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Is it better to have 4x2GB or 2x4GB of RAM?

Given the choice between either 2x2GB or a single 4GB RAM module with all other advertised aspects being equal (same price, same CAS latency, same frequency, etc.) which one is preferred?

It seems to me that buying the single stick gives greater opportunity to upgrade (especially with a motherboard with only 2 slots anyway), however people recommend matching pairs, and I guess picking up a matching stick at a later date is somewhat unlikely.

It also seems to make sense that two sticks would give twice the performance of a single stick. However does this actually happen? The motherboard in question is dual channel (currently running 2x1GB) which I guess makes a difference here?

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2x4GB, of course! –  Daniel Beck Aug 15 '11 at 14:18

3 Answers 3

Reasons to go for two:

Main reason considered is for the extra throughput of the dual channel (accessing both RAM modules at once). The actual improvement from this is a subject of some debate though e.g. http://www.tomshardware.com/reviews/PARALLEL-PROCESSING,1705-15.html

As expected, the performance difference between single channel and dual channel DDR2-800 memory using an up-to-date Core 2 Duo system Compare Prices on Core 2 Duo Processors is little to nil, depending on the benchmark - most tests show differences, but they are really small. For games and enthusiast PCs, we recommend sticking to high-performance dual channel RAM, because the memory is one of those components that you want to perform best for a smooth experience. For regular applications, though, it doesn’t really matter much whether you run single or dual channel. Two 1 GB DIMMs typically are cheaper than a single 2 GB module, but a single DIMM will reduce your power consumption by several watts (which might just be more interesting than it is important).

Cost is also potentially a factor as two smaller DIMMs are often cheaper than the single large DIMM (as mentioned in the quote above)

Reasons against are those of extra heat and system load; and also the upgradability point that you raised initially.

There's debate on similar lines in the following SU question: Is it better to have 4x2GB or 2x4GB of RAM?

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+1 for dual channel, but you say that's only one reason? It's usually the only reason to run multiple DIMMs, otherwise you're just spreading extra heat and using more power in the system. –  Breakthrough Aug 15 '11 at 13:37
    
edited to add the second main reason I can think of (cost), although this one can be very variable and less of a factor than it used to be I think. As you say, the other arguments are generally to go for a single large DIMM –  Kris C Aug 15 '11 at 13:41
    
So the idea that 2 sticks is twice as fast as 1 is pretty much wrong? Heat/energy usage is somthing to consider since the computer in question is on a lot of the time, but even in the case of say a 24/7 server, does the RAM actually add up anyway? I only ever considered the PSU, CPU, GPU, etc power usage in my builds previously. Not sure what your getting at with system load? Are you saying that in certain uses 1 stick is faster because it loads the CPU less or somthing? –  Will Aug 15 '11 at 14:04
    
On system load I was referring to the extra signalling load of addressing two modules on the bus, which is described in a lot more depth in the "Is it better to have 4x2GB or 2x4GB of RAM?" question. On '2 sticks twice as fast' - maybe in some situations, but in general usage (going by the studies I can find, haven't done my own benchmark!), not the case. On the heat energy, it probably is fairly negligible. Given the cost of RAM and all the stuff above, the actual answer is probably "It doesn't really matter"! –  Kris C Aug 15 '11 at 14:26

Two factors playing here:

  • 2*2G gets some benefit from dual channel operation but
  • 1*4G leaves more memory slots free for future upgrades.

Per my experience having enough memory makes machine always feel faster than having dual channel operation. For that reason I personally prefer larger stick over smaller ones; You lose some small percent of theoretical bandwidth now, and win significantly later on, by being able to populate the free slots without throwing anything away.

With possible exception of gaming with AMD APUs, most current CPUs do not benefit overtly much from additional memory bandwidth, they mostly care about latency, which explains why the option 2 might be better right now. Look at the speed effects on Intel CPUs and AMD CPUs. As you can see, the difference between 800 and 1600MHz sticks is in most cases unnoticeable, some 16-17% at worst. This tells that the bottleneck for the application is something other than bandwidth.

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Is that using unmatched sticks at a later date, or did you manage to get another of the same to upgrade? –  Will Aug 15 '11 at 14:11
    
The sticks don't in practice these days need to be that matched to work in dual channel configuration; the capacity needs to be the same and the pair works at the speed of the slower one, but if those two are matched, on current processors it's usually enough. –  Zds Aug 15 '11 at 14:19
    
I'd agree that RAM throughput is not a big factor in general computing. Once there is a cache miss in the core and a RAM fetch has to be made, the performance hit is roughly the same unless you are dealing with massing streaming data. Most benchmarks shows that slight increases in RAM bandwidth, CPU clock speed, etc., only result in slight performance increases. L2/L3 cache is probably the only real factor in performance. –  goofrider Jul 29 '13 at 6:37

Almost all systems with onboard shared-RAM GPUs (nForce, Intel express graphics) will utilities dual channel RAM. The CPU will access the RAM through one channel while the GPU will access the shared VRAM in another. The performance boost is pretty significant on pre-sandy bridge systems.

Have not looked at whether on-die GPU behaves similarly, though I'd assume they should.

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