Does a 4GB + 8 GB DIMM outperform a single 8GB DIMM?

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I observed a Windows 10 x64 + OS build consuming 3 GB of the installed 4 GB RAM when idle (before I ran much of anything else).

When I initially opened the case, I thought I only saw one slot so I ordered one 8 GB DIMM figuring doubling down would solve my problem. After I removed the Fan-shroud as I went to replace the DIMM I saw that there was actually two slots so I left the already installed DIMM in and also installed the new one. and Windows 10 corroborated 11.9 GB in Task Manager.

Now, I have read that I might get better performance if I had a pair of identical DIMMs installed so it would have been most cost effective just to buy a second 4 GB.. However, correct me if I am wrong but 12 GB of RAM still beats merely using the one 8 GB DIMM as 12 GB at single dimm speeds is still way faster than having to swap/page out to disk for additional virtual memory, right?

Tom

Posted 2016-10-05T13:45:48.380

Reputation: 1

Possible duplicate of How does the number of sticks of RAM influence speed, for the same total amount of RAM?, Is it better to have four times 2 GB or two times 4 GB of RAM?, etc.

– Ƭᴇcʜιᴇ007 – 2017-05-11T17:28:56.677

Answers

If you have a motherboard that uses a dual channel memory controller, then two DIMMs will give you better performance than one 8GB DIMM. However, if one DIMM has less MHz than the second DIMM, The second DIMM will match that MHz speed meaning you will only lose performance.

The reason for this is that the two-channel controller can read or write two DIMMs in parallel, much like a road with two traffic lanes in each direction can handle twice as many cars.

That being said, because of CPU caches the overall performance of the system is not very dependent on RAM speeds and you probably won't notice any difference at all unless you run benchmarks.

Dylan Rz

Posted 2016-10-05T13:45:48.380

Reputation: 658

Darn the price on the 8 GB 1600MHz DDR3 jumped from $35 to $45. I should have looked for a MHz rating on the four when I had it in my hand.. For reference the newer Eight GB DIMM was about a half inch high and the original four GB DIMM was closer to an inch so it is reasonable to assume there was a technology gap making the older Four GB slower such as 800 MHz --- which is apparently slowing my Twelve GB down to the slower speed. I will monitor RAM consumption and prices. If I use less than eight GB, I will remove the four. If I use more than eight GB, I will buy a second 8 GB 1600MHz DDR3. – Tom – 2016-10-05T14:55:45.660

@Tom, I was about the suggest the same thing. See how your machine works with just the 8 GiB. I must surely feel faster - as your have 1. doubled the RAM 2. Have a higher speed RAM installed. – Prasanna – 2016-10-05T16:29:11.063

I just opened 4x Office product, 4x different browsers, Adobe Acrobat READER, & Amazon Music were using 6.3 GB. I had to start opening all of those little "Apps" that are actually designed for Surface tablets to consume 8.9 GB. I am not runing the big Adobe Publisher, a CAD/CAM App, or a SQL/Exchange DB here so I am wondering if my NORMAL usage will ever crest 8GB.. It is probably worth another shutdown, peel the fan shroud off to look for a sticker on that 4 GB DIMM... If it or google market 1600 MHZ the FOUR can stay.. If not it should almost certainly be removed.. – Tom – 2016-10-05T17:53:36.270

I should have examined the BOTTOM of the Task Manager / Performance / Memory report as it reports I am using 2 of 2 slots @ 1600 MHZ. I MIGHT have gotten better performance and saved $20 to just order a second FOUR GB. However, I am not slowing my Eight down since the Four must be rated for the same speed for Windows to report it as thus. I am best off leaving all twelve in there since they must both support 1600 MHZ..... – Tom – 2016-10-05T23:35:47.700

Yes, 12GiB of memory is usually faster than just 8GiB.

On most motherboard this will come down to:

8GiB (normal) vs 8GiB (normal) + 4 GiB (normal)

On some motherboards it might even be:

8GiB (normal) vs 8GiB (first half used as dual channel) + 4 GiB (part of dual)

But even if it does not manage to use a dual channel memory interface it will still be faster than just 8GiB, also without dual channel.

I might get better performance if I had a pair of identical DIMMs

Yes, that would be even better. But both solutions will trump using just one DIMM.

Disclaimer for a very unlikely situation:

If you have a very memory intensive task addressing large amounts of memory (read: way bigger than gets cached).
and the second DIMM is significantly slower than the other
and the CPU/memory controller can make use of the faster memory.
and the OS and program and data does not use more memory than available (so no use of the pagefile is needed)

then a single faster DIMM might outperform a fast+a slow DIMM setup. Really unlikely though.

Hennes

Posted 2016-10-05T13:45:48.380

Reputation: 60 739