PCIe 2.0 and RAM speed comparison question

Discussion in 'PowerPC Macs' started by RedCroissant, Oct 25, 2015.

  1. RedCroissant Suspended

    Joined:
    Aug 13, 2011
    #1
    I love the PowerPC Macs that I own and have owned but I have a question for the group sort of about the newer Intel Macs since I work at Best Buy and only deal with Intel machines when it comes to Apple products.

    Is PCIe 2.0 almost as fast as RAM to the point where using the swap or paging out is almost irrelevant?

    I know when not enough RAM is available that the data will be temporarily written to the HDD which in turn slows down the system drastically. With the use of SATA SSDs and the faster R/W rates, that is probably less of a concern but still likely unless the amount of RAM is sufficient.

    With each advancement of PCIe, is it likely that actually having separate RAM will no longer be a necessity? Because if the PCIe-based flash storage is just as fast as RAM, then wouldn't using the swap have an almost negligible effect on performance?

    Thanks in advance for any clarification of this since me looking this up before wasn't making it very clear.
     
  2. Anonymous Freak macrumors 601

    Anonymous Freak

    Joined:
    Dec 12, 2002
    Location:
    Cascadia
    #2
    PCIe 2.0 is approximately 500 MB/s per lane. The average PCIe SSD uses 2 or 4 lanes, so 1-2 GB/s.

    For comparison, the fastest PCIe there is, 16 lanes of PCIe 3.0, is about 16 GB/s.

    Memory is still much faster.

    The DDR2 RAM at 533 MHz that shipped in the fastest G5s could do 4.2 GB/s per channel (G5s used two memory channels, so 8.4 GB/s. Still well in excess of a standard 4 lane PCIe 2.0 port.)

    The DDR3 RAM in the latest Mac Pro uses quad-channel 1866 MHz memory - a whopping 15 GB/s per channel, or about 60 GB/s. Still much faster than even the full 16 lanes of PCIe 3.0. (Which the Mac Pro only has for its GPUs.)

    And DDR4 just hit the market, promising even faster speeds.

    PCIe isn't going to catch up to RAM any time soon.
     

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