Question on installing 2 pairs of RAM w differing capacity

[mda]

Hi All,

Just as a question --

I have 2 sets of RAM:

2x16GB Corsair LPX 3200
2x8GB Corsair LPX 3200

They both have the same primary timings so XMP should run them properly.

My question is, how should I populate the memory slots for optimal dual channel configuration/performance?

Typically, the motherboard arranges its DIMM/channels in the following way:
A1, A2, B1, B2

Right now, I have the DIMMs installed as follows.
A1 - 8GB
B1 - 8GB
A2 - 16GB
B2 - 16GB

So it's basically arranged like this on the board:
8GB / 16GB / 8GB / 16GB

I'm not sure at this point if the same capacity DIMMs should be in the same channel? (16GB Dimms in Channel A, 8GB DIMMs in Channel B) or no?

Which tests can I use to help prove which configuration will provide the best performance?

The PC is used for VM work so RAM capacity is more important than the actual speed right now, buuuut extra performance wouldn't hurt either!

Can't seem to find any info floating around on the web.

Thanks!

 

[viivo]

Same capacity modules should be on the same channel. Other than that, with the same timings, I can't imagine there being any functional or real-world performance differences.

 

[BinarySynapse]

Same capacity modules should be on the same channel. Other than that, with the same timings, I can't imagine there being any functional or real-world performance differences.

This is incorrect. Each channel should have matching total capacity for best performance. If you have 16GB (8+8) in one channel, and 32GB (16+16) in the other, the top 16GB in the second channel will be slower. If you put 24GB (16+8) in each channel, then the entire 48GB will be accessed in dual channel mode.

 

[mda]

This is incorrect. Each channel should have matching total capacity for best performance. If you have 16GB (8+8) in one channel, and 32GB (16+16) in the other, the top 16GB in the second channel will be slower. If you put 24GB (16+8) in each channel, then the entire 48GB will be accessed in dual channel mode.

Thanks! I actually had both trains of thought going on in my head. So I think I'll leave it where it is at the moment.

 

[kirbyrj]

Do a memory bandwidth test and see what gets you better performance. I think if you don't have matching capacities in each channel you will revert to single channel.

I think that the answers given are slightly vague. The way you have your setup is the way that ryan_975 described it. viivo sounds like he actually means what ryan_975 said, he just used the term "channel" incorrectly. You can have more than one set of memory in a channel (I don't know what the technical term is, but "rank" maybe?). So basically you want 1x8GB stick and 1x16GB stick in each of the dual channels, and match the 8GB sticks and 16GB sticks together in the same rank of each channel (like you did).

 

[BinarySynapse]

Do a memory bandwidth test and see what gets you better performance. I think if you don't have matching capacities in each channel you will revert to single channel.

In the early days of dual channel memory controllers, that was true, but modern memory controllers have independent channels and can intelligently interleave access to them up to the capacity of the lower channel.

I think that the answers given are slightly vague. The way you have your setup is the way that ryan_975 described it. viivo sounds like he actually means what ryan_975 said, he just used the term "channel" incorrectly. You can have more than one set of memory in a channel (I don't know what the technical term is, but "rank" maybe?). So basically you want 1x8GB stick and 1x16GB stick in each of the dual channels, and match the 8GB sticks and 16GB sticks together in the same rank of each channel (like you did).

Rank is a specification of the DIMM modules themselves. Bank might be the closest... but slot number is what we have. So A1 B1, etc.

I’m not entirely sure how today’s systems would handle mismatched DIMM configurations with equal total capacity. I’m going to have to try some things out and see.

 

[xx0xx]

^ Yeah to save money but still upgrade, I've been considering upgrading my 16GB (2x8GB) to 32GB by just buying a 16GB stick and putting it in a third slot, so I would have 2x8gb on one channel and 1x16gb on the other channel. This is because one of my slots is blocked by my cooler.

My other option would be to just buy a 2x16gb kit to replace the 2x8gb. I'm not sure the performance difference would be enough to justify that or not.