The V Distributed System

David R. Cheriton

One-line summary: Use a modular micro-kernel to make IPC fast and the OS
simple and to cluster your workstations.

Overview/Main Points

   * Kernel is distributed, in that separate copies of the kernel executing
     on each network node cooperate to provide a single system abstraction
     of processes and address spaces (using a base set of communication
     primitives). The computing power of a mainframe is provided, while the
     idle of single-user machines is reduced.

   * Small kernel approach is good for implementing the basic protocols and
     services in a modular way. In V, system-provided services are provided
     in the run-time libraries.

   * High performance communication is the most important factor for
     distributed systems. They worked to optimize the common cases and
     structure the kernel for optimal communication (fixed-sized messages,
     VMTP protocol for request-response, initialize processes with template
     header, ...). Optimizations helped to reduce the cost of multi-cast,
     which was important for the handling of shared state and group
     communication.

   * Protocols, not the software, define the system. Especially important is
     the ability to support groups in the protocol (for distributed systems,
     migration, and replication).

   * Kernel provides time, process, memory, communication, and device
     management in separate kernel modules, each of which is linked to the
     IPC facilities. All servers are accessed similarly whether they are
     local or remote. Modularity allows for fine-tuning of individual
     components.

   * Kernel implements only the amount of caching and consistency for memory
     and files and device management required to protect its integrity.
     Scheduling is primarily performed outside the kernel by a process that
     manipulates priorities.

   * I/O system is designed to impose a standard structure on messages
     between devices. Instead of open files, they use block-oriented,
     stateful UIO objects for everything (displays, devices, printers,
     internet protocols, pipes, etc.).

   * Object identifiers are used to identify objects such as files, address
     spaces, and directories in a host-independent manner. Clients find
     objects using multi-cast and caching. A global name server keeps track
     of services as well, in order to differentiate non-existence from
     failure.

Conclusions

The modular kernel does look like a good way to try out and test new
components/services, and to avoid the loading of unnecessary modules.
However, all of the modules that are given as examples are necessary and
would probably be loaded anyway, so I would think that a monolithic modular
kernel would offer better performance over their basic system.

Flaws

Scalability?

Performance? (these guys are really bad at this...)

What permissions are needed to add a new kernel server?

It is claimed that process termination is simplified because resources are
managed at the process level by various servers using garbage collection.
This would seem to take more time... (servers poll clients to see whether
they still exist).
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