2-2-1-1GB or 2-2-2GB for 2009 quad-core Mac Pro?

[macaleph]

I have a 2009 (Nehalem) quad-core Mac Pro with the base memory setup:

Slot 1: 1GB
Slot 2: 1GB
Slot 3: 1GB
Slot 4: empty

I'd like to upgrade to 6GB and am considering moving to either

1
Slot 1: 2GB
Slot 2: 2GB
Slot 3: 2GB
Slot 4: empty

or, reusing two of my 1GB DIMMS,

2
Slot 1: 2GB
Slot 2: 2GB
Slot 3: 1GB
Slot 4: 1GB

Setup 2 is a cheaper upgrade, and it gives each of the three channels (Slot 1; Slot 2; Slots 3 & 4) the same amount of memory. But is it as fast as Setup 1?

Thanks.

 

[Hellhammer]

Real world gains of triple-channeling are from tiny to non-existent. I would just go with the cheaper option, I doubt you would notice the difference between them unless you use apps that can really take advantage of the extra memory bandwidth that triple-channeling offers (only very few apps).

 

[macaleph]

Real world gains of triple-channeling are from tiny to non-existent. I would just go with the cheaper option, I doubt you would notice the difference between them unless you use apps that can really take advantage of the extra memory bandwidth that triple-channeling offers (only very few apps).

Would the second setup give me triple channeling though?

 

[Hellhammer]

Would the second setup give me triple channeling though?

It would give you dual-channeling. Theoretically tri-channel is 50% faster but as I said, only very few apps benefit from the extra memory bandwidth. Even dual-channeling is faster than what most apps can take advantage of.

 

[macaleph]

It would give you dual-channeling. Theoretically tri-channel is 50% faster but as I said, only very few apps benefit from the extra memory bandwidth. Even dual-channeling is faster than what most apps can take advantage of.

Thanks.

So we only get triple channeling if the first three slots are filled with equally sized DIMMS and the fourth is empty, right?

How can I tell whether a given combination of sizes will give me single or double channeling?

 

[Hellhammer]

So we only get triple channeling if the first three slots are filled with equally sized DIMMS and the fourth is empty, right?

Yes, I think so.

How can I tell whether a given combination of sizes will give me single or double channeling?

If you install them in pairs (2x2GB in upper slots and 2x1GB in bottom slots IIRC), then dual-channeling will be enabled. If you installed only one module or one in first and one in third slot, then they would operate at single-channel speed.

Maybe someone can confirm that. I'm not 100% sure about the slot organization (what slots are "pairs" with each other)

 

[C. Alan]

Isn't there a read-write bench test that could confirm if triple channeling is enabled? I run 2-2-1-1 in my 2009 quad core.

 

[VirtualRain]

Isn't there a read-write bench test that could confirm if triple channeling is enabled? I run 2-2-1-1 in my 2009 quad core.

The only reliable memory benchmarks I'm aware of run on Windows... Sisoft Sandra and Everest.

If you install them in pairs (2x2GB in upper slots and 2x1GB in bottom slots IIRC), then dual-channeling will be enabled. If you installed only one module or one in first and one in third slot, then they would operate at single-channel speed.

Maybe someone can confirm that. I'm not 100% sure about the slot organization (what slots are "pairs" with each other)

The fact is no one knows for sure how the memory controller manages different mixed-matched DIMM's with any certainty. All anyone can do is guess or benchmark different combos to try and draw some conclusions.

The only thing we know for certain is that if you want tri-channel operation your best bet is 3 matched DIMMS.

 

[RebootD]

Or get 3x 4GB for 12GB, or get 4x 2GB for 8GB total. With a MacPro I would highly suggest taking the 8GB minimum. Also as others have stated triple channel has been tested without much in real world gain.

 

[macaleph]

Another possibility I am considering is to buy a single 2GB DIMM to use with all three of my 1GB DIMMs to give a total memory of 5GB.

5GB is enough for my current needs. However I suspect that even double channeling wouldn't be possible with this combination of DIMMs, and that hence there would be a significant degradation in memory performance.

Does anyone have an opinion?

 

[Hellhammer]

Another possibility I am considering is to buy a single 2GB DIMM to use with all three of my 1GB DIMMs to give a total memory of 5GB.

5GB is enough for my current needs. However I suspect that even double channeling wouldn't be possible with this combination of DIMMs, and that hence there would be a significant degradation in memory performance.

Does anyone have an opinion?

As I said earlier, it's unlikely that you would notice difference between single, dual and tri-channel. Why? Because even single-channel is faster than what most apps can take advantage of. If you're fine with 5GB of RAM, then you shouldn't worry about channeling, it's overhyped.

Besides, if I'm not mistaken, your 3x1GB would run at tri-channel and only 1x2GB would be single-channel. Also, IF you notice performance degradation because of this new memory setup, you could just get two 2GB modules and run 3x2GB in tri-channel.