Design issues

Allocation policies:

• local allocation:
  • replaces only pages of current process
  • assumes static process memory allocation size
  • problems:
    • growing working set leads to trashing
    • shrinking working set leads to wasted memory
• global allocation:
  • replaces pages owned by any process
  • strategies:
    • treat every global page equally, or prioritize
    • dynamic memory balancing using working set size estimation
    • selectively swap out processes, i.e. hurt process efficiency since it’s not as important (or OOM kill)

Working set model:

• working set estimation:
  • aging-based
  • scanning-based: like aging, but without temporal dimension. divide time in discrete slots, scan all reference bits in page table entries, decide how many slots
  • active lists (e.g. WSCLOCK): amount of pages in working set is known, want to decide which set of pages is in working set. active list of n elements (size of working set) which contains active pages
  • applications: memory prepaging, page replacement, checkpoint-restore, live migration
• working set size estimation:
  • sampling-based
  • monitoring-based (e.g. page fault freq)
  • Miss Rate Curves (MRC)
  • applications: dynamic memory balancing, garbage collection, WS estimation

Cleaning policy:

• paging works well if there are a lot of free pages
• direct reclaim: when you have the need to allocate new page, you directly reclaim an existing page
• indirect reclaim: paging daemon sleeps and then evict pages if free pages are in short supply (don’t wait until the last minute)
• another way is to leave them in memory, easy reuse

Virtual memory interface:

• allocator interface: malloc family, mmap family
• memory-mapped files
  • treat files as memory objects for faster IO
  • facilitate code sharing for programs/shared libs
• copy-on-write semantics
  • lazy copying for fork(), deduplication, checkpointing, etc.
• shared memory
  • MAP_SHARED mappings, key-based, file-based
• distributed shared memory
  • shared memory semantics over network