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Linux 5.1 Will Let You Treat PMEM Like 3DXPoint Optane NVDIMMs Back As System RAM

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  • Linux 5.1 Will Let You Treat PMEM Like 3DXPoint Optane NVDIMMs Back As System RAM

    Phoronix: Linux 5.1 Will Let You Treat PMEM Like 3DXPoint Optane NVDIMMs Back As System RAM

    With broader availability expected soon for Intel Optane NVDIMMs backed by 3DXPoint memory, which offers a new means of speedy persistent memory, patches have landed in Linux 5.1 to optionally treat this persistent memory just like system RAM...

    http://www.phoronix.com/scan.php?pag...ds-PMEM-As-RAM

  • #2
    This brings the possibility to add 480 GB of memory for less than U$500. It's a good initial step.

    Of course it would be better if you could raid your 3D XPoint to increase the speed and if it could be allocated like a L4/5 cache (use faster memory first, 3D XPoint next, and SSD swap last), but small steps first and more features next release.

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    • #3
      Wear on this type of device is going to be interesting. Will they wear-level on it?

      Some flags like mutexes, TAS, etc are going to change a lot on busy systems.
      Even if we assume 5-10 million of write cycles, it could probably wear down pretty fast on some systems.
      Are they assuming that caches and cache-coherency are going to cover most of these rapid changes?

      Also, how about security if wear leveling?
      If it is a flat memory space, couldn't wear leveling expose more interesting data in case of a buffer overflow?

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      • #4
        What is the difference between this and just using the drive as swap? The only thing I can think of is that as system RAM the kernel will use it for disk buffers.

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        • #5
          Originally posted by linner View Post
          What is the difference between this and just using the drive as swap? The only thing I can think of is that as system RAM the kernel will use it for disk buffers.
          Using it as swap drive wouldn't give out a lot of benefits. Remember that kernel swaps in/out pages from RAM to swap (it's not 0-copy), which could result in trashing of you main storage (or secondary, or whatever).

          If you use Optane DRAM sticks the latency of such memory will be slower, but you will not need to constantly swap in/out pages and thus it's 0-copy (no copies are made). This also means that your main storage is not trashed if there is high load on memory side. Think of it as marking part of your memory as slower-tier memory (I don't actually know how kernel treats it).

          I wonder if tmpfs (e.g. /dev/shm) could exclusive be on Optane.

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          • #6
            Originally posted by davidlt View Post

            I wonder if tmpfs (e.g. /dev/shm) could exclusive be on Optane.
            That.

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