Question regarding Quad Channel Memory Bandwidth with 8 DIMMS installed

[clayton006]

So this may seem like a simple question but so far I haven't able to answer it with my own testing.

Using the rig in my sig (2011 socket) I was experimenting with different RAM amounts. I needed 64GB for all of the VM stuff I was doing at the time (and, well, could use it again now). I noticed that when I would run Sandra benchmarks checking memory bandwidth, I would get almost exactly 1/2 the bandwidth had I only had 32GB installed.

At first I was surprised and then I realized it may have something to do with 2 DIMMs per channel vs 1 DIMM per channel and the possibility that if you had more than one DIMM per channel that bandwidth may be reduced?

I thought this was also a concern on SB (and later) Xeons with how their memory controllers worked for much larger amounts of RAM?

Does anyone know the answers to these questions or has observed the same behavior?

 

[HobartTas]

You sure you got them placed right? Socket 2011 channels come in adjacent pairs so to populate with 4 sticks you place them into slots 1,3,5,7 or 2,4,6,8. If you put them into 1,2,3,4 or 5,6,7,8 your only using 2 channels and this would account for half the expected bandwidth.

If you have placed them correctly then I'm guessing the board bios may be too old or the 8GB memory is somewhat funny in being double banked instead of single banked or something equally weird. Socket 2011 should easily handle 8GB sticks as I have three of them working in my X58 board. I had this hassle with double banked SDRAM 100 Mhz memory but in that case you put in a 128MB stick and it would only see 64MB of it.

If its not any of these then I'm all out of ideas amd can't help you any further.

Cheers

 

[clayton006]

Yeah I placed them in correctly. I used Crucial Ballistx and Corsair RAM and both had the same issue. This very well could be this ASRock board being weird, but I did notice that the Asus Maximus Extreme board I was using I believe did the same thing.

I just didn't know if this was a technical limitation of having all 8 DIMMs populated or if it should always run roughly at 46 ~48 GB/sec bandwidth?

 

[HobartTas]

I wonder what accounts for the lower bandwidth? does memory speed get reduced or are timings increased perhaps? its strange that its exactly half the bandwidth unless perhaps it does a memory access to the first stick and then does the same for the second one in that channel leading to a doubling of time taken and hence lowering the bandwidth by exactly half, if not any of these things then at this point I have no real idea.

Also does a similar thing happen with

(a) say Gigabyte manufactured boards, or
(b) any manufacturers server 2011 boards, or
(c) has this been observed in even older X58 boards?

I think the only place to ask this question is to post this on the Intel Community forum boards in the Chipset sub-forum and see if you might get an answer there. If not the send an e-mail to Intel directly.

Cheers