CAS Latency and RAM on MacPro

Discussion in 'Mac Pro' started by RAMtheSSD, Mar 26, 2013.

  1. RAMtheSSD, Mar 26, 2013
    Last edited: Mar 27, 2013

    RAMtheSSD macrumors member


    Mar 23, 2013
    High on a Mountain Looking for Wisdom
    First, the "Facts" I've been able to Established: E5520 does not support 1333 ram at native speed
    The -Chipset- does not support 1333 ram at native speed - unless I change the processors first.
    The CAS latency of the 1066 Ram (Currently, Apple ram) 7-7-7-20 as is the CAS latency of OWC 1066 ram.

    CAS latency of 1333 ram is 9-9-9-24 for all the ram I've been able to find.

    I did the Boot Rom Flash to 5,1 with the intent of replacing the processors in the future. However, I am upgrading my 8 gb to 32gb of ram (dire need of more ram) and I (depending on what is said here) would like to buy ram at the speed of the future processors so that I don't have to eat gobs of ram at that time.

    The Questions:
    Will the ram downclock ok?

    Is the higher latency offset by the wider window (21 for 1333 vs 17 for 1066)

    Has anyone done the Boot Rom Flash and upgraded the ram before the processors?

    I have seen this sort of latency difference make an observable difference on WinTel boxes (maybe due to window difference?) in the other direction (upgraded ram had lower latency, higher clock speed -still downclocked- and wider window) but all the Mac ram i've seen so far has higher latency for higher clock speed; what gives?

    I plan on doing this soon (next 5 days)
  2. VirtualRain, Mar 27, 2013
    Last edited: Mar 27, 2013

    VirtualRain macrumors 603


    Aug 1, 2008
    Vancouver, BC
    If its JEDEC compliant RAM it will have SPD timings for the lower clocks. So as long as its a reputable vendor, it should down clock just fine... Ideally with tighter timings depending on how they setup the table. However, if I recall from my over clocking days, SPD timings are often set pretty lose to ensure maximum compatibility between memory controllers and main boards and without any way on a Mac to measure or tweak them, you're likely stuck with this. For that reason, if RAM latency is critical to you (not likely) you should buy the tightest rated CAS latency RAM for the memory bus speed your system is rated for. For example, you could buy DDR3-1600 9-9-9, which has CAS latency of 11ns, but it may be programmed to run at a rather loose 8-8-8 at 1066 for latency of 15ms. And unfortunately, there's no way to know before you buy. Your best bet would be to buy RAM with the tightest timings you can get for your memory bus frequency (eg DDR3-1066 6-6-6... or more likely... 7-7-7). Only then will you really know what you're getting. Although having said all this, there's probably not much point as you're really splitting hairs on performance when you get into RAM latency an even memory bus frequencies and even channel interleaving.
  3. wonderspark macrumors 68040


    Feb 4, 2010
    I'll add that I did 32GB of RAM from OWC prior to both the 5,1 firmware flash and the CPU upgrade.

    Went from 1066 RAM to 1333 RAM on 4,1 firmware --> ran at 1066 just fine.
    Ran the 5,1 firmware flash on the old quad core 3.33 CPU --> ran at 1333 just fine.
    Swapped in the 6-core CPU --> Still runs at 1333 just fine, but more cores, yay.

    The original W3580 runs at 1333 normally, but the old 4,1 firmware was holding it back.
  4. RAMtheSSD, Mar 29, 2013
    Last edited: Mar 29, 2013

    RAMtheSSD thread starter macrumors member


    Mar 23, 2013
    High on a Mountain Looking for Wisdom
    Updating the Forum

    I got the OWC ram in PC-10600 DDR-3 1333 with the 9-9-9-24 Latency and the 21 Window. I have yet to make some of the experiments that would really confirm this but it does appear, at first glance, that the bigger window does make a difference -I will post the Geekbench 64 printouts with the memory performance when I try running with just the 1333 sticks- and that the difference is enough to make up for the latency.

    The improvement in the numbers is not stratospheric but the reduction in swap has made a noticeable difference. Oddly enough, the difference is measurable in OS X but it is NOTICEABLE in Windows 7 Pro 64! I really did not expect that!

    Incidentally, because there was so much talk about Crucial and Corsair ram, I checked with them just in case: neither had memory that was "compatible" by their scanner and what they had that might have been compatible was more expensive than OWC and not by a dime or two!

    I will also say that OWC customer service is beyond anything I have ever experienced and they get better every time! That alone would suffice for me to recommend them but there is more. Since it was not all that clear that just any ram would do -because of the potential for future upgrades- both a tech rep and an engineer got on the chat. After patiently exploring my plans, they made sure that the ram would work with all the plans before pointing me in the right direction.

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