Exokernel: An Operating System Architecture for Application-Level Resource Management

Motivation

• Traditional OS architectures are slow and too restrictive: they constrain applications to using a single interface
• Different applications have different requirements; performance studies show greater application control has benefits (e.g. virtual memory, file caching)
• Kernel can be made small and efficient by making it as close to hardware as possible

• Exports instruction set plus system calls to encapsulate privileged instructions
• Secure bindings for reservation of resources by applications
• Application-visible revocation and abort protocols for resources
• Protected control transfer between applications as a basis for IPC, as with Lightweight Remote Procedure Call
• Packet filters and Application-specific Secure Handlers to allow general processing on receipt of packets

• Small kernel runs in physical memory
• Low domain crossing overhead: 18 instructions
• Dynamic packet filter for ASHes, created and compiled to object code at run-time, 10X faster than interpreted filters
• Round-robin scheduling with reservation of timeslices, can be passed to another process on protected control transfer
• Applications can opt for contiguous or dispersed time slices!
• Stack and non-stack paths for system calls

• Provides application-level virtual memory
• Easily extended, e.g. choose between regular or inverted page table versions for best performance
• Outperforms Ultrix on matrix-multiplication, Appel and Li benchmarks

• Comparison to Ultrix is made without making clear what is left out! How compatible is the Exokernel architecture with existing OS design?
• Does the Exokernel approach make sense when we all have computers on our desktops?
• Does Exokernel make kernel hackers out of application designers? Don't we just want multiple operating systems?
• How portable is a library OS?
• Paper is vague on what a Unix-style process maps to. Does an Exokernel "processor environment" have 1 Unix process, or several? (Coordination versus protection)