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Triple channel, why four slots?

mattmower

mattmower

macrumors regular
Original poster
Aug 12, 2010
119
19
Berkshire, UK
As I understand it the 2010 Mac Pro has a triple-channel memory controller. So I am wondering... Why the single-CPU has 4 memory slots, instead of 3 (or, better yet, 6)? It seems an odd design choice.

I'm also not clear what the penalty is for using the 4th slot which, presumably, takes it out of triple-channel mode if you have to use matched pairs rather than a set of 3.

Is it anything to worry about in real-world performance terms?

Matt.
 
If you put a fourth stick in, it will switch to dual-channel mode. You lose a channel, but odds are if you're adding an extra stick any negligible performance loss from adding the fourth stick you gain back from having more RAM.
 
mattmower said:
As I understand it the 2010 Mac Pro has a triple-channel memory controller. So I am wondering... Why the single-CPU has 4 memory slots, instead of 3 (or, better yet, 6)? It seems an odd design choice
Click to expand...
Because Apple has proven recent years not to care one bit about pro users' needs and wants to intentionally cripple their less extremely expensive models in order to force us to be more extremely expensive models.
 
mattmower said:
As I understand it the 2010 Mac Pro has a triple-channel memory controller. So I am wondering... Why the single-CPU has 4 memory slots, instead of 3 (or, better yet, 6)? It seems an odd design choice.

I'm also not clear what the penalty is for using the 4th slot which, presumably, takes it out of triple-channel mode if you have to use matched pairs rather than a set of 3.

Is it anything to worry about in real-world performance terms?

Matt.
Click to expand...

It's called "conception failure". The same goes for putting 8 slots on the dual CPU mac pro instead of 12. But in real-world performance it simply doesn't matter at all.
 
psychometry said:
Because Apple has proven recent years not to care one bit about pro users' needs and wants to intentionally cripple their less extremely expensive models in order to force us to be more extremely expensive models.
Click to expand...

Very dramatic, something tells me Steve is not twisting his mustache on this one. I am sure its a Intel architecture thing.
 
Apple probably went with 4 as 6 was too close to the 8 on the DP system. Apple logic.

Using the 4th slot puts it in to a sort of dual channel mode and you lose about 30% bandwidth performance, but the real world performance impact will often be negligible.

The capacity of 4 DIMMs is going to be better than the speed improvements on the capacity of 3 DIMMs for most people. So I wouldn't worry about it.
 
mattmower said:
As I understand it the 2010 Mac Pro has a triple-channel memory controller. So I am wondering... Why the single-CPU has 4 memory slots, instead of 3 (or, better yet, 6)? It seems an odd design choice.

I'm also not clear what the penalty is for using the 4th slot which, presumably, takes it out of triple-channel mode if you have to use matched pairs rather than a set of 3.

Is it anything to worry about in real-world performance terms?

Matt.
Click to expand...

Go 3x8GB and you'll be golden.................... and a bit poorer to boot. :p
 
eponym said:
Or not.

We don't know if the 8GB DIMMs will even work (with the hex anyway). Specs are implying that 4GB DIMMS are the max.
Click to expand...

I've seen this, yet don't Intel specify a maximum of 24GB themselves?

Or perhaps they are assuming 6x4GB somehow? Is it common for motherboards to have 6, rather than 8, memory slots?

M.
 
Umbongo said:
Apple probably went with 4 as 6 was too close to the 8 on the DP system. Apple logic.

Using the 4th slot puts it in to a sort of dual channel mode and you lose about 30% bandwidth performance, but the real world performance impact will often be negligible.

The capacity of 4 DIMMs is going to be better than the speed improvements on the capacity of 3 DIMMs for most people. So I wouldn't worry about it.
Click to expand...

30% seems quite a hit. Is this negligible because memory bandwidth is already so high that, for all practical purposes, it's not saturated?

M.
 
mattmower said:
I've seen this, yet don't Intel specify a maximum of 24GB themselves?

