Machine-Independent Virtual Memory Management for Paged Uniprocessor and Multiprocessor Architectures R. Rashid, et al. 2nd Symposium on Architectural Support for Programming Languages and Operating Systems Palo Alto, October 1987

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