[Colonel Panic]

Are there any downsides to using mixed RAM modules in a computer (specifically an iMac?). The machine has four slots, currently containing 2x 4GB modules; I have an additional 2x 16GB modules.

Am I better off using all four (total 40GB, or just removing the 8GB and settling on a total of 32GB. Does it matter which slots are used for each module?

FWIW, the 4GB modules are Apple supplied, the 16GB come from Crucial. All are DDR-4-2400 SODIMM.

TIA

[stevelup]

It's inadvisable to mix them.

I would remove your Apple RAM and just fit the new Crucial stuff.

[OCB]

Why not try with all 4, and run some speed test app? As an engineer I tend to like seeing figures though

They're the same speed and latency rating most probably - Crucial are generally very good for that.

I was in similar situation 3 or so years back. iMac had 2x2gb (slowwww....)
Added 2x4gb, bringing it to 12gb. Night and day difference.

Certainly on my (2010) iMac adding ram is astoundingly easy. 2 Philips screws. I saw how much dust the thing had consumed, so it got a good vacuuming as well.

[Colonel Panic]

Why not try with all 4, and run some speed test app?

Any suggestions as what might be a good test app (and easy to use, i.e. not needing Terminal etc.). It'll be running a fresh install of Sierra 10.12.

TIA

[Lindsayp]

If it has been shown that you can put 16Gb into a 2010 iMac and it uses the memory, why does Apple tell me my 2009 iMac can only have a maximum of 8Gb? It doesn't particularly need to be upgraded but the extra speed would be nice.

[stevelup]

Without you quoting the exact model, I can't be sure. But that year's models should accept 4GB of RAM per slot. 4 slots... 16GB.

CP, if you really want to leave all the RAM in, go for it, but I don't think there's any point. The performance delta you may see between 32GB and 40GB will be imperceptible and therefore not worth the instability risk.

[OCB]

CP, fire over the exact model number. That might help.

True that you'll rarely even get close to 20gb of memory usage unless you're heavily into video editing etc.

I'll check over the weekend when I'm back home what app I used on my iMac to check performance.

For the question on 2009 iMac, probably processor or motherboard chipset limitations. Not much you can do other than live with it or flog it. Old iMac still get a few quid on eBay etc.

[Colonel Panic]

27" iMac with 5K display (mid 2017)
4.2GHz QC i7 (turbo boost up to 4.5GHz)
8GB 2400MHZ DDR4 (2x4GB)
1TB Flash storage
AMD Radeon Pro 580 (8GB)

I _think_ it is "model 18.3"

[Lindsayp]

I'm not too bothered about upgrading the 2009 iMac, I don't really need extra grunt from it for its current job, and I bought a new one earlier this year for the heavier workload. It was just curiosity. Thanks Steve