About memory mode

[fyc]

Hello SteveScargall

Thanks for your reply.
Sorry I can't post my reply with my phone successfully, so I make a new question.
The PMem we purchased has not arrived yet, we need to use simulation methods to do some solution verification in the early stage.
Our purpose is to build a memory pool with PMem.
If PMem is configured in memory mode, the memory seen by the operating system or the third-party tools is
(1) The original DRAM + PMem is used as memory
(2) Only PMem is used as memory
Which form of the above will we see?
Can the original DRAM still be seen?

[SteveScargall · Accepted solution]

When the system is configured in Memory Mode, the DRAM is used by the memory controller as a cache, so the OS does not have access to it. The total memory capacity seen by the OS will be the capacity of PMem only. For example, if your system uses an 8+8 memory population configuration with a 4:1 ratio of PMem:DRAM, you would have (8x128GB PMem = 1024GB) + (8 x 32GB DDR4 = 256GB). The OS would see 1024GB of memory using commands such as `lsmem`, `free -m`, etc and treat it as volatile memory. It cannot see or directly use the DRAM as this is handled by the memory controller. 

Using the same memory population and configuring the platform in AppDirect, the OS would see and have access to both the DRAM and PMem for a total of 1280GB. However, it's now up to the application or system administrator to decide and configure how the memory is accessed as this is not automatic. If the application is not PMem aware, there is a growing set of Memory Tiering solutions available that can assist unmodified applications to consume both DRAM and PMem, including:

• Linux Kernel Memory Tiering - Available in Kernel v5.15 or later
  • If you want to build your own Kernel with all the latest features, see the Intel branch
  • The README-tiering.md has more information for the features. 5.15 has the demotion code. The Intel branch has both demotion and promotion features. 
• Linux System RAM - Available in Kernel v5.1 or later
  • Read How To Extend Volatile System Memory (RAM) using Persistent Memory on Linux
• MemVerge Memory Engine
• MemKind Transparent Tiering
  • See the MemTier library
• Many research allocator papers and solutions

The above solutions achieve transparent memory, much like Memory Mode, but using software instead.