Memory Virtualization

• Memory Virtualization
  • Shadow Page Table
    Direct Mapping from GVA → HPA
    VMM handles updating the pages
    Traps to VMM thus require context switches and lot more cycles
  • Extended Page Tables
    Mapping from GVA → GPA and then GPA → HPA
    This is done with the help of MMU thus requires hardware support
• Memory Reclamation Techniques
  • Uncooperative Swapping
    Forcefully free pages
    can free important pages thus leading to performance issues
  • Ballooning
    Make a dummy device inside the guest os and ask for pages (inflating the balloon)
    whatever pages gets allocated, free them up
    has the advantage of cooperating with the guest os
    can take more time to reclaim the space
  • Memory Sharing
    Any pieces of memory which is same for multiple guest os’s can be mapped to the same physical memory.
    Method: Scan pages and match based on hash-based similarity