RAM is 4x2 faster than 8x1 ?

Discussion in 'Mac Pro' started by Gonk42, Feb 16, 2008.

  1. Gonk42 macrumors 6502

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    #1
    I realise this subject has been done to death on various threads but I've not seen a comparison of like for like. BareFeats state that 8 are better than 4 but don't compare the same amount of RAM in each case, i.e. they don't compare 8GB with 8GB.

    I would expect that 4x2GB would have less latancy than 8x1GB so on average would be a tiny bit faster. Has anyone seen any test results?

    (I should say that I'm posting this out of scientific curiosity - from the practical point of view its better to go with 4x2 anyway because it leaves slots free.)
     
  2. kirkbross macrumors 6502a

    kirkbross

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    #2
    I think it's technically true, but I already bought a 6GB (6x1) kit from OWC so I am prone to want to believe that the difference in latency and performance is negligible.:rolleyes:
     
  3. QCassidy352 macrumors G3

    QCassidy352

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    #3
    I thought having 8 slots filled was actually faster than having an equivalent amount of memory with only 4 slots filled?
     
  4. barefeats macrumors 65816

    barefeats

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    #4
    I should have an answer by Monday. I received additional 1GB modules so I can do the 4 x 2 vs 8 x 1 scenario and test it with our 64 bit parallel multithreaded copy of STREAM.

    My hypothesis is that the 8 x 1 should be faster, but it doesn't "future proof" you if you plan to expand to 8 x 2 in the near future.
     
  5. ajx22 macrumors regular

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    #5
    Ram works better with parity - so having all slots filled is faster/better then not. The OS still 'looks' for RAM to be present in ALL present slots.

    This is the reason that the MBP's (etc.) ship with all slots filled (2x 1GB); vs only a single 2GB stick.
     
  6. tyr2 macrumors 6502a

    tyr2

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    #6
    I'd expect that 4x2 is faster than 8x1. Mostly because the memory is serially accessed. Any requests for the second pair in each riser must go via the memory controllers on the first pair first, thus increasing latency.

    You need 4 slots filled to achieve memory interlacing and the wider bus, increasing this to 8 won't increase this further.

    Will be interested to see the results of your testing barefeats.
     
  7. Gonk42 thread starter macrumors 6502

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    #7
    I look forward to seeing your results. People seem to have convincing arguments both ways. I tend towards the second set being slower as they are only accessed when the memory is not found in the first set which must lead to some delay but I am no expert.

    By the way, barefeats, by raising the matter of the tests not being like for like in terms of both being the same RAM size I was not trying to detract from the very interesting results that you've published to date - I'm just a slightly pedantic scientist:)
     
  8. orpheus1120 macrumors 65816

    orpheus1120

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  9. fridgeymonster3 macrumors 6502

    fridgeymonster3

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    #9
    take a look at barefeats.com. They look at the performance difference between 4x2 GB (8GB Total), 4x2GB + 2xGB (10GB total) and 8x2 (16GB Total). They concluded that the best performance comes from running either 4 matched or 8 matched DIMMS. The 10GB setup only saw a very slight performance increase over the 4x2 GB and the 16 GB was best overall. They suggest either 8x2 or 8x1 for best performance. However, if you are ever thinking about adding more RAM, the 8x1 isn't upgrade friendly.
     
  10. Umbongo macrumors 601

    Umbongo

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    #10
    Will you be benchmarking mixed sets also? The 6GB (2x2Gb & 2x1GB) and 10GB (4x2Gb & 2x1GB) configurations that many people see to have bought would be of interest I'm sure.
     
  11. Father Jack macrumors 68020

    Father Jack

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  12. student_trap macrumors 68000

    student_trap

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    #12
    I certainly hope so:D (4x2 on the way!)
     
  13. fridgeymonster3 macrumors 6502

    fridgeymonster3

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    #13
    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!:D


    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.
     
  14. gazfocus macrumors 68000

    gazfocus

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    #14
    I don't think the difference will be that noticeable.

