Triple channel, why four slots?

Discussion in 'Mac Pro' started by mattmower, Aug 13, 2010.

  1. mattmower macrumors member

    mattmower

    Joined:
    Aug 12, 2010
    Location:
    Berkshire, UK
    #1
    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.
     
  2. alphaod macrumors Core

    alphaod

    Joined:
    Feb 9, 2008
    Location:
    NYC
    #2
    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.
     
  3. psychometry macrumors regular

    Joined:
    Oct 5, 2006
    #3
    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.
     
  4. Major Reeves macrumors regular

    Joined:
    Jun 24, 2009
    Location:
    EUSSR
    #4
    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.
     
  5. iRobertM macrumors regular

    Joined:
    Jun 17, 2009
    Location:
    Seattle WA.
    #5
    Very dramatic, something tells me Steve is not twisting his mustache on this one. I am sure its a Intel architecture thing.
     
  6. skiffx macrumors 6502a

    Joined:
    Feb 5, 2008
    #6
    Or Apple simply riding an old design...
     
  7. Umbongo macrumors 601

    Umbongo

    Joined:
    Sep 14, 2006
    Location:
    England
    #7
    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.
     
  8. johnnymg macrumors 65816

    johnnymg

    Joined:
    Nov 16, 2008
    #8
    Go 3x8GB and you'll be golden.................... and a bit poorer to boot. :p
     
  9. eponym macrumors 6502

    Joined:
    Jul 2, 2010
    #9
    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.
     
  10. mattmower thread starter macrumors member

    mattmower

    Joined:
    Aug 12, 2010
    Location:
    Berkshire, UK
    #10
    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.
     
  11. mattmower thread starter macrumors member

    mattmower

    Joined:
    Aug 12, 2010
    Location:
    Berkshire, UK
    #11
    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.
     
  12. mattmower thread starter macrumors member

    mattmower

    Joined:
    Aug 12, 2010
    Location:
    Berkshire, UK
    #12
    Here in the UK Crucial are listing a 16GB part (2x8GB) for the 6-core 2010 MP:

    Are they just being super-confident that nothing has changed from the 2009 model?

    Matt
     
  13. Vylen macrumors 65816

    Joined:
    Jun 3, 2010
    Location:
    Sydney, Australia
    #13
    Max Memory Size of 24GB - presumably the maximum spread of 8GB modules across 3 memory channels.
     
  14. hugodrax macrumors 6502a

    Joined:
    Jul 15, 2007
    #14
    Pointless and poor value buying a single cpu mac pro.

    Either go Dual cpu or go iMac.
     
  15. Vylen macrumors 65816

    Joined:
    Jun 3, 2010
    Location:
    Sydney, Australia
    #15
    Poor value... maybe...

    But pointless? Pointless? ....
     
  16. eponym macrumors 6502

    Joined:
    Jul 2, 2010
    #16
    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.
     
  17. MacSince1985 macrumors regular

    Joined:
    Oct 18, 2009
    #17
    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!
     
  18. eponym macrumors 6502

    Joined:
    Jul 2, 2010
    #18

    No, your comment was pointless. You don't know what you're talking about.
     
  19. mattmower thread starter macrumors member

    mattmower

    Joined:
    Aug 12, 2010
    Location:
    Berkshire, UK
    #19
    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.
     
  20. Vylen macrumors 65816

    Joined:
    Jun 3, 2010
    Location:
    Sydney, Australia
    #20
    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...

    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.
     
  21. mattmower thread starter macrumors member

    mattmower

    Joined:
    Aug 12, 2010
    Location:
    Berkshire, UK
    #21
    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.
     
  22. mattmower thread starter macrumors member

    mattmower

    Joined:
    Aug 12, 2010
    Location:
    Berkshire, UK
    #22
    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
     
  23. Umbongo macrumors 601

    Umbongo

    Joined:
    Sep 14, 2006
    Location:
    England
    #23
    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.
     
  24. mattmower thread starter macrumors member

    mattmower

    Joined:
    Aug 12, 2010
    Location:
    Berkshire, UK
    #24
    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
     
  25. Vylen macrumors 65816

    Joined:
    Jun 3, 2010
    Location:
    Sydney, Australia

Share This Page