The Duality of Memory and Communication in the Implementation of a Multiprocessor Operating System

One-line summary: This paper discusses Mach's object-oriented semi-user-extensible memory system .

Overview/Main Points

• High Level Details:
  • This is an attempt to create an extensible operating system that can provide the functionality of UNIX when desired but is generally lighter weight.

  • Communication and virtual memory can play complementary roles in both distributed and parallel applications as well as in the implementation of the operating system kernel. Memory management can be performed by user programs, and performance can be improved for multiprocessors.

  • Memory Objects: In the object-oriented Mach system, memory is represented as an abstract object (containing a collection of data bytes) upon which several operations are defined. A data manager (which can be separate from the kernel) handles initial values and permanent storage.

  • Mach inherits Accent's single level store, in which primary memory acted as a cache of secondary storage. File data can be read in directly from memory instead of from disk.

  • Mach also inherits from Accent the ideas of tasks, threads, ports and messages. Each object has an associated port, through which its functionality may be accessed using messages (Messages can be used to transport data, pointers, or to make remote procedure calls). Each task (similar to a UNIX process) can have multiple threads running in its address space (made up of memory objects). Tasks and threads use their associated ports to communicate with the objects that they need.

  • Virtual Memory: Tasks allocate regions of memory by allocating memory objects and mapping them into their space. The kernel maintains control of paging and cache management. For instance, on page faults, the kernel makes a call to the memory object requesting the new data, and on write-back, the kernel makes a call prompting the object to flush the page to secondary storage. There is a detailed API given in the paper for all of the kernel and manager calls.

  • External memory management makes secondary storage objects accessible through message passing.

• Lower Level Details:
  • Mach uses address maps, memory object structures, resident page structures, and a set of page-out queues. The address map maps the valid address ranges to memory objects. The other structures contain things like object-specific info, physical mappings, and status info.

  • The kernel page fault handler is still responsible for validity and protection, page lookup, copy on write, and hardware validation.

  • Problems included blocking on user tasks, failure to release memory, malicious data changes, deadlock, and maintaining sufficient memory for the kernel pager (there must be available memory to send messages to request memory...).

  • Different types of multiprocessor architectures were presented. Uniform Memory Access (UMA) share all memory. Non-uniform Memory Access (NUMA) share all memory, but have associate memory with individual CPUs that takes longer for other CPUs to access. No Remote Memory Access (NORMA) machines are similar to NUMAs but provide no hardware supplied mechanism for remote memory access (ie NOW). Mach can do shared-memory (example) using message passing.

  • Several examples and sample applications are given in the paper. Since we have already seen the benefits of active messages, exokernel, and SPIN, I won't waste paper on the pluses here.

Conclusions

Questions

Their performance information was completely unsupported. They stated how their cache could be useful in improving the performance (UNIX only uses 10% of mem to cache). However, they gave no comparison numbers, nor did they give any load information (upon which the results would depend heavily).