Speed penalty for single sided RAM on Athlon 64?

[Slaimus]

It is well known and even documented by Intel that using single sided memory instead of double sided leads to a performance hit with the P4. Does the K8 suffer from the same thing?
Also, seeing how single sided memory appears to be far more stable for the K8 memory controller, I am leaning towards single sided even if there is a performance hit. Is this justified?

 

[joelslaw]

this is a quote from an article I found:

If you read AMD's specifications for the Athlon 64 processor, you'll discover the admission that it can't handle the timing issues involved in running more than one double-sided module at the full DDR 400 speed. According to the same specifications, it states that systems with two or more double-sided modules can only be operated at the speed of DDR 333 modules.

As usual, AMD is being rather conservative with its specifications, probably in order to avoid as many problems as possible. Some Athlon 64 motherboards will run as much good-quality double-sided memory (i.e., made by Crucial) as the board can be fitted with without any problems, and others support a maximum capacity of particular brands of DDR 400 RAM that should run without problems.

here is the full thing:

http://www.pcbuyerbeware.co.uk/RAMProblems.htm#double

I don't know that I nessisarily agree with this. It may TECHNICALY run less stable, but I think in practice it's a negligable problem, deffinately not one I would let affect my purchase. I personally run DDR400 with my 64, and have NO stabillity problems (as far as 24 hours of prime95 is concerned) And I know a lot of people would say the same (even those with higher speed ram than 400)

It also sounds like a lot of this problem can be chipset related, so I guess choose your chipset carefully!

I guess in the end it's your call, I'd go dual sided because I like having a full gig (and if your thinking of 4x256 to keep it single sided, just remember, the more parts you have the more can go wrong, thus naturally increaseing instability) But maybe you're a little more picky than me. At any rate, thanx for the ? This one made me think!

 

[Peter]

Double-sided RAM has an advantage and a disadvantage:

+ The DRAM controller gets another rank, can keep pages open there, thus reducing overall access latency by a little bit. You won't notice as much of an effect with an AMD64 machine, since worst case access latency there is a lot lower than on Intel systems, and the AMD64's own performance itself by far isn't so bound to low latency, high throughput RAM as the P4's super-long-pipeline design is.
- You bring twice as much parasitic capacitance onto the common bus. There is a limit to what's acceptable here, mostly depending on the frequency of the bus operation. That's why DDR400 is (in general, not only at AMD!) designed for single-DIMM operation (per channel), and that's also why AMD recommends stepping down to a 2T command rate when you're using many chips at DDR400 speed (200 MHz).

 

[joelslaw]

man pete, you know too much! no wonder you're elite. Hope you work somewhere that appreciates your talent. Thnx for the answer, I wasn't really sure on this.

 

[Slaimus]

I settled on a pair of single sided 512MB Crucial sticks, the only 512MB sticks that I can find that are confirmed to be single sided. They will not run stable at CL 2.5 even at 2.8v at 200MHz, but have good headroom at CL 3 to about 240MHz. Low latency on high density chips is probably too much to ask. At least I left myself room to get another pair of single sided sticks without needing to relax timings.