[lug] memory configurations
stimits at comcast.net
Sat Nov 11 20:14:23 MST 2006
Hugh Brown wrote:
> D. Stimits wrote:
>> D. Stimits wrote:
>>> I'm looking at some advertisements for RAM, and I see there is a
>>> "single rank" and "dual rank" rating (I'm looking at Kingston, don't
>>> know if this is just their own thing or if it applies to DDR2 in
>>> general), and also an x4 and x8 rating. What's the difference
>>> between these, especially with respect to adding to SMP machines?
>> Well...reply to my own email. I *think* from what I've found that
>> single rank is better, that dual rank refers to essentially two
>> separate ram modules being on one physical slot. E.G., a dual rank
>> would logically look like two separate slots to the system, and a
>> single rank would look like just 1 logical slot (both would require 1
>> physical slot). Anyone know if this is basically correct?
>> I'm still trying to figure out the x4 and x8 rating, I'm wondering if
>> this is just some latency rating...anyone have an interpretation on
>> the x4 or x8 ratings? Which is better?
>> D. Stimits, stimits AT comcast DOT net
> We got told by a sales guy that using dual rank meant that you could
> fill fewer slots, but that all of the memory would be directly
> addressable (and also have less total memory). Single rank you could
> have more but you wouldn't be able to address all of it.
Did some research, still a lot of questions. The gist seems to be that
dual rank really is a single physical device with 2 electrical memory
devices, whereas a single rank really is just 1 electrical device.
The consequences of this I'm not sure about, except that on some Intel
chipsets have a limit to the total number of ranks, regardless of how
many slots are on the board. If the system could hold the same amount of
total megabytes regardless of rank, and one system used dual rank, while
the other used single rank, the single rank seems to have a 5 clock
latency, while the dual rank has a 4 clock latency (this is just a
generalization of some of the Kingston specs on DDR2 samples), so there
are counterbalancing issues as to total memory possible and latency.
I still don't know what the x4 and x8 ratings are, but it seems it is
proportional to whether the module is single rank or dual rank. Perhaps
with dual rank it has an effect like reading from both rank at the same
time, but I'm a bit frustrated trying to figure the exact meaning of the
x4 and x8, and which one is a better performance rating.
> This was with regards to intel woodcrest cpu based machines.
> It smacked of sales weasel, so I don't know that I trust it. I
> couldn't find anything useful on the web, so if there are any hardware
> types out there, I'd be interested in the answer too.
With regards to the sales pitch, I'm thinking the guy might have been
partially right, at least for a few limited chipsets (the conversation
seems to depend on the specific chipset). If a motherboard in general
could access only a certain maximum size of an individual memory module,
then a dual rank could effectively allow double the memory to be visible
in a given physical slot (because it's really two modules of half the
total size)...unfortunately rank count is just as important on those
boards as are physical slots, so this can leave half of the slots
unusable if the maximum rank count is 1x the total slot count (rank
count would be maxed when half the slots are populated).
It looks like there might be quite a bit of variation on what chipsets
and architectures (thinking of Intel versus AMD) can handle all slots
populated with dual rank memory, versus those which have limits based on
rank. Most of the rank count limitations I read about today specifically
referred to Intel chipsets. Unfortunately, I've been finding that for
single rank memory you have to suffer another clock cycle of latency, so
there might be cases where a board not populated to max would benefit
from dual rank.
D. Stimits, stimits AT comcast DOT net
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