Speed hit when adding 4th RAM stick to 4,1/5,1 6-Core?

Discussion in 'Mac Pro' started by Sharky II, Jul 26, 2015.

  1. Sharky II, Jul 26, 2015
    Last edited: Jul 27, 2015

    Sharky II macrumors 6502a

    Sharky II

    Jan 6, 2004
    United Kingdom

    I'm currently running a 4,1 (flashed to 5,1) 2009 Mac Pro (6-Core 3.46GHz) with 24GB RAM (3 x 8GB 1333MHz), and would like more RAM to use with Logic X.

    Can someone please confirm which of these following scenarios, if any, apply when adding a 4th stick (8 or 16GB) to my system:

    1) Adding the 4th stick immediately reduces the speed of all RAM sticks regardless of how much RAM is used

    2) Adding the 4th stick allows the system to run at triple channel speed until >24GB RAM usage, and the data on the 4th stick is slightly 'slower' - but under 24GB usage, there is no performance hit.

    3) Adding the 4th stick forces slots 3 and 4 to share the channel, so you get a speed hit on >16GB usage (or greater than whatever you have in slots 1 and 2) - but under 16GB usage, there is no performance hit.

    Or, something else?!

    I realise that most people say that the speed drop is 'negligible', but memory speed is the main bottleneck for many of the tasks I run, so I'm just trying to understand how it works.


  2. crjackson2134 macrumors 68040


    Mar 6, 2013
    Charlotte, NC
    3) As far as I know. You aren't actually doing anything but sharing bandwidth between memory slots 3/4.
  3. mikeboss macrumors 65816


    Aug 13, 2009
  4. ActionableMango macrumors G3


    Sep 21, 2010
    I don't know, and it would be interesting to learn more, but it doesn't really matter because there are a couple of points that trump any of that:
    1. If your memory usage is exceeding available RAM and you are paging out to the hard drive, that will be incredibly slow compared to any reduced speed by adding a 4th stick.
    2. If you aren't exceeding your available RAM, then adding more RAM isn't going to speed up your computer in the first place.
  5. Fangio macrumors 6502

    Jan 25, 2011
    Not sure in what kind of real world applications this would actually have a noticeable impact. With 32 vs 24 GB the difference here was barely measurable in Geekbench

  6. AlexMaximus macrumors 6502a


    Aug 15, 2006
    A400M Base
    For me it was exactly scenario 1)

    I have tried several options with no success. If you fill up all four slots, you will loose 10 - 15 % overall speed.
  7. Thessman macrumors regular

    Dec 8, 2005
    The speed hit depends on the apps you'll use and how intensely the RAM is being used.
    As per the MacRumors guide:


    The Nehalem Mac Pros use DDR3-1066 (PC3-8500) ECC DIMMs (not FB-DIMMs), no large heatsinks, with temperature sensors. There are 4 memory sockets (2009 Quad-core 2.66GHz, 2.93GHz, 3.33 GHz and 2010 2.8GHz and 3.2GHz) or 8 memory sockets (2009 8-core 2.26, 2.66GHz, 2.93GHz and 2010 8-core 2.4GHz). They can implement dual channel or triple channel memory access modes. Fastest performance is using triple-channel mode, with three identical modules in sockets 1 through 3 and leaving socket 4 empty.

    That said the actual speed decrease, so long as you're not using the extra ram is not more than 2%.
  8. Sharky II thread starter macrumors 6502a

    Sharky II

    Jan 6, 2004
    United Kingdom

    Sure, but what i want to know is what's going on with projects that use between 16-24GB of memory. Or, in fact, if even on smaller projects (way under 16GB), the speed of the memory is compromised at all!


  9. Sharky II thread starter macrumors 6502a

    Sharky II

    Jan 6, 2004
    United Kingdom
    Aha, quite the difference!

    For anyone wondering, I use Logic X all day, with some huge projects.

    The memory speed (and not just the raw core speed of the CPU) is a prime factor in low latency (32-64 sample buffer) performance for VST/AU instruments, especially large sample libraries (Kontakt etc). That's why going from a 2008 to a 2009+ Mac Pro gives a huge speed boost of low latency large sample libraries, because of the absence of the FSB bottle neck. If the machine can't keep up, you get lots of pops and crackles, stuttering etc.

    But, when my (usually very large) projects get to the mixing stage, low latency isn't a concern (I up the buffer size to 1024 instead of 64 when recording/writing) and projects with a lot of Flex-time pitch correction use tons and tons of RAM and it can easily exceed the 24GB.

    I'm not even sure if it's a memory leak bug, but apparently, right now when typing this, Logic X is using 28GB of my 24GB of RAM :) Generally everything is fine until you try to undo, and then you have to wait a little. I've seen Logic use 48GB of ram according to iStat!

    So, I'd like optimal performance in both scenarios, but can't quite afford 3 x 16GB sticks! Maybe one 16GB stick and two 8GB (32GB total, and selling one of my 8GB sticks) would be a good compromise (I think it's ok to mix sizes).


  10. IowaLynn macrumors 65816


    Feb 22, 2015
    MacPerformanceGuide did a lot of graphic tests with various amounts of memory. Adding in some PCIe-SSD blades (2-3x better performance than SATA III, near zero ? latency and great queue depth and IO)

    Mixing RDIMMs with others (16GB DIMMs are registered) is a no-go, so if you want to hold that door open for the future maybe start with 2x16GB, plan for more than even 32GB.
  11. js2704 macrumors member

    May 20, 2015
    I have the 8 core (dual 2.4) and bought a bunch of 4GB sticks from eBay to give myself 32GB. The speed difference was actually noticeable when I read one of these articles a year or so ago and yanked one stick from each bank. I've only got 24GB now and it's faster.

    The caveat is that if you're hitting swap because you're running out of memory it'll be way slower - I checked and was never going over 18-19GB.

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