Machine-independent VM: Mach

Description of Mach

   * Mach: a Unix-compatible OS intended for distributed environments.
   * Primitive elements in Mach: tasks, threads, ports, messages, memory
     objects. Provide location-independence.
   * Mach VM implementation separated into machine-dependent and
     machine-independent parts. Most of functionality is in
     machine-independent section.
   * Implementation's data structures: resident page table, address map,
     memory object, pmap. Only pmap is machine-dependent.
   * Features: sparse address spaces, memory-mapped files, more.

What to take away

   * Kernel VM code can be portably implemented with a minimum of
     machine-dependent code. This allows easy porting, and also improves
     performance since code isn't targeted for just one canonical processor.
   * Shared memory uses copy-on-write with shadow pages to reduce memory
     footprint. Some additional implementation complexity ensues.
   * User-level pagers are possible for advanced applications and
     sophisticated requirements.
   * Efficient VM optimizations can make message-based kernels more
     reasonable in terms of performance.
   * The concept of memory objects & associated user-mode pagers (e.g. the
     idea that physical memory is just a cache for virtual memory) enables
     distributed computing and other nifty functionality.
   * Performance is still good-- at least as good as Unix-- or so they
     claim. Their benchmarks are not very comprehensive or convincing.

Criticisms

   * More discussion and benchmarks of the distributed computing scenario
     would be nice.
   * The paper made a big point out of how the Mach VM implementation was
     mostly machine-independent, and claimed that this ``makes possible an
     unbiased examination of the pros and cons of various hardware memory
     management schemes''. However, they never did much of a comparison.