[TripleSeven]

Hello all,

I'm interested in purchasing additional RAM for my new Mac Pro 8-core. If I wanted to add 8GB, what are some of the considerations of installing 4x2GB modules vs.2x4GB modules (I'm aware of the significant price difference)? Also, any opinions about "Mac Pro Certified" vs. "Apple qualified" modules? OWC offers both, and had this to say when I asked them what the difference was:

Hi,
This is XXXXX from the OWC Tech Support Dept.

They both meet or exceed Apple's requirements for memory. Certified chips have been
approved by Apple as well and Netlist uses the same chips for Apple as well.

Thanks for the feedback!

~ 777

[mduell]

4x2GB gives you twice as much memory bandwidth as 2x4GB, with the same latency.

Unless you're going to go to 24-32GB, or your apps are extremely sensitive to memory latency and want to go above 8GB, I can't see any reason to buy 4GB modules.

[reader50]

I checked Apple's specs page for the 8-core systems to confirm my recollections. The maximum RAM is listed at 16 GB, 8 slots (4 slots x 2 risers) at 2 GB per slot. It doesn't say anywhere that it can take 4 GB DIMMs.

[Lateralus]

Originally Posted by reader50 

I checked Apple's specs page for the 8-core systems to confirm my recollections. The maximum RAM is listed at 16 GB, 8 slots (4 slots x 2 risers) at 2 GB per slot. It doesn't say anywhere that it can take 4 GB DIMMs.

It became known quite a while back that 4GB DIMMs are compatible.

Apple's listed specifications for RAM limits are almost always wrong historically.

[mduell]

From the Intel 5000X chipset datasheet:
Supports up to 16 dual-ranked FB-DDR2 4GB DIMMs, that is, 64GB of physical
memory in non-mirrored configuration or 32GB of physical memory in mirrored
configuration.

Since the Mac Pro only has two 4 slot risers, I think you're limited to 8x4GB, but 8x8GB may work (if and when anyone ships 8GB FB-DDR2 DIMMs).

[~bash $]

Regarding Apple Qualified v. Certified designations, it's worth asking Apple or OWC if either one might void Apple's warranty if installed.

And it's worth echoing the sentiment that you should go with 4 dimms instead of 2 for the same amt of total memory, for the bandwidth considerations mduell states above.

[ninahagen]

Originally Posted by mduell 

4x2GB gives you twice as much memory bandwidth as 2x4GB, with the same latency.

Does this remain true furtherdown the spectrum. Would 8 x 1GB give twice as much bandwidth, with the same latency?

If so, then as long as one were sure they didn't want to go over 8GB, then 1GB modules would be the best choice.

Gosh, who could even use 64GB of RAM? Maybe the folks at Pixar.

[reader50]

Originally Posted by ninahagen 

Does this remain true furtherdown the spectrum. Would 8 x 1GB give twice as much bandwidth, with the same latency?

If so, then as long as one were sure they didn't want to go over 8GB, then 1GB modules would be the best choice.
...

No, this is incorrect. You get max bandwidth by filling the first 4 slots - that is, the closer two on each riser. The 2nd 4 have the same bandwidth as the first 4, but higher latency. Their bandwidth is not additive - data from the 2nd 4 actually daisy-chain through the first 4, which is where you get the extra latency from.

In other words, your best performance comes from the first 4 slots. Ideally, max them out up to the 4 GB / slot before using the next 4 slots. Note that this method can be very expensive, so people do compromise. ie - buying 8 x 2 GB instead of 4 x 4 GB because it saves a lot of money.

If you use more than 4 FBDIMMs, put the biggest 4 into the first 4 slots. That way you get lower-latency access to as much of your memory as possible.

[ninahagen]

Thanks for clearing that up.

I am a G5 Quad user planning to upgrade to a Mac Pro on Day 1 of the next rev.

Right now I use 8 x 1GB RAM, more than enough for CS3, even with heavy multi-tasking.

However, when CS3 goes 64-bit, more RAM will equal more speed. Can anyone guess how much more RAM would be usable? What factor would set the limit? Now its a limit in the CS3 software itself, which, for example, only allows Photoshop to use up 3G of RAM.

The reason I ask is that if the 64-bit had no limiters in the software, and more RAM converted with relative linearity to more speed, then 64GB RAM could make 30 minute batching jobs into a small fraction of that. In that case I would get 2 x 4 GB now and keep that until the 64-bit CS3 comes, and reserve 6 slots.

[mduell]

Photoshop will be able to use more than 3G RAM when Adobe releases a 64-bit version for Leopard. Who knows when that will be; could be next year or in 3 years.

