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I have a system that is using 4 GB of memory in two DIMM slots for dual channel memory. Is it okay to add another single stick of memory that is not pair with another memory module?

Are there any benefits to this?

What are the drawbacks?

Are there any performance charts that details the differences?

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What motherboard do you have? – Dennis Mar 25 '13 at 21:19

Are there any benefits to this?

There is the obvious benefit of having more memory to work with.

What are the drawbacks?

  • Either all memory access drop to single channel mode. slowing things down slightly, or
  • Only this single DIMM will be accessed in single channel mode.

Are there any performance charts the detail what is going to happen?

Dozens, and none tell precisely what will happen because it varies per program.

If you program is CPU bound (that means it is waiting a lot on the CPU) then speeding up memory access will usually not do much. If it is IO bound (e.g. waiting on a disk) then the program will not speed up either. If it is memory bound (e.g. huge matrix multiplication which are way to large to fit into the CPU's cache) then speed can theoretically double.

Theoretically. Practical speed difference for average usage is about 5%.

You will need to balance loss of those 5% (on average) with the speed increase by getting more memory. Usually the extra memory wins. But once more: That differs a lot on what you do.

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Is there an easy way to tell which of the two drawbacks will occur? – David Mar 25 '13 at 21:39
No. It depends on the motherboard (actually, the memory controller on the northbridge or in the CPU) and the way dual channel is implemented. And possibly also on the way the BIOS/UEFI configures things. – Hennes Mar 25 '13 at 21:48

How much memory is utilized by the system also depends on the Operating System, and whether it is a 32bit version or 64 bit version etc.. running on it. and the Memory it can support.

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