 for real.

> 4) Any recommendations of a good dual Xeon board with dual channel DDR
>    for beowulf applications using a lot of memory bandwidth?

depends on your price/performance and aesthetics.  i850 with 2xRD1066 is
still fastest, though I'm not completely clear on whether that support
is official, or merely "overclocking works".

there's an upcoming e7205 (granite bay) that is uniprocessor, but supports
dual-DDR2100; I expect that'll be very fast and cost-effective if your app
really needs the bandwidth.