RAM does not relate linearly to speed. It either does very little (if you go from swapping to still swapping, or not swapping to not swapping) or a huge gain (if you go from swapping to not swapping).

Given the crazy prices of FB-DIMMs in general, and the especially crazy prices of 4GB modules, I'd go with 4x2GB to start and then add some 4GB modules when you need more memory.

[ninahagen]

Originally Posted by mduell 

Given the ... especially crazy prices of 4GB modules

How much are the 4GB modules (Crucial)?

Originally Posted by mduell 

I'd go with 4x2GB to start and then add some 4GB modules when you need more memory.

That sounds good. It will limit me to 8GB in the upper riser and 16GB in the lower one. Can you imagine any scenario in which CS3 64-bit for Leopard working Photoshop batchs with 100 or more 1GB files would be limited by 24GB RAM as opposed to 32?

Thanks very much... always enjoy your posts.

[TripleSeven]

Hello again everyone,

Thanks very much for the excellent feedback and the education following my original post concerning the installation of 8GB RAM on my new Mac Pro machine. "mduell" and "reader50," I particularly appreciate your technical insights and explanations. Very informative indeed. Accordingly, I chose to purchase 4x2GB modules, vice 2x4GB modules. I selected the 2GB "Mac Pro Certified" chips from OWC. Although they were about $50 per module more expensive than the "Apple Qualified" chips, they were still $20 per module cheaper than the 2GB modules at Crucial ($80 net savings).

ninahagen: a quick review of the crucial website (see links,below) reveals just how terribly expensive the 4GB modules are at this time: $1,734USD for a matched pair, or $867USD per module! The OWC 4GB modules are not cheap either, but at $1,300USD for a matched pair ($650 per module), they are a full 25% less painful than those from crucial. If you were to purchase FOUR 4GB modules (in order to populate the upper and lower riser cards with matched pairs), you'd be looking at $3,468USD(!) for 4x4GB from Crucial, vs. $2,600USD for 4x4GB from OWC. (So...the Crucial 4GB chips are about $217USD per GB, while the OWC 4GB chips are about $163USD per GB). I'm sure we can all think of many other good uses for the $868USD saved by going with OWC for the 4GB chips.
Alternatively, going with the 4x2GB configuration as has been recommended in this thread, or even with an 8x2GB configuration, you would save a considerable amount of money (versus any 4GB module configuration) and still have a wickedly potent RAM capability: Crucial 2GB modules are $210USD each ($420USD matched pair); OWC gets $190USD each ($380 matched pair) for their "Mac Pro Certified" RAM, and $140USD each ($280USD matched pair) for their "Apple Qualified" RAM (the difference between the "MP Certified" and the Apple Qualified" monikers appears to be that the former is OEM, or at least OEM identical; both have specially designed and tested heat sinks appropriate for the Mac Pro).
In any case, with either Crucial or OWC, the difference between a 4x2GB configuration and a 2x4GB configuration (and again, the 2x4GB configuaration appears to be non-optimal - see other posts, above) is financially significant: 4x2GB = $840USD (Crucial), or $760USD (OWC), while the 2x4GB = $1,734USD(!) (Crucial), or $1,300USD (OWC). As you can see, the 4GB chips are about TWICE as expensive per GB as the 2GB chips. Stepping up to 16GB (8x2GB vs. 4x4GB) only further dramatizes the (price) differences. Hence, it seems difficult to justify the 4GB chips at this time, unless both (a) you really have a performance requirement to run more than 16GB RAM, and (b) money is of no object.

References:
1. Computer memory upgrades for Apple Mac Pro (8-core) Desktop/PC from Crucial.com
2. Mac Pro Memory - Mac Pro Quad Xeon Memory, Mac Pro 8-Core Xeon Memory DDR2 667MHz Fully Buffered 240 Pin DIMM PC5300 Upgrades at OtherWorldComputing.com
3. Apple Mac Pro Memory Upgrade Kits for up to 32.0GB using 4.0GB Memory Modules at OtherWorldComputing.com

Thanks again to all. I look forward to returning soon to provide feedback on my purchase decision, and to learn more about the fascinating world of computing with Mac.

~ 777

[mduell]

Originally Posted by ninahagen 

That sounds good. It will limit me to 8GB in the upper riser and 16GB in the lower one. Can you imagine any scenario in which CS3 64-bit for Leopard working Photoshop batchs with 100 or more 1GB files would be limited by 24GB RAM as opposed to 32?

You'd want to put 12GB on each card (2x4 on the inside slots and 2x2 on the outside slots).
If you use a RAM disk as your scratch disk, you may find the extra 8GB useful; I'm not all that familiar with the memory requirements of PSCS3.