[lug] memory configurations

D. Stimits 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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