Thank you Barefeats, you've answered my question. It is an interesting result as to me it is counter-intuitive given that there are only 4 serial channels though given that these operate at a much higher frequency than the memory itself I guess what happens on the modules themselves is more important than being first in line.
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RAM is 4x2 faster than 8x1 ?
- Thread starter Gonk42
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Thank you Barefeats, you've answered my question. It is an interesting result as to me it is counter-intuitive given that there are only 4 serial channels though given that these operate at a much higher frequency than the memory itself I guess what happens on the modules themselves is more important than being first in line.
I guess nobody is interested in the actual truth - you all just want 4x2 GB to be better than 8x1 GB when that is not the case:
Barefeats.com :
"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."
I rest my case!
See article: http://barefeats.com/harper3.html
Maybe the Thread Starter should look at previous threads before posting a completely redundant topic that has been covered extensively here.
In my defense, I did read the previous threads and referred to them in my OP. I couldn't find any test results, at the time, where 4 slots were compared with 8 with the same total RAM (the barefeats results that I saw went up in RAM as the slot occupancy went up so it may have been that more RAM led to an improvement instead of more slots).
Barefeats has now answered the question in a very comprehensive manner and I think I was not the only one interested in the new results.
(I hope the above doesn't sound peevish fridgeymonster - it is not meant to be.
)Interesting thread, as I've just added my 7th and 8th sticks of RAM to my Mac Pro. Mine are as follows:
Riser A: 4 x 1Gb
Riser B: 2 x 512Mb (original memory) in Slots 1 & 2, 2 x 2Gb in Slots 3 & 4
All RAM slots are filled, they're all in matching pairs, OS X is recognising that there is 9Gb RAM in the system... So, my question is: is there an optimum order to install these RAM chips? For instance, is having 4Gb on Riser A and 5Gb on Riser B gonna cause any problems? Should the 2Gb sticks be in slots 1 & 2 on Riser B rather than slots 3 & 4? Should I just content myself with the fact that OS X is regognising 9Gb RAM and get on with things??
Phil
Riser A: 4 x 1Gb
Riser B: 2 x 512Mb (original memory) in Slots 1 & 2, 2 x 2Gb in Slots 3 & 4
All RAM slots are filled, they're all in matching pairs, OS X is recognising that there is 9Gb RAM in the system... So, my question is: is there an optimum order to install these RAM chips? For instance, is having 4Gb on Riser A and 5Gb on Riser B gonna cause any problems? Should the 2Gb sticks be in slots 1 & 2 on Riser B rather than slots 3 & 4? Should I just content myself with the fact that OS X is regognising 9Gb RAM and get on with things??
Phil
Interesting thread, as I've just added my 7th and 8th sticks of RAM to my Mac Pro. Mine are as follows:
Riser A: 4 x 1Gb
Riser B: 2 x 512Mb (original memory) in Slots 1 & 2, 2 x 2Gb in Slots 3 & 4
All RAM slots are filled, they're all in matching pairs, OS X is recognising that there is 9Gb RAM in the system... So, my question is: is there an optimum order to install these RAM chips? For instance, is having 4Gb on Riser A and 5Gb on Riser B gonna cause any problems? Should the 2Gb sticks be in slots 1 & 2 on Riser B rather than slots 3 & 4? Should I just content myself with the fact that OS X is regognising 9Gb RAM and get on with things??
Phil
Riser A should probably be:
slot1: 2GB
slot2: 2GB
slot3: 1GB
slot4: 1GB
Riser B should probably be:
slot1: 1GB
slot2: 1GB
slot3: 512mb
slot4: 512mb
That's based on the first slots starting with riser A being the fastest. You probably wouldn't notice any real world performance differences though. On the 2006 Mac Pro memory chart that often gets posted it's suggested you have the 1GB sticks all on Riser B and the 2GB in slots 1 & 2 on Riser A. However I don't know if that chart is designed to display optimized setups.
Some newbie questions:
1) Does this mean that I've just wasted my cash buying 8x2GB as this is just as fast as the 8 x 1GB?
2) If I retain the supplied 2 x 1GB to take myself to 18GB this will actually result in a performace drop?
Many thanks
Miles
Not if you need more than 8GB of memory.
Well without 4GB DIMMs you can't get 18GB anyway.
I'm buying RAM to make the machine run faster but (to my inexperienced eyes) the excellent data suggests that there is no speed difference which seems odd! Or have I got the wrong end of the stick in terms of how RAM makes the machine faster!?
DUH!
Ebay here I come
That's right. Though they come from the factory with 1 x 1GB on the top riser and 1 x 1GB on the bottom riser (base 2GB config), if you only want to add 2 x 2GB, you have to put the two 1GB FB-DIMMs on the same riser. That's what we did when we tested.
Xeon has 256 bits memory-bus
Interesting that 8 chips is faster than 4.
Xeon has a 256 bits bus, meaning that you need 4 chips, better have them the same size, if you take like 2x 1GB, 2x 2GB, when your test program will make its test within the first 1GB, it will be faster. But when in the real world, you really use that memory, it will become slower when you in the second GB (so like in the 5th GB and 6th GB.
I'm not sure why 8 chips is faster, because it will still have 256bits access... I could imagen that it is faster because of having 2 xeon chips. Too bad, I just ordered 4x 2GB, and I really don't need those 8GB extra after the first 8GB....
Interesting that 8 chips is faster than 4.
Xeon has a 256 bits bus, meaning that you need 4 chips, better have them the same size, if you take like 2x 1GB, 2x 2GB, when your test program will make its test within the first 1GB, it will be faster. But when in the real world, you really use that memory, it will become slower when you in the second GB (so like in the 5th GB and 6th GB.
I'm not sure why 8 chips is faster, because it will still have 256bits access... I could imagen that it is faster because of having 2 xeon chips. Too bad, I just ordered 4x 2GB, and I really don't need those 8GB extra after the first 8GB....
I've been looking at the Wikipedia entry on FB-DIMMs and this refers to a MSc Thesis which I also looked at (skim read). The main thing of interest that struck me was a statement saying that having two DIMMs in a channel reduces read after write and write after read delays if they are from different DIMMs because each DIMM module has a buffer as part of the serial to parallel conversion. This will tend to overcome the inherent latency of the second DIMM module only being accessed when the first has had a look in.
In other words, in some circumstances the two DIMM modules in the same channel can act in parallel.
(I apologise to any hardware engineers for my rather simplistic and probably wildly inaccurate reading of the facts.)