Papers

SPIN -- An Extensible Microkernel for Application-specific Operating System Services (1995)

Summary. Many operating systems were built a long time ago, and their performance was tailored to the applications and workloads at the time. More recent applications, like databases and multimedia applications, are quite different than these applications and can perform quite poorly on existing operating systems. SPIN is an extensible microkernel that allows applications to tailor the operating system to meet their needs.

Existing operating systems fit into one of three categories:

1. They have no interface by which applications can modify kernel behavior.
2. They have a clean interface applications can use to modify kernel behavior but the implementation of the interface is inefficient.
3. They have an unconstrained interface that is efficiently implemented but does not provide isolation between applications.

SPIN provides applications a way to efficiently and safely modify the behavior of the kernel. Programs in SPIN are divided into the user-level code and a spindle: a portion of user code that is dynamically installed and run in the kernel. The kernel provides a set of abstractions for physical and logical resources, and the spindles are responsible for managing these resources. The spindles can also register to be invoked when certain kernel events (i.e. page faults) occur. Installing spindles directly into the kernel provides efficiency. Applications can execute code in the kernel without the need for a context switch.

To ensure safety, spindles are written in a typed object-oriented language. Each spindle is like an object; it contains local state and a set of methods. Some of these methods can be called by the application, and some are registered as callbacks in the kernel. A spindle checker uses a combination of static analysis and runtime checks to ensure that the spindles meet certain kernel invariants. Moreover, SPIN relies on advanced compiler technology to ensure efficient spindle compilation.

General purpose high-performance computing, parallel processing, multimedia applications, databases, and information retrieval systems can benefit from the application-specific services provided by SPIN. Using techniques such as

• extensible IPC;
• application-level protocol processing;
• fast, simple, communication;
• application-specific file systems and buffer cache management;
• user-level scheduling;
• optimistic transaction;
• real-time scheduling policies;
• application-specific virtual memory; and
• runtime systems with memory system feedback,

applications can be implemented more efficiently on SPIN than on traditional operating systems.