Help on Dual Channel Memory Restictions

[burpnrun]

I am thinking of getting a newish (AMD AM3, AM2+, AM2 - capable) motherboard with 4 DDR2 slots. I also have 4 gigs (2 x 2) of one vendor's DDR2 memory (Patriot - CAS 5) and 2 gigs (2 x 1) of another vendor's DDR2 memory (G.Skill, also CAS 5). Actually, I have more sticks lying around, but these are the closest match and each pair has always worked without problem in dual channel configurations.

The timings vary only at the 5-5-5-x-2T level (and "x" differs only by 2 numbers or so), these values are in the SPD, and both are 1.8 ~ 1.9 volt stuff.

My question is whether I can dual channel these separately, to get 6 gigs? More clearly, 2+2 in one dual-channel pair (e.g., slots 1 and 3), and the 1+1 dual-channel pair in slots 2 and 4. Afterwards, hopefully, I would have 6 gigs of RAM operating successfully/quickly in dual-channel mode. Target OS would be Win7_64.

Any thoughts on repercussions or gotchas? TIA.

Edited January 31, 2010 by burpnrun

[puntoMX]

You can leave the memory to "Auto" on most motherboards, and swap the 1GB modules with the 2GB to the first or second group. Double channel isn't really the deal any more, even diferent sticks can be put in 128bit memory access mode (even different size sticks). When talking about memory performance on a AMD system you will have to play with the ganged or un-ganged memory access modes.

Any way, what CPU are you going to use? There are still some good boards that support DDR2 with a AM3 socket; just be prepared to run the CPU on a bus of 2000MT/s and not what is stated for the AM3 CPU (up to 4000MT/s). The bus speed doesn't affect the speed the CPU works on, a 2.5GHz CPU will be a 2.5GHz CPU...

A nice chipset is the 785G with the 710 southbridge; Inexpensive and fast (good onboard video as well with optional 128MB DDR3 (1066/1333) sideband RAM) and combine that with a Athlon II X4 620/630. With most of those CPUs the L3 cache can be unlocked giving you a Phenom at 60-70% the cost, but it's not warranted you can.

Any need on picking the mobo? Just ask . In most cases I would go with Gigabyte but ASUS and some others make good AMD chipset based mobos as well...

[burpnrun]

Thanks for the reply. I did a lot of googling today, and found that the best way to deploy the sticks is 2 - 1 in one channel, and 2 - 1 (gig) in the other channel. Seems it shouldn't make too much difference.

I've ordered a Gigabyte GA-M785G-UD3H and a Phenom II 550BE from NCIX, and it is supposed to be here tomorrow. What I plan to do is see how the current M2NPV-VM with 2 x 2 of DDR2-800 (X2-5000BE @ 2.8GHz) stacks up against the stock configuration of the new board and CPU, first using the 2 x 2 and then adding the 1 giggers to each channel. I suppose I'll use memtest+ for this, since I have to do it on the new board anyway, and it will give a quick snap of read/write and bandwidth, etc. If I remember, I'll post the results here.

I plan to set them up unganged. According to what I have read, it shouldn't make too much difference. The new machine's for my wife, who plays WOW 99% of the time (4830/512m on her current M2NPV-VM), and surfs and looks at her e-mail the other 1%. Will use Win7x64 Home Premium. Love XP, but all good things come to an end eventually. I really expect Win7 to be a bear to get "usable" without the glitz and useless fluff.

[burpnrun]

Per my previous post, I've had a chance to do some light comparisons between the previous and current board, and memory configurations:

Previous: M2NPV-VM, Athlon X2-6000 (Brisbane) @ 3.1 GHz, 2 x 2 gigs G.Skill DDR2-800 @ 880 Dual Channel

Current: MA785G-UD3H, Phen II/550BE @ 3.6 GHz (18x200), 2 x 2 gigs G.Skill DDR2-800 @ 800 Dual Channel, with & without extra 2 x 1 gigs Patriot DDR2-800 (also in dual channel mode)

G.Skill 2 gig pieces: 5-5-5-15-23-2T (run at 1.9v, both boards)

Patriot 1 gig pieces: 5-5-5-16-21-2T (run at 1.9v, both boards)

Results using Memtest86+ v4:

M2N: 3,530 (remember, this is at 880 MHz, a 10% o/c, for the 2 x 2 sticks)

785: 3,895 (2 x 2 sticks, 550BE=3.6 GHz (18x200), Bus = 200, i.e. stock except for CPU multiplier)

785: 3,839 (as above, but with 2x2 and 1x1 gig sticks populating all 4 slots, dual channel)

(loss = 56 for slightly mismatched 1 and 2 gig sticks)

I can get the 550BE and memory way above that, juicing the bus, voltage, etc. But I thought it would be interesting to report back on the effect of slightly mismatched memory stick timings in dual channel mode. All runs "unganged".

My conclusion is that the difference is so minor that it might even be a statistical blip, so there's no problem. The 785 board runs the combined pairs at the slower Patriot's speed, evidently. I don't know whether the difference would be so minor if the CAS was 4 on one, and 5 on the other, etc. Maybe someone could comment.

One other observation: The Nvidia M2N board and the AthlonX2 did very well in memory bandwith vis-a-vis the 785 chipset, considering the former is 3 or 4 year old technology. Of course, the above 785 timings are at stock settings except for the CPU multiplier (550BE normally 15.5 x 200), and a quick test of leaving the 18 multiplier stick, and raising the bus to 220 (10% overclock), yields a stable memtest of about 4,300 on the 6 gig memory configuration.

That's all, folks.

(edit) P.S. I was NOT successful at unlocking the 550BE. Despite (even) reducing the multiplier to 15 and using the AOC + Hybrid option (all cores or auto), combined with a + .15v increase, the board would not post on the subsequent reboot. Needed to short the battery and reboot to clear CMOS to defaults. Guess that I lost the lottery. The BE was a C2, manufactured 0949EPMW, DGI boxed.

Edited February 5, 2010 by burpnrun

[puntoMX]

CAS 4 or 5 would not make much difference at all, you could get some lower latencies but that's all; only with benchmarks you would see a small difference but in usage you would not see much, however, onboard video could benefit from it (you are not using it so leave that aside). Higher bus speeds would do much more with AM3 (I think you can get the Southbridge to some 2500-2700MHz ).

Too bad you could not unlock any thing, but I'm sure that CPU will do some 4GHz. The single core Sempron 140 can most of the time be unlocked to 2 cores, a 3 core AMD has a high change to work with the 4 cores, but a dual core to 4 cores is hard to get. What I mostly do is to buy a Athlon II and enable the L3 cache (on x2 2xx and x4 6xx).

Unganged will do best in multitasking as it will use 2 64bit lanes to the memory so that would give you better overall performance. You could set it to ganged and get better write speeds and lower read speeds with single treated memory benchmarks. To tell you the truth; I'll keep all my configurations on unganged as it gives the smoothest performance, and that smoothness results in a more responsive computer, and that's what counts for most people: A smooth responsive computer .