32GB v 40GB for 2017 iMac

[mreg376]

In a review over on Amazon someone said they did benchmarks and discovered that he got 7-8% more speed out of two 16GB RAM modules alone for a total of 32GB, instead of adding the two 16GB modules to the stock two 4GB modules for 40GB total. Can there be any truth to that?

 

[Darmok N Jalad]

Possibly so. The RAM runs in dual channel, and when you populate all 4 slots, the system will set the RAM to run at the speed/timings of the slowest RAM installed, or possibly even slower for the sake of stability. This is not by Apple's design, but that of the Intel (like any other) memory controller. Also, there's usually more complications with running mixed capacity, even if you're running 20GB x 20GB in dual channel. And what your Amazon reviewer refers to as "more speed" is probably actually "more bandwidth."

All that said, the gain you'll see in virtually all usage scenarios from that extra 7-8% bandwidth is not going to be realized in anything other than synthetic benchmarks. A quad core iMac simply doesn't need that much bandwidth, so even a slightly handicapped memory system isn't going to be a problem. You will see benefits from the extra 8GB, on the other hand. How much so depends on how much memory pressure you put on your system.

Lastly, I added an extra 8GB to my 5K iMac, to go with 4 sticks of 4GB each. I actually saw a small increase in memory bandwidth, so it's possible the Amazon reviewer had his system misconfigured, where 8GB was in one channel, and 32GB was in the other. The system likely dropped to single channel mode.

 

[davidmartindale]

W/O knowing the spec of the RAM it is hard to give a yes or no. But I will say that if the RAM in the Mac was say 1866MHz DDR3 and then the replacement modules were 2066MHz and you left the default RAM in it will only run at 1866MHz but if you swap it out entirely and all the RAM is 2066MHz than it will run at that speed.

In other words all the RAM will run at the speed of the slowest RAM module. Also Dual and Quad Channel RAM is something to consider as well

 

[Darmok N Jalad]

I should add that you could always try to remove the extra 8GB and see if you notice a difference. I'm betting no. I had a similar issue with my old Mac Pro, where the CPU had triple channel mode. If you used 4 sticks, it dropped to dual channel. Even with the fastest 6-core/12-thread CPU available installed, having more RAM was better than having triple channel memory. I couldn't find a real-work benchmark (like Handbrake) that showed a performance gain outside the margin of error for such testing.