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JNavas
WWAN
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Registered: ‎08-07-2010
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Message 1 of 6

Memory: dual rank vs single rank

When you add memory to your ThinkPad, you may have a choice between single rank and dual rank modules, often at the same price. For example, to expand the T460p from 8GB single module to 16GB two modules, either the Crucial CT8112709 (single rank) or the Crucial CT8112711 (dual rank) for the same price.

 

Dual rank is a bit like dual channel in that interleaving (on module in the case of dual rank) can boost performance, although dual rank is not as efficient as dual channel, since the data bus is shared between ranks.

 

When there is no difference in price, choose dual rank over single rank:

  • Dual rank can have a small performance advantage over single rank (of up to 5% or more) in certain applications.
  • There are no issues with dual rank in one channel (slot) and single rank in the other channel (which might happen with an original single rank module).
  • Replacing single rank with dual rank probably isn't worth the money, since the typical performance gain is usually quite small.

Even more important, choose dual channel (two modules) over single channel (one module) for best performance, a difference of up to 20% in certain appliations.

 

Notes:

  • For best performance, match dual channel modules as closely as possible; e.g., two 8GB dual rank modules for a total of 16GB.
  • There is neither benefit nor drawback to using compatible memory that is faster than specification; e.g., DDR4-2400 vs DDR4-2133 in the case of the T460p. The faster memory will be clocked at the specification speed, not the faster speed, and timings (e.g., CL) will be set according to the specification speed.
  • The benefit of using more expensive memory with slightly lower timings (e.g., CL16 vs CL17) is usually too small to care about.

References:

John ~ ThinkPad T460p
[ Win8.1Pro64 | i5-6440HQ | 16GB | 1TB SSD | WQHD | 802.11ac | BT | Cam | FR ]
pjc123
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Message 2 of 6

Re: Memory: dual rank vs single rank

Quote "Dual rank can have a small performance advantage over single rank (of up to 5% or more) in certain applications."

 

This is completely opposite to everything that I have read about while deciding which memory to get a few days ago.  I even chatted with Crucial about this issue to make sure, and they verified that single rank is slightly faster than dual rank.

JNavas
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Message 3 of 6

Re: Memory: dual rank vs single rank


@pjc123 wrote:

Quote "Dual rank can have a small performance advantage over single rank (of up to 5% or more) in certain applications."

This is completely opposite to everything that I have read about while deciding which memory to get a few days ago.  I even chatted with Crucial about this issue to make sure, and they verified that single rank is slightly faster than dual rank.


Did you check my references (including Crucial)?

There are some cases where single rank is preferred:

  • From What is a memory Rank (Crucial)?
    "The drawback with higher Ranked modules is that servers sometimes have a limit on how many Ranks they can address. For example, a server with four memory slots may be limited to a total of eight Ranks. This means you can install four Single Ranked modules or four Dual Ranked modules but only two Quad Ranked modules, as installing more would exceed the amount of Ranks that can be addressed." That applies only to (certain) servers, not laptops.
  • From Upgrading a Server or Workstation? Read this first! (Crucial):
    "Higher rankings can also impact your memory bandwidth, forcing your memory to run at lower speeds when high-ranked parts are present. This trade-off may be required to utilize higher capacities of memory in some systems, though, if single- or dual-rank parts are not available at capacities desired, or if reduced ranking parts are not compatible with a system at higher memory totals. If none of these restrictions apply, there is otherwise no functional difference between rankings of parts." That applies only to quad rank or higher -- single and dual rank usually have the same timings -- and thus isn't relevant to laptops.

On dual rank versus single rank performance:

  • From DIMMs: Single vs. Double vs. Quad Rank (quoting IBM):
    "... Whenever possible, it is recommended to use dual-rank DIMMs in the system. Dual-rank DIMMs offer better interleaving and hence better performance than single-rank DIMMs." [emphasis added]
    "For instance, a system populated with six 2GB dual-rank DIMMs outperforms a system populated with six 2GB single-rank DIMMs by 7% for SPECjbb2005. Dual-rank DIMMs are also better than quad-rank DIMMs because quad-rank DIMMs will cause the memory speed to be down-clocked."
    "With the Xeon 7500/6500 processors in the x3850 X5, having more ranks gives better performance. The reason is because of the addressing scheme, which can extend the pages across ranks thereby making the pages effectively larger and therefore more page-hit cycles."
    Confirmed in SERVER -- memory rank -- single, dual, quad ?

Explanation (see Memory Timing Analysis): Memory read timing is a sequence of Preparation (latency), Data read, and Recovery (next cycle cannot be started until memory has recovered).

  • In sequential read of locations 0 and 1 from single rank memory, this results in something like P0P0P0P0P0D0R0P1P1P1P1P1D1R1.
  • With dual rank memory organized with even addresses in rank 0 and odd addresses in rank 1, the read of location 1 can be started in rank 1 while rank 0 is recovering, something like P0P0P0P0P0D0P1P1P1P1P1D1R1.

All that said, it's not worth worrying about, as stated in my initial post: "Replacing single rank with dual rank probably isn't worth the money, since the typical performance gain is usually quite small."

 

UPDATE:

  • Crucial CT8112709 (single rank) is also known (same part) as CT8G4SFS8213.
  • Crucial CT8112711 (dual rank) is also known (same part) as CT8G4SFD8213.
John ~ ThinkPad T460p
[ Win8.1Pro64 | i5-6440HQ | 16GB | 1TB SSD | WQHD | 802.11ac | BT | Cam | FR ]
pjc123
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Message 4 of 6

Re: Memory: dual rank vs single rank

Thanks for clearing that up.

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beavis87
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Message 5 of 6

Re: Memory: dual rank vs single rank

Is there comparability issues when installing dual rank RAM into a laptop with just one slot? I believe the RAM installed in my Ideapad 510s is single rank. When I installed an 8GB Crucial module it said dual rank (used their compatibility tool), and it would not post. Is there 16GB single rank? I'm finding that 8GB isnt enough and while Crucial's website says it can support 16GB, which is dual rank, I cant even get a 8GB dual rank to work. 

Ideapad 510S-14IKB (80UV001BUS)
JNavas
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Message 6 of 6

Re: Memory: dual rank vs single rank


@beavis87beavis87 wrote:

Is there comparability issues when installing dual rank RAM into a laptop with just one slot? I believe the RAM installed in my Ideapad 510s is single rank. When I installed an 8GB Crucial module it said dual rank (used their compatibility tool), and it would not post. Is there 16GB single rank? I'm finding that 8GB isnt enough and while Crucial's website says it can support 16GB, which is dual rank, I cant even get a 8GB dual rank to work. 


There should not be comparability issues when installing dual rank.

Ideapad 510s spec8GB max / 2133MHz DDR4, one 260-pin SO-DIMM socket

However, 16GB should work (single or dual). Likewise 8GB dual rank.

You may have gotten a bad module. Or you may not have seated it properly.

John ~ ThinkPad T460p
[ Win8.1Pro64 | i5-6440HQ | 16GB | 1TB SSD | WQHD | 802.11ac | BT | Cam | FR ]

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