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Dell r710 Ram stuck running at 800mhz


We have an interesting scenario, were we have 18x 8gb DDr3 RAM (identical)  capable of 1333mhz. Anyhow for some reason when we install all 18 and fill up the banks memory is only configured to run at 800, but if we remove a few it will bump up to 1333.

We have 2 x X5570 and 18 x 8GB PC3L-10600R 2Rx4 all the same brand. All firmware has been updated and ram has been set to optimizer mode within the bios.

We are wondering where the bottle neck is that is causing the ram to be reduce to run at 800mhz.

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Gary CaseRetiredCommented:
It's behaving exactly as detailed in the technical specifications.

See Page 33 of the Technical Guide for details:

Gary CaseRetiredCommented:
... by the way, this is fairly common.   When you have more than a specified number of active ranks, the system slows down the memory speed a bit to ensure reliable memory operations.
PlumInternetAuthor Commented:
Thanks Garycase, that then raises the question, is more but slower ram better than less but fast ram?
For most commercial applications more slow RAM is better than less fast RAM. See http://globalsp.ts.fujitsu.com/dmsp/Publications/public/wp-sandy-bridge-ep-memory-performance-ww-en.pdf page 15 (19). The chart shows the STREAM benchmark running at 1.82 times what it would run at with the lower speed RAM but the SPECint benchmark only shows 1.15 times performance boost at the higher speed. Commercial apps behave like SPECint, STREAM is designed to test RAM speed and the only things that behave like that are heavy duty maths programs such as weather forecasting.

It's a great whitepaper, Fujitsu deliberately configured their server badly to show that it doesn't really have that much effect despite Intel telling you to balance the RAM or performance will suffer. Applies equally to 55/5600 CPUs, they've done a whole bunch of them if you google for "fujitsu ram speed whitepaper".
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