Why more RAM if Mac is accessing SSD at 2gb/s?

Discussion in 'Mac mini' started by FilmIndustryGuy, Dec 14, 2018.

  1. FilmIndustryGuy macrumors regular

    May 12, 2015
    Manhattan Beach, CA
    why would one need more than 8gb of ram if OSX is accessing RAM at more than 2gb/s? or am I wrong as to how this works? I'm under the impression that mac uses up the 8gb ram quickly and compresses everything else but accesses it off the SSD at high speed simulating more ram.
  2. stooie3000 macrumors newbie

    Feb 24, 2014
    The SSD speeds in modern macs certainly make paging less noticeable than it used to be with spinning HDD’s but modern RAM is still around 10x faster (please someone correct me if I’m wrong). I think DDR4 is around 20GB/s while the apple SSD’s are around 3GB/s read and 2GB/s write.

    Depending on your workflow, and how much you end up paging to the SSD, it could make a huge difference to usability (but entirely depends on what you’re doing!)

    Edit: I’m not an expert on this, but I suspect there’s more to it than simply max transfer speeds. Latency may well be a significant factor as well. Hopefully someone more knowledgeable than me will chime in and clear things up!
  3. Ploki macrumors 68040

    Jan 21, 2008
    Mac Mini RAM is around 22000mb/s r/w.
    Mac Mini is:
    128gb: 600mb/s write
    256: 1200 mb/s write
    512: 1900 mb/s write
    1tb: 2600 mb/s write
    2tb: 2700 mb/s write

    Read is consistent across the SSD range.
    10 years ago memory was already at 6-10gb/s... so SSD is not even on par with with decade old memory.

    and yes, as @stooie3000 mentioned, RAM has a faster direct to CPU interface while SSD is connected to CPU via PCIe. In some cases PCIe is connected directly to CPU, and in some via PCH (platform controller hub), which adds some additional latency.

    in any case, you can expect 10-20x worse performance with doing paging from SSD as opposed to using RAM.
    In case of HDDs, that was painfully uselessly slow. It was the point where everyone said "i need to restart my computer"
  4. SoCalReviews, Dec 15, 2018
    Last edited: Dec 15, 2018

    SoCalReviews macrumors 6502a

    Dec 31, 2012
    It's not just about memory access speed. The system RAM provides a very different function than the SSD memory. In the Mac Mini RAM is allocated by the system for video RAM and it is engineered to be a stable memory resource directly accessed by the CPU in real time. RAM is designed to be accessed over and over with a high level of reliability and has a very high MTBF (mean time before failure). The SSD has a lower MTBF. In other words the more the SSD is accessed the more likley to fail from that usage over time.
  5. pl1984, Dec 15, 2018
    Last edited: Dec 15, 2018

    pl1984 Suspended

    Oct 31, 2017
    There are several factors:
    • As stooie3000 already mentioned SSD speed is still considerably slower than RAM.
    • The multi GB/sec numbers we see used in these discussions is representative of sequential speeds. Random speeds are considerably slower...in the 10s to low 100 MB/sec range... resulting in the first bullet being more applicable than before (RAM access contains a significant amount of random access).
    • Paging requires overhead. The system has to manage the task of paging which itself consumes resources.
    • SSDs are not "byte" level addressable. SSDs need to be written in blocks (typically 4K in size). When a byte needs to be updated on the SSD the systems needs to read the relevant 4K block, update the single byte, and then write the updated 4K block back to the SSD (unless the data to be written is 4K in size and aligned on a 4K boundary which means the block can just be written).
    • An SSD block cannot simply be overwritten, it needs to be erased before it can be reused. A modern SSD would write to an existing free block and allow garbage collection (i.e. TRIM) to free the block. Given the nature of memory use such activity could quickly consume the existing free blocks on a drive and therefore force the drive to have to erase blocks before they could be reused.
    It is the last two bullets which, IMO, are the largest factors in using an SSD as a replacement for RAM and, to a lesser degree, paging. The explanation is greatly simplified to illustrate the point and get too confusing.
  6. jinnyman macrumors regular

    Sep 2, 2011
    Lincolnshire, IL
    Set aside the technical reasons, I can confirm that as soon as my 2018 Mac mini starts to swap heavily, slowness kicks in and you can definitely feel it. If you plan to do heavy memory stuff, get more ram.

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5 December 14, 2018