Upgrading Memory

I ordered mine with 2gb then place another 4gb in, giving me 6gb.

Just make you place memory in correct. They have to be split equally on each board.

"A better scenario, if you insist on using those two factory 1GB FB-DIMMs is to order four 2GB FB-DIMMs, pull out the two factory 1GB FB-DIMMs, put the four 2GB FB-DIMMS in slots 1 and 2 on each riser, and put the two 1GB FB-DIMMs in slot 3 & 4 of the top riser. That will give you a total of 10GB.

But if you want maximum memory throughput under all situations, we recommend either 8 matching 1GB FB-DIMMs, 8 matching 2GB FB-DIMMs, or 8 matching 4GB FB-DIMMs."
<http://www.barefeats.com/harper3.html>

We got our hands on eight 1GB FB-DIMMs and eight 2GB FB-DIMMs. We tested the different combinations with a special 64-bit parallel multi-threaded version of STREAM. We averaged the results of Copy, Scale, Add, and Triad to produce an overall speed rating in gigabytes per second:

4 x 1GB = 4GB = 6.5GB/s

6 x 1GB = 6GB = 6.7GB/s

2 x 1GB + 2 x 2GB = 6GB = 6.5GB/s

4 x 2GB = 8GB = 6.5GB/s

8 x 1GB = 8GB = 7.5GB/s

2 x 1GB + 4 x 2GB = 10GB = 6.8GB/s

6 x 1GB + 2 x 2GB = 10GB = 7.5GB/s

6 x 2GB = 12GB = 6.8GB/s

4 x 1GB + 4 x 2GB = 12GB = 7.5GB/s

2 x 1GB + 6 x 2GB = 14GB = 7.5GB/s

8 x 2GB = 16 GB = 7.5GB/s

Conclusion: Any combo of matching pairs that fills all 8 slots = fastest.

Thanks! That is very helpful! As I am still debating how to upgrade the memory this list makes it easier but also more difficult. As I've read all your memory benchmarks 8x1GB seems the way to go. But I've also read that increasing the number of DIMM's will also increase the latency, by a lot.

So what is wisdom... Is increasing the (theoretical) bandwidth more important than increasing the latency? It would seem to me that the computer will more often do "small" reads and writes to the memory than storing big chunks of info.

When using a PC the real-life benefits never were in the same league as the theoretical low-level benchmarks.

Would be very handy to see some (real life (Photoshop, iTunes, etc) benchmarks using 4x2 or 8x1GB!

But I've also read that increasing the number of DIMM's will also increase the latency, by a lot.

I remember reading that it caused a small increase in latency for the previous Mac Pro, but the new ones have a different memory system, and I haven't seen any mention of increased latency for them.

Okay! Now that would be very good news, but also not very logical, as also on the new Mac Pro the fbdimm's handle their data through the AMS. And the more dimm's you use, the lower the latency will get. It's just a part of the design.

How much this will influence real-life applications I alas do not know. As 1GB sticks are also much cheaper I still tend to go that route (buy an additional 6x1GB). Upgrading will be less pleasant, I do recognize that fact 😉

This is all very difficult stuff. They just should've gone with regular ddr2 memory. Cheaper, much less heat, faster in practice (that is, much more efficient due to the lower latency). There are not a lot of people actually using 32GB's anyway I think. Except for Rush and the lottery winner I just read about anyway 😉

I wouldn't worry about latency, I would worry about quality and heat/temps. You need ECC etc that is Mac Pro 2008 compatible. There is no 'regular' or generic FBDIMM DDR2.

This I understand! There is a shop over here (Netherlands) that sells 1GB fbdimm's for a reasonable price. Kingston modules with the "Apple approved" heatsinks on them. As the prices of the 2GB modules are (relatively) much higher I guess it's "best" to go for an additional 6x1GB after all. I seriously doubt that I would ever need more than 8GB.

This is all very exciting. I hope the PC-to-Mac switch will bring only sheer joy 😉 (with Vista I am very happy, by the way (it's all a matter of preference/habit I guess))

One more point to go with the results posted above. Though in our test, any combination that filled all 8 slots was fastest, I understand that in the real world, to get maximum interleaving effect, you'll want 8 matching FB-DIMMs like 8 x 1GB or 8 x 2GB.

What a fascinating insight into memory and processing in the Mac Pro! I have just bought one of these lovely beasts, and I notice that you didn't include the basic memory configuration with just 2 RAM modules - did you test it like that, and just how bad is it with the bandwidth not properly utilised?

If I understand you correctly then if I wanted to upgrade the new Mac Pro to 8Gigs, and use the 2 X 1G modules that come with the new Mac Pro, I should just buy 3 pairs of 1G modules (or six 1Gig modules that are paired)? And then fit each 1G module as pairs in the 8 slots?

Well, that was what I gathered from the discussion, so I've gone ahead and ordered 6 x 1GB modules to fill the other RAM slots, and, hopefully, get the best bandwidth! We'll see when they arrive - if you can hold your breath for 5-7 days, I'll post back here as to just how impressed I am.

Thanks Pete and do, please, come back and report. I'm very interested.

John

Pete, did you order the 6 1GB modules off of OWC for $379.99? That is what I was just looking into as I'm about to purchase a new Mac Pro within a month...

I just upgraded my MacPro (bought with 2x1gb) with 4x2gb dimms. I purchased them from memorytogo.com (i've purchased from them many times, always with great success).

But after reading this post, I should just fill the darn thing with 2gb dimms and ditch the 1gb dimms???

How fun!