thatmdvideoguy
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Nov 24, 2007, 10:50 PM
Post #15 of 29
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Re: [Ron Priest] MacPro RAM Upgrade
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http://anandtech.com/mac/showdoc.aspx?i=2832
"As we mentioned and proved in our previous articles, the number and configuration of FB-DIMMs in your Mac Pro can affect performance. The Intel 5000X chipset in the Mac Pro features two 144-bit FBD branches, each being the width of two FB-DIMMs (effectively giving the chipset four "channels"). Therefore you need at least two FB-DIMMs in the system (the width of a single FBD branch), but ideally you'd need at least four to have a hope of attaining peak bandwidth.
As some of our readers (and Intel) pointed out, the story doesn't just end at needing four FB-DIMMs. The rank of the FB-DIMMs can impact performance as well, and ideally each of your FB-DIMM modules would be dual rank modules. The rank of a DIMM is determined by dividing the width of all of the devices on the module by the width of the module itself. For example, a single rank FB-DIMM would have 9 DDR2 devices each being 8-bits wide. A dual rank FB-DIMM would be composed of 18 DDR2 devices, each still being 8-bits wide. All of our 512MB FB-DIMMs are single rank modules, while our 1GB and 2GB modules are dual rank.
The story doesn't end with rank though. Because of the dedicated read and write lanes between the memory controller and the AMBs on FB-DIMMs, you can be reading from one FB-DIMM while writing to another. So in theory, if you're running an application (or combination of applications) that have a lot of concurrent reads and writes going on you could stand to benefit from having more than one FB-DIMM per channel.
Based on all of the above information, it would seem like your best bet is to stick as many dual rank FB-DIMMs as you can afford in your system, and if that were the case then we'd be able to move on from here. Unfortunately it's not, because as we mentioned in previous articles, the more FB-DIMMs you have in your system, the higher access latencies will be to those additional FB-DIMMs.
What we then end up with is a tradeoff between more bandwidth and higher latency, so which route do you take? We've done a lot of testing and most of our tests seem to favor the four dual-rank FB-DIMM module configuration, but the number/configuration of modules really depends on your particular needs. We're still testing to figure out what the tangible real world performance differences are between the multitude of memory configurations, but for now just know that if you need maximum bandwidth you'll want 8 dual rank FB-DIMMs, but if you want lower latency you'll want fewer modules. Whether or not you'll see a performance difference will depend mostly on the application(s) you're running."
http://diglloyd.com/diglloyd/free/MacPro/memory.html
"Recommendations—
Modules must be installed in pairs of matched size (see Apple’s Replacement Instructions and RAM Expansion Product-Specific Details). So there can be 2, 4, 6 or 8 modules installed. The riser boards are nicely labeled as to which slot is which (slots 1/2/3/4 on each riser board). - Aim for a configuration of either 4 or 8 modules, not 2 or 6. Each group of 4 modules should be the same size, and occupy the same slots on each riser board.
- Avoid single-rank modules. Apple’s 512MB modules are single-rank. However, 4 of them do offer 90% of the performance of the dual-rank modules, so they’re still speedy in groups of 4.
- If you have mixed sizes, install the fastest modules in the lowest-numbered slots. Mac OS X appears to allocate memory from the lowest-numbered slots first.
Keep in mind that the performance of any specific program might or might not benefit from the optimal configuration. On the other hand, there is no reason not to purchase the optimal configuration, assuming your budget allows it."
I don't know if this explains it, but it seems they are saying to put in either 4 like sticks or 8.
Allan
2 VX2000s, 3.0 8-core Mac Pro, Dual 1Gb G4, G3, PowerBook G4, FCStudio 2 not 3, PS CS3, Samson U32 Q mic, Edirol R-09, 4 iRivers, NRG lights, Epson R300 and Stylus 600 and HPC5280 printers, Bravo II publisher, a sense of humor, little hair and occasional flashes of creativity
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I had read the barefeats article before; these two articles were linked there. 
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