Mac Pro - Memory Configuration

[NATO]

Hi,

I'm just looking for some information regarding the memory configuration of a Mac Pro.

I would like to buy one of these systems but because of the prohibitively high price of RAM, I'm goin to go with 2GB to start with and add later. I am aware that to get maximum performance, BOTH riser cards need to have some RAM installed to enable quad-channel mode, however:

If I go with 2GB (Apple-installed), I will end up with 4x512MB. If I then buy a 2GB (2x1GB) pack from Crucial at some point in the future, will there be any loss of performance due to having 2x1GB on Riser card 1, and 4x512MB on Riser Card 2, or should I ideally go for 4x1GB (a much more costly option)?

Many Thanks for your help, it's much appreciated.

 

[Trekkie]

Yes there will be a loss of performance. How much? Hard to say depending on what you're doing.

If you're not taxing the memory subsystem, there won't be much of a loss. However the loss is compared to if you bought 4 1GBs instead of 2.

What you're doing is with the four DIMMs of the 4x512MB is that you have two people in two rooms doing some work. The system gives them each work to do, and they both accomplish it in the same amount of time.

When you put some bigger people in one of the rooms, the system gives the two rooms something to do and one room gets done faster than the other, and the system could end up waiting on the one room to get something done even though the other room is finished. This is where the 'depends on the workload' comment comes from.

But if you're not taxing the system, you won't notice it near as much.

You'd be better off going with another 4x512MB since there is enough slots for it for ideal performance. But then again that depends on your workload and if you really need more than 4GB of RAM or can handle chucking 8x512MBs two years from now when you need 8GB of RAM. eBay is a wonderful resource.

 

[TangoCharlie]

Mixing DIMMs...

Personally, I try never to mix ram of different sizes, speeds or from different manufacturers.

As you say, FB-DIMMs (esp. those from Apple!) are rather expensive at the moment; however, expect RAM prices to drop over time**, so get the 2Gig now and wait for prices to drop, and then go for the 8x1Gig for that 8Gigs.



** Expect a blip in RAM prices when Vista comes out....

 

[Trekkie]

Don't expect memory prices to drop like in the past. With AMD in the market now you'll have a mix of DDR2 and FBD. FBD is basically a DDR2 DIMM with buffering chips put on them. According to the industry it should maintain a price premium for the next two years at least by about 30%.

Unless AMD dies and everyone quits buying them tomorrow of course. Which I doubt will happen.

 

[NATO]

Thanks for the replies, I'm just weighing up getting 2GB or 4GB, but at least I'm a little more informed about the problem.

I'm also interested to hear how FB-DIMM RAM prices are likely to change for the forseeable future especially if they're unlikely to change for a few years.

 

[L int.]

I'm a little confused now:

is it better to have 4 x 512MB instead of 2 x 1GB?

personally I'd prefer the second option, since 2GB is enough to start with, but gives me the possibility for 8GB in the future.

 

[miniConvert]

I thought for optimum performance the Mac Pro should be fitted with 4 sticks of RAM, 2 on each card. Does 8 identical sticks, 4 on each board, not attract a performance hit at all?

 

[waremaster]

I thought for optimum performance the Mac Pro should be fitted with 4 sticks of RAM, 2 on each card. Does 8 identical sticks, 4 on each board, not attract a performance hit at all?

Technically if you increase the number of sticks you also increase latency.

if you want 2 gig best config would be 4 x 512

if you want 4 gig best would be 4 x 1gig

if you want 8 gig best would be 4 x 2gig

you want 4 chips to give you 256bit with the lowest latency