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A summary by Nick Rayner for PSU CS533, Spring 2006

Machine-Independent Virtual Memory Management for Paged Uniprocessor and Multiprocessor Architectures R. Rashid, et al. 2 nd Symposium on Architectural Support for Programming Languages and Operating Systems Palo Alto, October 1987. A summary by Nick Rayner for PSU CS533, Spring 2006.

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A summary by Nick Rayner for PSU CS533, Spring 2006

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  1. Machine-Independent Virtual Memory Management for Paged Uniprocessor and Multiprocessor ArchitecturesR. Rashid, et al.2nd Symposium on Architectural Support for Programming Languages and Operating SystemsPalo Alto, October 1987 A summary by Nick Rayner for PSU CS533, Spring 2006

  2. Overview of Contribution • Portability • Fixed, simple hardware interface • Capability • Advanced memory management in software • Integration • Memory management is message passing • Customization • User-defined memory handlers

  3. Mach Review • Task: resource allocation unit • Paged virtual address space • Thread: CPU utilization unit • Independent program counter within task • Port: protected queue for messages • Message: typed collection of data objects • Memory Object: managed data collection • Can be mapped into task address space

  4. VM Features • Address ranges map to memory objects • 9 standard task operations available • Copy-on-write sharing between tasks • Read/write sharing with child tasks • “Pager” tasks associated with memory objects control faults and page-outs

  5. Data Structures • Resident page table • Indexed on physical page number • Address map • Virtual ranges to object regions • Memory objects • Backing storage, kernel or user managed • Pmap • Machine dependent memory mapping

  6. Memory Paging • Physical pages ≠ hardware pages • Physical size a boot time parameter • Must be power of 2 multiple of hardware size • Physical pages ≠ memory object pages • Object handlers may use their own policies

  7. Resident Page Table • Modified and Reference bits • Memory object list links table entries for each object • Allocation queues for free, reclaimable, and allocated pages • Hash buckets based on object & offset

  8. Address Maps • Doubly linked list of entries sorted by virtual address • Byte offsets within a memory object for task virtual address ranges • Inheritance and protection attributes • Last fault “hint” pointers may be kept

  9. Memory Objects • Reference counters allow garbage collection • Cache maintained for rapid reuse • Pagers may request caching • Kernel manages pages in primary memory • Pager handles store and fetch • Standard ports and fixed interface govern pager--kernel communication

  10. Sharing • Copy-on-write: “shadow object” created • If no write, only contains link to source • On write, contains update and link • Chain develops with repeated copies • Kernel collects unneeded intermediates • Read/write: indirection via “sharing map” • Equivalent to address map entry • Pointed to by address maps of sharing tasks

  11. Machine Dependence: pmap.c • Must implement 16 specific routines (even if they have no function) • Must maintain pmaps • Must switch pmap appropriately • Not expected to have full accounting • Should have “frequently referenced task addresses” and current kernel map

  12. Hardware Assessment • By isolating architecture role to support of Mach-defined interface, the authors claim the system can provide a “relatively unbiased assessment” of hardware design alternatives • No mention is made of how one could control for the specific pmap.c implementation used in each case

  13. Performance Tables 7-1 and 7-2 of the paper • Net benefit shown for all machines • Note that user-space pagers or memory objects unlikely

  14. Weaknesses • Shadow chains can be lengthy and redundant. Kernel garbage collection may be incomplete and requires “complex” locking for multiprocessors (suggesting overhead and possible contention). • Paging services at user level require context switching away from kernel fault handler and then back again • TLB consistency issues in SSM

  15. Strengths • Porting requires altering only a single module, and has been successfully accomplished by a novice C programmer • Sharing facilities improve parameter passing possibilities • External memory objects with paging services support distributed systems as well as arbitrary backing storage

  16. Conclusion • Portability and capability demonstrated (at least with kernel-managed objects) • No clear sense of how the tradeoffs might play out with significant user-level additions

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