Will Mac Pro be slower if I only use one side of RAM?

[yalag]

I wanted to upgrade my new mac pro from the default config of 16GB ram (4x4) to 32GB. I went online and bought 2x16 but I'm just wondering if it matters if I'm only using 2 of the 4 slots in my mac pro? Do I suffer a performance penalty?

 

[AidenShaw]

I wanted to upgrade my new mac pro from the default config of 16GB ram (4x4) to 32GB. I went online and bought 2x16 but I'm just wondering if it matters if I'm only using 2 of the 4 slots in my mac pro? Do I suffer a performance penalty?

The improved performance from the extra memory will almost certainly far offset the theoretically slower 2-channel memory system.

There's a good thread at https://forums.macrumors.com/posts/18745317/ on performance with different memory configs. A couple of individual tests were sensitive to fewer channels, but the majority (and the composite) showed little effect.

The E5-x6xx v2 processors have very large caches (the L3 cache is larger than the first hard drive that I bought) so only apps that thrash the cache notice the memory bandwidth loss.

 

[sebseb]

Well most Xeon E5 V2 processors that are in the Mac Pro have 4 channels of ram, the difference with the old Mac Pro. Those had 3 channel but channel 3 and 4 used the same bus!

However since the E5 has 4 channels! it does not matter if you put the rams in one side! Each slot is a channel! you would be using 2 out of 4. Tt would have been better to purchase 4X8GB sticks! you still get 32GB but you use the max potential of the CPU.

 

[yalag]

Well most Xeon E5 V2 processors that are in the Mac Pro have 4 channels of ram, the difference with the old Mac Pro. Those had 3 channel but channel 3 and 4 used the same bus!

However since the E5 has 4 channels! it does not matter if you put the rams in one side! Each slot is a channel! you would be using 2 out of 4. Tt would have been better to purchase 4X8GB sticks! you still get 32GB but you use the max potential of the CPU.

I'm not sure what you are saying, it does or does NOT affect performance?

 

[flowrider]

^^^^To such a small degree that you may not even notice it.

Lou

 

[sebseb]

I'm not sure what you are saying, it does or does NOT affect performance?

As Lou said it's too little! But I honestly don't get why! Because the CPU has 4 slots of ram, so why would the performance even change if you put in 1&3 instead of 1&2?

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And about what I said, Rams connect directly to the CPU, therefore CPU's have memory channels! The old generation had 3, so you could only connect 3 rams to it! However apple put 4 slots. Slot 1 and 2 had their own channel and 3&4 shared a channel! That gives you 4 slots of ram with a 3 Channel CPU! So it was usually best if you only used slots 1,2,3! Using channel 4 would give you more Ram but lower the speed.

However the new CPUs have true 4 Channels! so in theory you should be able to use 1 through 4! You can put one ram, 2 rams or 4 and in any order! I would put 1&2 but I don't think 1&4 should cause performance decrease! you can try it out! use benchmark apps and see if it makes a difference and share the results!

 

[paulrbeers]

I'm not sure what you are saying, it does or does NOT affect performance?

I'm really not sure what sebseb is getting at. Why he is even bothering muddying the water with information about the previous Mac Pro is pointless and only adds confusion.

The flowrider and AidenShaw are right. It will have a minor impact, but (assuming you actually can use more than the base 12-16GB the Mac Pro comes with) the advantage of having the RAM will more than offset the few percentage points in overall speed loss.

 

[AidenShaw]

cache will benefit from more RAM

...but (assuming you actually can use more than the base 12-16GB the Mac Pro comes with) the advantage of having the RAM will more than offset the few percentage points in overall speed loss.

Be aware that even if you don't think that your apps need more than the base, the system will make use of it for filesystem and other caches. Getting rid of IO by caching data can be huge - RAM speed compared to typical SSD speed is like an SSD compared to a floppy drive.

I have a drive with several million JPGs and other images (mostly expanded by multiple sizes of thumbnails and some movies converted to stills). When I touch the drive for the first time after a reboot - slow as hell to scan for files. After one scan, though, it flies. If I look at the filesystem caches, I'll see 10 GiB of directories and file metadata in cache! The system basically turned under-utilized RAM into an in-memory filesystem.

 

[mcaswell]

I was initially hesitant to go the 2x16gb route instead of 4x8gb, but found a thread or two (I think one of them was the one posted above) that indicated the performance difference would be negligible.

One other benefit... if you ever decide to upgrade to 48gb or 64gb, you can simply add one or two more 16gb modules without having to remove your current RAM and starting over.