Mnemosyne

• an interface for programming with persistent memory
• write speed: 3 microseconds

Introduction

• Existing operating systems bifurcate volatile storage (memory) and disk storage. Mnemosyne wants to provide SCM through an abstraction of persistent memory.
• Not a replacement for files
• Uses (moderate data):
  • document edit snapshots
  • logs
• Key Points:
  • simple to use
  • system must allow data structure change recovery
  • compatibility with existing processors
• persistent memory regions: segments of memory stored in SCM
  • can be created automatically
  • to hold variables marked with keyword pstatic
  • virtualized by swap files
• persistence primitives: operations that support updating data
• durable memory transaction: enables in-place updates for arbitrary structures
• NV-Heaps: user-level persistent memory transactions
• Implementation: pair of libraries & changes to Linux kernel (for SCM pages)

SCM

• Existing Technologies:
  • PCM (phase-change memory)
    • low power requirements
    • closest to deployment
    • speeds
      • READ: currently 2x slower than DRAM
      • WRITE: currently 2000x slower than DRAM
      • prototypes project that read speeds will match DRAM
    • wears out after 10^8 writes
  • STT-RAM (spin torque transfer RAM)
  • memristors
  • above is only available in small sizes (< 1GB)
• User mode access is achieved by using persistent regions
• Consistent updates are achieved by use of persistent primitives and memory transaction support
• pmap: can be used to create a persistent region; similar to nmap
• Mnemosyne also provides a persistent heap (for dynamically allocated variables)
• Memory is virtualized by:
  1. Recording the mapping of virtual to physical persistent memory regions
  2. Swapping SCM to backing files
• Virtualization stops other programs from monopolizing finite space of SCM
• persistent keyword in C to mark pointers for type-safety
• atomic keyword makes sure a block of code is written in a memory transaction

Vocabulary

• SCM: storage class memory
  • provide the interface (load / store) of memory with the persistence of disks
• bifurcate: to divide into two parts / branches
• consistent update: a transformation of data from one consistent state to another