Memory Bandwidth Halved w/2nd DIMM

Brother Geeks,

There are vague hints on this matter scattered around the web, and I'm hoping someone here can help me nail it down. I recently put 1 512MB stick of Corsair VS512MB400C3 RAM in an Asrock P4VT8+ motherboard, ran SiSoft SANDRA's memory bandwidth benchmark, got a quite respectable score, and promptly bought another VS512MB400C3 DIMM -- exactly the same thing. When I placed the new DIMM in its slot, the SANDRA benchmark number was cut in half. I've arranged the DIMMs in every permutation possible on a 3-slot MoBo, changed the DRAM frequency to all three speeds available (DDR 266, 333, 400) in my BIOS, and the SANDRA score does not vary. Does it somehow involve dual channel memory (I'm not even really sure my MoBo has it) not functioning properly with fewer than 3 DIMMs? Is there anything I can do other than eat the cost of the 2 Corsair sticks and replace them with a 1 gig DIMM? Some tweak I might be missing in the BIOS? Am I screwed? I would be grateful for any advice. Thanks to all who read this.

System Specs:

Asrock P4VT8+ Motherboard
P4 2.80c GHz CPU
2x512MB Corsair VS512MB400C3 RAM
ATI Radeon 9600XT AGP Video
WinXP Pro SP1
Maxtor 5T030H3 30GB Hard Drive (boot)
Seagate Barracuda ATA IV ST380021A 80 GB Hard Drive (storage)
MSI CR52-M CD-RW

 

I don't believe your system has dual channel. No mobo with dual channel will have an odd number of memory slots. It would make no sense.

I've never used the VIA P4 chipset, nor have I ever used any Asrock products, but I suspect that's where the problem is. Really, you paid good money for a slew of great products (processor, RAM, vid card)... and you're hooking them up to a pretty mediocre quality motherboard. IMX, you should never use a non-Intel chipset, especially if you're running a Pentium instead of a Celeron. It's just a waste of money.

If it were me, I'd invest in an Intel-based solution. Preferably, the much lauded ASUS P4C800-E Deluxe. Failing that, you should try to get an i875 board from Intel, Asus, or Gigabyte. Trust me, you will not be dissatisfied. That board *does* have dual channel, so you'll reap the benefits of doubling your effective bandwidth.

 

actually there are many mobos with Dual Channel support and only 3 slots for DIMM's, mine (A7N8X Deluxe) included.

Dual channel requires 2 or more DIMM's, has nothing to do with pairing

 

Uh, yes it does. Dual channel uses two independant memory busses. That why when you look at the PC4800-E, there's two groups of slots, and there one black and one blue in each group. The first group is bus A, and it operates at DDR400, so you put PC3200 RAM into it. The second group is bus B, and it operates independent of bus A also at DDR400, so you put PC3200 in that one.

If you were to put two sticks into the first two slots for bus A, you get all the RAM but you would not get the benefits of dual channel. Your mobo does indeed have dual channel and an odd number of slots. And that's dumb, because if you put 3 GB into the system (the max) then 2GB will get half the bandwidth (2x1GB @ DDR400) and 1GB will get the other half (1GB @ DDR400). That's silly, and you're bottlenecking. It's an inherently flawed design.

Maybe the nForce chipset does things completely differently, but not as far as I know.

 

interesting, i'll have to look into how the nforce2 chipset handles it, as your explanation should seem valid. I suppose (unless there is a better explanation) that ASUS just figured that very few athlon XP users would ever use more than 2GB of ram, as its really not needed.

 

Sorry for being a bit short. I've had a rough week. Not an excuse for being mean, but just a reason for being dumb.

I looked it up in the manual for your mobo, and yes, they designed it like I said above. Attached is an excerpt for page 14 of the manual (page 28 of the .pdf document). Notice that they essentially say "don't use sockets 1 & 2 alone or you get no dual channel".

 

I think they're assuming that you'll buy 2x 512 MB PC3200 and put them into 1 & 3. Later, you'll upgrade with 1GB of PC3200, and put it into socket 3, and move the 512 into socket 2. That way you'll have 1 GB on each channel.