Question about the 4-core and 8-core nehalem MPs

Discussion in 'Mac Pro' started by 20eman, Mar 20, 2009.

  1. 20eman macrumors member

    Mar 10, 2009
    Hypothetical question here.

    Suppose we have a quad-core 2.93GHz Mac Pro and an eight-core 2.93GHz Mac Pro. Suppose the machines are otherwise configured the same, i.e., identical hard drives, identical graphics cards, identical RAM configuration such as 4x2GB, identical operating system.

    Would the performance of these machines be equivalent for all programs that use four cores or less? For all tasks that don't use multithreading?
  2. gnasher729 macrumors P6


    Nov 25, 2005
    Practically the same performance for most apps; there may be differences in either direction, and the differences may be large.

    Biggest difference: The 2 x 4 core has 2 x 12 MB L3 cache instead of 1 x 12 MB. That will help a lot if you have four threads doing completely different things. You would have 6 MB cache per thread instead of 3 MB. On the other hand, if four threads all access the same data, the OS may put them onto different chips, which slows communication between threads down. On the other hand, such an app can tell the OS that all the threads should run on the same processor.

    There is also "Turbo mode", where the processor can increase the clock speed if only one core is used. A 2 x 4 core could switch into Turbo mode with two threads running, one on each processor.
  3. alanlindsay macrumors member

    Mar 4, 2008
    The 2.66, 2.93 quad core xeons W35XX series have a memory bandwidth of 4.8GT/s while the 2.66, 2.93 Octo E55XX have 6.4GT/s. That would be the another difference in the scenario you mentioned.
  4. 20eman thread starter macrumors member

    Mar 10, 2009
    So, what I'm hearing is: if I'm not likely to be using multiple threads, then any benefit of the octo vs. the quad is 1) unclear 2) probably not worth $2,750.

    In other words, if I don't need more than 4 cores and I'm happy with only 8GB of RAM, then the quad-core is almost twice as good of a deal. If I've only got $3300 to spend, I'll probably do much better with the 2.93 quad than the 2.26 octo.

    Is there any concrete data on Turbo Boost? Precisely what conditions cause the processor to go into Turbo mode? What effect does this have on performance?
  5. nanofrog macrumors G4

    May 6, 2008
    1. Essentially non existant.
    2. A waste of money, unless you absolutely know you will need 8 cores soon.
  6. alanlindsay macrumors member

    Mar 4, 2008
    I do agree with the statement that if you don't need 8 cores it would be a waste of money to buy the octo mac pro.

    One thing that I think is important is to recognize that the quad core MP is basically just an i7 with ECC ram. The octo is really a different beast by virtue of its ability to manage two physical CPUS. Considering that an i7 machine can be had for around $1000 it's really a big ask at $2500. Imac not an option?
  7. 20eman thread starter macrumors member

    Mar 10, 2009
    The un-upgradability of the iMac is a deal-breaker for me, since one of my goals is to be able to upgrade the graphics card in the future. Even if that wasn't true, I see no reason to buy an Apple screen when I already have a perfectly good Dell screen.

    Since I have mac-only software, a PC is not an option, and Hackintosh is not an option due to ethical reasons.

    It's not the ideal situation, but for me the only SINGLE computer that works is an upgradable Mac desktop. (I've been limping along with a two-computer system, and I'm tired of it. I've held off upgrading for some 6-7 years at this point.) I'm not a pro, but the hole in the Mac desktop lineup means I have to buy a Mac Pro. But let's not talk about that.

    Are there any Turbo Boost test results?
  8. Lucibelle macrumors member

    Mar 12, 2009
    If you are currently using a 6-7 year old Mac, you will be VERY happy with any of the '09 MPs, quad or octo. Promise. :)
  9. 20eman thread starter macrumors member

    Mar 10, 2009
    Yeah, what most people are complaining about is the RAM restriction, but for my needs, 8 GB is more than sufficient. My two computers have a combined 2.5 GB of RAM between them, and that's been enough.

    The real reason I'm upgrading is to (finally) get an Intel Mac, and to consolidate my two computers into one. I'm kinda getting tired of 10.3.9. Plus the radeon 4870 will be a nice upgrade from my geforce 7800GS (an AGP card for dang sake).

Share This Page