    For me, I'll be getting 4 x 2GB because then it will give me the option to add more ram later where as if I filled all 8 slots, I would have no free slots.
     
  15. zdobson macrumors 6502

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    #15
    I would assume that you would want to leave the stock 2GB in if you order 4x2. Even though the difference isn't much, the stock 2GB will be faster in your machine than on the shelf.

    Also, I can't image the benefit of having all slots full with 8x1 would be greater than having the expandability of 4x2.
     
  16. fridgeymonster3 macrumors 6502

    fridgeymonster3

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    #16
    I agree with you completely, as you can see in my signature that is exactly what I did; however, the OP stated that he was interested in scientific curiosity not practicality in terms of expandability.
     
  17. zesta macrumors member

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    Jan 31, 2008
    #17
    Based on what I have read from multiple sources:

    2 GB modules should be slightly faster than 1 GB modules.

    8 dimms should have better throughput, but slightly more latency than 4 dimms.

    I don't think the difference will be noticeable, and I would recommend getting the 4x2, so you can upgrade easier later. That is what I am doing. You can also keep your original 2x 1GB for a total of 10 GB, which may be slightly slower than 4x 2GB, but probably not noticeable.

    -Randy
     
  18. barefeats macrumors 65816

    barefeats

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    #18
    Yes. I know that those two configs are very popular. They will be tested.
     
  19. barefeats macrumors 65816

    barefeats

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    #19
    That's a good point. If you are future proofing your investment and the difference turns out to be slight, then 4 x 2GB makes more sense that 8 x1.
     
  20. barefeats macrumors 65816

    barefeats

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    #20
    Test results for different combos

    We tested the different combinations with a special 64-bit parallel multi-threaded version of STREAM. I averaged the results of Copy, Scale, Add, and Triad to produce an overall speed rating in gigabytes per second:

    4 x 1GB = 4GB = 6.5GB/s

    6 x 1GB = 6GB = 6.7GB/s

    2 x 1GB + 2 x 2GB = 6GB = 6.5GB/s

    4 x 2GB = 8GB = 6.5GB/s

    8 x 1GB = 8GB = 7.5GB/s

    2 x 1GB + 4 x 2GB = 10GB = 6.8GB/s

    6 x 1GB + 2 x 2GB = 10GB = 7.5GB/s

    6 x 2GB = 12GB = 6.8GB/s

    4 x 1GB + 4 x 2GB = 12GB = 7.5GB/s

    2 x 1GB + 6 x 2GB = 14GB = 7.5GB/s

    8 x 2GB = 16 GB = 7.5GB/s

    Conclusion: Any combo of matching pairs that fills all 8 slots = fastest.
     
  21. FastMan macrumors member

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    Jan 9, 2008
    #21
    barefeats, any explanation for this? 10 as fast as 16?
     
  22. Spikeanator6982 macrumors 6502

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    Jun 13, 2007
    #22
    8GB 8x1GB the same speed as 10GB and 16GB. As Barefeats said, the speeds are the same if all slots are full. amount of ram and speed dont go together. As far an explaination, bc 10GB and 16GB both take up the 8 slots.:rolleyes:
     
  23. ksz macrumors 68000

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    #23
    Excellent! Now I will keep the stock 2 x 1GB when adding the new OWC 4 x 2GB for a total of 10GB. I was really expecting to discard the stock 2 x 1GB.
     
  24. zdobson macrumors 6502

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    #24
    I thought this combo wasn't possible according to info on this page:
    Memory Test

    "If you add two more sticks, they have to match those first two. You can add two more 1GB sticks in slot 2 on both risers for a total of 4GB. You can NOT add two 2GB sticks to that slot or your system will only report 2GB total."

    Let me know. I'm hope it is possible because it's what I'd like to do.
     
  25. zesta macrumors member

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    Jan 31, 2008
    #25
    You need to move the 1GB from one riser, to make a matched pair. So you end up with:

    in the top riser: 2GB, 2GB
    in the bottom riser: 1GB, 1GB

    They just need to be in matched pairs.

    -Randy
     

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