When I got my 27” iMac in 2017 it came with 8GB of RAM. Being one of the few Apple Macs with user upgradable RAM1, I decided to avoid that part of the Apple tax and buy my own. The stock 8GB came in two 4GB sticks to enable dual channel memory. The iMac had 2 spare slots, so I ordered 16GB more as two 8GB sticks.
It was only when I opened the package that I realised I bought a single 16GB stick instead of two 8GB sticks. This would give the iMac three sticks RAM, which meant it couldn’t use dual channel memory.
Being impatient, instead of exchanging it for what I really wanted, I kept it and thought I would just buy another 16GB stick when the price is lower.
###You don’t know what you’re missing
It turned out that for over 18 months RAM prices would remain above what I paid for my original 16GB stick. I didn’t really use all of the 24GB of RAM I had, so I decided to keep on waiting. I never really felt like my iMac was slow2, and didn’t think single channel memory would make much of difference.
Recently that the price dropped to a point where it didn’t feel like I was just burning money to satisfy a curiosity,
So now I had dual channel memory again, is there any real difference?
###You probably weren’t missing much
I ran Geekbench 4 just before and after installing the final 16GB RAM stick, and I was surprised to see over 10% increase with dual channel memory. Single-core test resulted in 5,799 vs 5,249, and multi-core 20,403 vs 17,485. The detailed results show small increases across the tests, with memory bandwidth almost doubling. Certain multi-core tests had a big difference, probably ones which were being limited by the memory bandwidth rather than CPU performance.
I also tried testing the time it took to do a clean full archive of Hackers, but it was only faster by one second3 with dual channel memory. Apart from playing games4, compiling swift is probably the most taxing thing I do on this Mac. Adding that new memory stick will make no noticable difference to that.
Does dual channel memory make a difference? Only if you’re doing something that is constrained by memory bandwidth. If you don’t know if you’re doing something that is memory bandwidth constrained, that means you probably aren’t. I’ve spent £60 and gained nothing except a slightly better Geekbench score, but your mileage may vary.
Hi, I'm Weiran Zhang. I work as a Senior Software Engineering Manager at Capital One. I have a passion for technology and building thriving software teams. This blog is a place for me to document things I've learned and things I find interesting. You should follow me on Twitter.