[MarkLT1]

I heard that the optimal RAM setup for the Mac Pro's is filling 4 of the memory slots. I am on a bit of a budget, but for my purposes, processor power is more important than RAM (a typical simulation I run has about a 512meg RAM footprint). The plan is to do the 3.0ghz, w/4 gigs of RAM, as I can always upgrade the memory later.

The way I see it is I have 2 options:
1) Configure the MacPro with Apple's 4x1gig for memory.
2) Configure the MacPro with Apple's 2x512meg for memory, and buy 2x2gig memory sticks. Put the 2x2gig pair in the first set of slots, and put the 2x512meg in the second set, to fill 4 slots, for a total of 5 gigs.
3) Configure the MacPro with Apple's 2x512meg for memory, and buy 2x2gig memory sticks. Drop the 2x512s, and just put in the 2x2giggers.

After doing the pricing for RAM with apple qualified heat sinks, it looks like Option 2 and 3 are actually slightly cheaper, and would leave me with room for more RAM expansion in the future (i.e.- it is easier to justify dropping the 2 512 meg chips, to get to 16gigs if needed, than to justify dropping all 4 1gig chips).

So, the first question is, how much of a performance drop will I see if I only use 2 ram slots? Second question is, how much of a performance drop if the first set are 2x2gigs, and the second set are 2x512megs?

[brokenjago]

The fastest option and also most cost effective would be option 2. With 4 RAM slots filled, it runs in quad channel mode, which allows you to get the most memory performance.

To answer your questions, I have no idea and virtually None, respectively.

[all2ofme]

I agree - go for the 2x512MB and the 2x2GB.

I did that with mine, and swapped the DIMMs around in order to have the 2GB DIMMs first in line for access (i.e. in the slots that the stock RAM came in) because I read somewhere that there'd be lower latency if the system only accesses the 512MB DIMMs when it needs to.

And besides, why would you drop the two 512s? You'd lose the quad channel performance gains and a gig of RAM!

[MarkLT1]

Great.. it sounds like the 2 512s + 2 2 giggers is the way to go.

Originally Posted by all2ofme 

And besides, why would you drop the two 512s? You'd lose the quad channel performance gains and a gig of RAM!

I wasn't sure 1) if quad channel required the same size ram sticks to be in all 4 slots, and 2) if it did require them to all be the same, if having different size sticks would hinder performance. Luckily it sounds like this isn't the case!

[HyperX]

Wait- so my mac came with 2x512 - i bought 2x1 gigs (for three total)

Then today I said what the hell and ordered 2x2 gigs (two of them). So I will have

1 + 2 + 4 + 4 = 11 gigs of ram. With all slots used up.

By doing this, I am actually hindering the performance of my mac pro 3.0 gigahertz?

Should I just run 8 gigs? and sell of the 3 gigs?

[Leonard]

Originally Posted by HyperX 

Wait- so my mac came with 2x512 - i bought 2x1 gigs (for three total)

Then today I said what the hell and ordered 2x2 gigs (two of them). So I will have

1 + 2 + 4 + 4 = 11 gigs of ram. With all slots used up.

By doing this, I am actually hindering the performance of my mac pro 3.0 gigahertz?

Should I just run 8 gigs? and sell of the 3 gigs?

Yes, you take a slight penalty when accessing the 2 slots away from the motherboard on the riser cards. Optimally you want to have 4 DIMMs installed with 2 in the top riser and 2 in the bottom riser in the 2 closest slots to the motherboard. You also want to have the highest capacity DIMMs closest to the motherboard. Having more than 4 DIMMs gives a slight penalty, but I'm not sure how much. See AnandTech: Apple's Mac Pro - Upgrading CPUs, Memory & Running XP


So in your case you should have:

2 2 1 1

2 2 512 512

or

2 2 512 512

2 2 1 1

[brokenjago]

You'll take a 3-5ns (as in nanosecond) hit in memory access times.

[mduell]

There's no reason to go with 2 memory modules instead of 4. 4-8 modules is the optimum configuration, depending on your applications relative sensitivity to latency vs capacity.