Mac Pro 2010 8GB ram configs?

[iondot]

I'm about to order the 3.33 hexacore Mac Pro and money is tighter than I would like. It is feasible to put in only a single 8GB stick? Can I put in just two?

I know three is theoretically ideal, but for now can I get away with less and, if so, what sort of performance hit would I be looking at with one or two sticks?

 

[Umbongo]

You can use any number of DIMMs.

If we say three DIMMs give you 100% bandwidth performance, two give around 80% and one gives around 45%.

If using 1333MHz DIMMs the difference between three and four is around 30-35% less. That is due to the limit of 1066MHz memory speeds with registered DIMMs when using more than one on a channel and the drop to a dual channel type interleave.

The real world performance impact can be negligible however, so if you are planning to add more in the near term then I would buy what you can afford and not worry too much.

 

[xgman]

Only if you plan on upgrading to 3 8gb sticks later. Otherwise you would get better performance with 3 smaller sticks.

 

[iondot]

You can use any number of DIMMs.

If we say three DIMMs give you 100% bandwidth performance, two give around 80% and one gives around 45%.

If using 1333MHz DIMMs the difference between three and four is around 30-35% less. That is due to the limit of 1066MHz memory speeds with registered DIMMs when using more than one on a channel and the drop to a dual channel type interleave.

The real world performance impact can be negligible however, so if you are planning to add more in the near term then I would buy what you can afford and not worry too much.

So if I bought one 8GB stick, would my computer run at 45% the speed it would if there were three? That would be beyond my ability to accept.

 

[Umbongo]

So if I bought one 8GB stick, would my computer run at 45% the speed it would if there were three? That would be beyond my ability to accept.

Your memory bandwidth would be 45% of what it would be at triple channel. Real world performance? I couldn't say. The difference between 2, 3 and 4 DIMMs for real application usage is often negligible. Real world between dual channel and single were always very close back in the DDR2 days too. I wouldn't worry about it, it won't cripple your system, it just isn't optimal for memory intensive usage.

 

[ahavriluk]

Is it OK to use different size sticks? Let's say 4, 4, 1 or 8, 1, 1 Gb sticks or any comdination of 4 or 8 Gb sticks?

 

[iondot]

@Umbongo - I really appreciate your help here.

I'll be using my machine mostly for work in Photoshop on RAW files from my 5Dii. I know Photoshop can be memory sensitive. Would the single 8GB stick be hobbling me in that particular scenario?

 

[iondot]

Ah, this appears to provide some good concrete info:
http://macperformanceguide.com/Reviews-MacProWestmere-MemoryBandwidth.html

 

[Umbongo]

Is it OK to use different size sticks? Let's say 4, 4, 1 or 8, 1, 1 Gb sticks or any comdination of 4 or 8 Gb sticks?

The 8GB DIMMs are registered memory and so can only be used with other registered DIMMs. 1GB, 2GB and 4GB DIMMs that are certified for the Mac Pro are all unbuffered. You could use non-certified registered DIMMs of other sizes though.

@Umbongo - I really appreciate your help here.

I'll be using my machine mostly for work in Photoshop on RAW files from my 5Dii. I know Photoshop can be memory sensitive. Would the single 8GB stick be hobbling me in that particular scenario?

I'd go for at least two. I couldn't say what your real experience would be between, say, 1x8GB vs. 4x2GB, but it is far from the ideal performance that can be attained. If you are thinking about buying another in a month or so then I'd just go with one now.

 

[mrbard]

So is there a performance impact if you mix registered with unbuffered RAM? Will you have memory problems, or?

 

[2contagious]

So is there a performance impact if you mix registered with unbuffered RAM? Will you have memory problems, or?

I think it just plain doesn't work, not sure though
Anyway, this might be of interest to you:

okay, I have asked diglloyd via eMail he says:

"I've run it with 1/2/3/4 modules (8GB). All combos work fine. Bandwidth with 1 module is poor, about 1/3 that of max."

So only using a single stick of 8GB from OWC worked, but the memory bandwidth was a third of the max. possible that is achieved by having 3x 8GB sticks and one slot empty.

I am referring to this graph:

Highest memory bandwidth on the 6-core configuration is with 3x 8 GB RAM. According to what 'diglloyd' says about running it with one stick, it seems like it runs with a third of the capacity (of 24GB) and a third of the bandwidth that's possible (21481 MB/s), so about 7000 MB/s I guess.