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Memory ranks with Opterons
Posted on 11-05-2005 03:28:00 UTC | Updated on 11-05-2005 03:28:00 UTC
Section: /hardware/memory/ | Permanent Link

I've been dealing with AMD dual-core Opterons lately and have learned some valueable information about the memory it uses. If your trying to use DDR 400 with a Opteron 246 or higher you might have problems running it at DDR400 speeds if you don't pay attention to memory rank limitations of the Opteron's. First lets read about what memory ranks are:

### Definition of memory ranks ###

   #  Single Rank Memory
   defines a set of DRAM chips (on a module) comprising 8 byte wide (64 bits) data,
   or 9 bytes (72 bits) with ECC.  All devices in a Rank are connected by a single
   Chip-Select.  The actual memory size is not defined.  Single-sided memory
   modules are always Single-Rank.  Double-sided unbuffered DIMMS and SODIMMS are
   always Dual-Rank.  Server DIMMS may have up to 4 ranks.  Single-rank memory
   modules will allow a server to reach its maximum memory capacity and highest
   performance levels, whilst enabling support for the latest server chipset and
   motherboard features.

   #   Dual Rank Memory
   defines 2 sets of DRAM chips (on a module) each comprised of 8 byte wide (64
   bits) data, or 9 bytes (72 bits) with ECC.  All devices in a Rank are connected
   by a single Chip-Select.  The actual memory size is not defined.  Normally a
   module will have one rank per PCB side.  Dual-rank modules can be used if a
   server supports them.  These modules generall offer the best price/performance
   point but can limit overal system capacity or restrict future upgrade options.

   #   Quad Rank Memory
   defines 4 sets of DRAM chips (on a module) each comprised of 8 byte wide (64
   bits) data, or 9 bytes (72 bits) with ECC.  All devices in a Rank are connected
   by a single Chip-Select.  The actual memory size is not defined.  Normally a
   module will have two ranks per PCB side.

Kingston says that certain Opteron motherboards will only work with so many ranks of DDR400 per CPU. But the techs from the AMD fourms say it's a Opteron memory controller issue with DDR400 not a mobo issue. I beleive it's a Opteron issue and the motherboard type does not matter. That is what Kingston are not saying of their page. Either way it's a big problem. In linux of you don't follow the memory rank rules you will get machine check exception errors (MCE) doing any memory intensive task. The kernel will panic and you get a screen dump of memory addresses. What needs to be done is if more than 4 ranks of DDR400 memory are used per physical CPU then the memory must be clocked down to 333Mhz. So if your using an Opteron 246 or higher follow this rule.

If you go to Kingston's web page or other memory manufacture's webpages you will see memory being sold for Opteron boards with descriptions like "2GB 400MHz DDR ECC Reg DIMM Dual Rank x4" or "1GB 400MHz DDR ECC Reg Kit Single Rank x4". Kingston will sometimes tell you the limitation of the board on it's page if you searched for memory by motherboard.

For example Kingston says the Supermicro H8DA8 board can only support four ranks of memory per CPU running at DDR400 speeds. That would be 2 sticks of 2 rank memory or 4 sticks of 1 rank memory. If you put in more than 4 ranks total per CPU you must clock the bus speeds down to 333Mhz. If not you will hit the memory timing issue mentioned above. Usually if you want to make it affordable you will have 4 sticks of 2 rank memory. This is because your trying to get lets say 16Gig of RAM on a two way server (8Gig per CPU) then you might use 2Gig sticks. The sticks of 4 - 2Gig 1 rank or 2 - 4Gig 2 rank for the motherboard mentioned above cost an arm and a leg. But filling the memory banks with 2Gig 2 rank is a much cheaper way to get to 16Gig total but you won't be able to do it at DDR400 speeds. So you would be better off buying the 8 - 2Gig 2 rank DDR333 and taking the 66Mhz hit. Which for the money to performance ratio is not that big of a deal.

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