Ask: Using identical RAMs (dual channel) but the only difference is Capacity

[bakanisan]

I know there are plenty of answers out there but there's no answer to this specific question:

Is there any differences between using identical RAMs vs using RAMs with the same specs except for capacity?

IE: 2 sticks of DDR3 2GB 1600MHZ vs 1 stick of DDR3 2GB 1600MHZ + 1 stick of DDR3 4GB 1600MHZ? Please ignore the difference in total capacity. I know it has some impact but aside from that, what are the differences?

[Jurrunio]

Different capacity in different channels means you lose speed when 1 channel runs out of memory

 

There are more if you're talking about higher frequency ones. But I dont remember seeing any 2GB DDR3 1600, those top at 1333 from my memory

[Eigenvektor]

If your board does not support flex mode, you'll be running in single channel, meaning you lose half of your memory bandwidth. If your board does support flex mode, then only some of your RAM will run in dual channel mode, the rest will run in single channel mode.

 

https://www.compuram.de/blog/en/single-dual-and-multi-channel-memory-modes/

[bakanisan]

20 minutes ago, Jurrunio said:

Different capacity in different channels means you lose speed when 1 channel runs out of memory

 

There are more if you're talking about higher frequency ones. But I dont remember seeing any 2GB DDR3 1600, those top at 1333 from my memory

It means I'm good for 4GB in Dual channel then? After that it gets throttled?

[Jurrunio]

18 minutes ago, bakanisan said:

It means I'm good for 4GB in Dual channel then? After that it gets throttled?

yes

[Eigenvektor]

18 minutes ago, bakanisan said:

It means I'm good for 4GB in Dual channel then? After that it gets throttled?

Depends on whether your board/CPU supports flex mode.

 

If it does and you mix 4 + 2, then half of the 4gb stick will run in dual channel mode with the 2gb stick, while the remaining 2gb will run in single channel mode.

[Raintech]

32 minutes ago, bakanisan said:

It means I'm good for 4GB in Dual channel then? After that it gets throttled?

Short answer: Yes

Long answer: Kind of. It depends alot on your motherboard and how its RAM modules are laid out. Assuming you have a typical motherboard with 4 total ram slots, (2x dual channels), then you should be fine to run in Flex Mode, in which case the 2 sticks will be treated as completely non-identical ram modules. Just make sure you're using Slot#2 and Slot#4.

Your motherboard will check the 2 sticks and run both at the lower frequency, (in this case its the same frequency), and it will run in dual-channel up until the 2gb capacity on both sticks, (aka 4gb total, 2gb per stick), with the last remaining 2gb running unpaired, and therefore losing about half your memory bandwidth.

Another way of thinking about it is like this: You have 6gb total capacity, 4gb capacity is run at full speed, the last 2gb is run at half speed.

If your motherboard doesn't support flex mode, then you may run into issues like instability, all 6gb being run in single-channel, or one of the sticks not being detected whatsoever. (though these are unlikely with a modern motherboard). In this worse case scenario, you can usually overcome the instabilities by changing which slots your 2 ram stacks are plugged into, ie switching the 4gb stick from Slot#4 to Slot#2, but that's going into non-flex channel territory and proprietary bios behaviours which changes on a brand by brand basis.