理解Computer中memory virtualization

• memory virtualization

  wikipedia

  In computer science , memory virtualization decouples volatile random access memory(RAM) resources from individual systems in the data centre, and then aggregates those resources into a virtualized memory pool available to any computer in the cluster.

  The memory pool is accessed by the operating system or applications running on top of the operating system.

  The distributed memory pool can then be utilized as a high-speed cache, a messaging layer, or a large, ahsred memory resource for a CPU or a GPU application.

• Description

  Memory virtualization allows networked, and therefore distributed, servers to share a pool of memory to overcome physical memory limitations, a common bottleneck in software performance.

  The memory pool may be accessed at the :

  • application level

    the pool is accessed through an API or as a nerworked file system to create a high-speed shared memory cache

  • operating system level

    a page cache can utilize the pool as a very large memory resource that is much faster than local or networked storage

  Memory virtualization implementations are distinguished from shared memory systems. Shared memory systems don’t permit abstraction of memory resources, thus requiring implementation with a single operaing system instance(i.e. not within a clustered application environment)

  Memory virtualization is also different from storage based on flash memory such as solid-state drives (SSDs) - SSDs and other similar technologies replace hard-drives(networked or otherwise), while memory virtualization replaces or complements traditional RAM.

  上面這麼多話，核心就是，memory virtualization 旨在擴展RAM的能力。

• 前情回顧

  根據上面兩段，需要先了解：

  • 《理解Computer中shared memory systems》
  • 《理解Computer中flash memory》
  • 《理解Computer中virtual memory》

• Backgroud

  Memory virtualization technology follows from memory management architechtures and virtual memory techniques.

  In both fields, the path of innovation has moved from tightly coupled relationships between logical and physical resources to more flexible, abstracted relationships where physical resources are allocated as needed.

  Virtual memory systems abstract between physical RAM and virtual addresses, assigning virtual memory addresses both to physical RAM and to disk-based storage, expanding addressable memory, but at the cose of speed.

• 擴展

  • Virtual memory - Traditional memory virtualization on a single computer, typically using the translation lookaside buffer (TLB) to translate between virtual and physical memory addresses
  • In-memory database - Provides faster and more predictable performance than disk-based databases
  • I/O virtualization - Creates virtual network and storage endpoints which allow network and storage data to travel over the same fabrics (XSigo I/O Director)
  • Storage virtualization - Abstracts logical storage from physical storage (NAS, SAN, File Systems (NFS, cluster FS), Volume Management, RAID)
  • RAM disk - Virtual storage device within a single computer, limited to capacity of local RAM.
  • InfiniBand
  • 10 Gigabit Ethernet
  • Distributed shared memory
  • Remote direct memory access (RDMA)
  • Locality of reference
  • Single-system image
  • Distributed cache