Persistent Virtual Memory in the Great New Operating System In the Sky -- James Larkby Lahet

Talk by James Larkby Lahet.

Despite its age, the GNOSIS/KeyKOS system is like a time capsule from the future. KeyKOS was designed from the ground up to support multi-tenant utility computing; it represents a vision of the cloud from the days before dial-up.

My talk will target people with some familiarity with operating system design, but I will introduce all relevant storage concepts. There will be some brief background on the KeyKOS system in general because I think that its design, particularly in a historical context, is too cool to pass over. I will then focus on how a key component, Persistent Virtual Memory (PVM), functioned and the many unique benefits it provided. Finally I will discuss how new memory and storage technologies might help address the challenges of PVM, and how its benefits apply to modern server and mobile applications.

KeyKOS used capabilities (unforgeable resource names combined with access rights) to provide a level of security between users we often associate with hypervisors. It used a stateless message-passing kernel with library OSes to support execution of unmodified binaries of Unix and several other contemporary systems, without explicit hardware support for virtualization.

However, there is one piece of the KeyKOS system that is utterly alien. In order to achieve the desired level of reliability, KeyKOS employed a single-level store, or Persistent Virtual Memory, which will be the focus of this talk.

In KeyKOS, ephemeral process memory and persistent, on-disk files were not distinct. Instead all memory was treated as persistent, and checkpointed periodically. Processes were not directly aware of disk vs. RAM, although a journaling mechanism allowed explicit control of persistence for state external to the system, such as network requests.

The benefits of PVM were system-wide. This approach allowed a crashed system to quickly resume execution of all running tasks. In addition, the change to the nature of storage workloads in this system was profound; the authors of one paper tout greater than 90% disk bandwidth utilization, greatly improving I/O speeds. Finally, PVM simplified system design because tying state and execution together eliminated the risk of inconsistencies.

PVM is not simply "the road not taken", but it is an idea that is uniquely positioned to exploit new and upcoming storage technologies, such as Shingled Magnetic Recording disk drives and non-volatile byte-addressable memory.

In addition to being suited to the needs of servers, Persistent Virtual Memory also has a striking resonance with the persistence model of mobile applications and mobile platforms' approach to resource management.