Or perhaps they are assuming 6x4GB somehow? Is it common for motherboards to have 6, rather than 8, memory slots?

M.
Click to expand...

Max Memory Size of 24GB - presumably the maximum spread of 8GB modules across 3 memory channels.
 
Pointless and poor value buying a single cpu mac pro.

Either go Dual cpu or go iMac.
 
mattmower said:
I've seen this, yet don't Intel specify a maximum of 24GB themselves?

Or perhaps they are assuming 6x4GB somehow? Is it common for motherboards to have 6, rather than 8, memory slots?

M.
Click to expand...

Yes, there's an implication that the limit of 24GB is achieved via 6 slots. It also doesn't help that the Dell 6-core precision workstations come with 6 slots.

See here. We'll know soon enough for sure.

I'm guessing they will work.
 
I haven't seen the riser cards in person, but they look large enough to handle 6 slots each. Maybe a third party will build replacement riser cards allowing you to put 12x8Gb (96Gb:eek:) in dual processor MP and 6x8Gb in single processor units!
 
hugodrax said:
Pointless and poor value buying a single cpu mac pro.

Either go Dual cpu or go iMac.
Click to expand...

From my perspective it's neither pointless, nor poor value (in respect of the rest of the 2010 line-up).

I am after raw performance and am willing, even if it stings a little, to pay the £640 differential between the 3.2GHz Quad & the 3.33GHz Hex. But not the additional £2000 I'd have to pay, over that, to get the 2.93GHz 12-core.

Matt.
 
eponym said:
No, your comment was pointless. You don't know what you're talking about.
Click to expand...

Oh dear. My Hex Mac Pro is pointless according to hugodrax.

Because it was such a pointless purchase, I may as well use the machine as a doorstep when it arrives.

Never mind the fact I'm working on a 1.83Ghz Core Duo iMac... because it was pointless to buy my single cpu Mac Pro.

But to move away from the bitter sarcasm...

MacSince1985 said:
I haven't seen the riser cards in person, but they look large enough to handle 6 slots each. Maybe a third party will build replacement riser cards allowing you to put 12x8Gb (96Gb:eek:) in dual processor MP and 6x8Gb in single processor units!
Click to expand...

Less than likely but an interesting thought.

Too bad it's pretty much impossible to fit 288Gb (36x8Gb) of RAM into a Mac Pro... cause that's the maximum memory size according to intel with their multi-cpu Xeons.
 
It's probably pointless (no pun intended) but I have contacted Crucial to ask them about the 2x8GB part they have on sale. Maybe they have done pre-release testing with Apple and guaranteed it will work.

M.
 
And my thanks to the original commenters about the triple-channel/4-slots point.

I might lament that it doesn't have 6-slots since that would seem to be a more sensible choice.

But as long at it will, finally, take 8GB modules, I will relax in the knowledge that I can go over 16GB at some point in the next 2-3 years.

Matt
 
mattmower said:
30% seems quite a hit. Is this negligible because memory bandwidth is already so high that, for all practical purposes, it's not saturated?

M.
Click to expand...

Pretty much. There are a lot of dual channel vs. triple channel benchmarks out there if you want to see how it might impact specific usage.
 
eponym said:
Yes, there's an implication that the limit of 24GB is achieved via 6 slots. It also doesn't help that the Dell 6-core precision workstations come with 6 slots.

See here. We'll know soon enough for sure.

I'm guessing they will work.
Click to expand...

Update August 12: OWC Larry says “85/15 chance on 8GB working in 4/6-core— I share a cautious optimism now with comparison/expectation to the 2009 model design chipset and expectation from”.
Click to expand...

I'm not sure quite sure how to read this, do you think it means:

85% chance that 8GB will work in the 4/6 core model, 15% that they wont.

or

85% chance that 8GB will work in the 4 core model, 15% chance that it will work in the 6 core model.

?

Matt
 
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