[OS Notes] Virtual Memory OS

Table of Contents

Paging

  • Use disk to simulate larger virtual than physical memory

Working set model

  • Disk much, much slower than memory
    • Goal: run at memory speed, not disk speed
  • 8020 rule: 20% of memory gets 80% of memory accesses
    • Keep the hot 20% in memory
    • Keep the cold 80% on disk

Paging challenge

  • How to resume a process after a fault?
    • Need to save state and resume
    • Process might have been in the middle of an instruction!
  • What to fetch from disk?
    • Just needed page or more?
  • What to eject?
    • How to allocate physical pages amongst processes?
    • Wchich of a particular process’s pages to keep in memory?

Re-starting instructions

  • Hardware provides kernel with information about page fault
    • Faulting virtual address (In %cr2 reg on x86–may see it if you midify Pintos page_fault and use fault_addr)
    • Address of instruction that caused fault
    • Was the access a read or write? Was it an instruction fetch? Was it caused by user accss to kernel-only memory?
  • Hardware mush allow resuming after a fault
  • Idempotent instructions are easy
    • E.g., simple load or store instruction can be restarted
    • Just re-execute any instruction that only accesses one address
  • Complex instructions must be re-started, too
    • E.g., x86 move string instructions
    • Specify src, dst, count in %esi, %edi, %ecx registers
    • On fault, registers adjusted to resume where move left off

What to fetch

  • Bring in page that caused page fault
  • Pre-fetch surrounding pages?
    • Reading two disk blocks approximately as fast as reading one
    • As long as no track/head switch, seek time dominates
    • If application exhibits spacial locality, then big win to store and read multiple contiguous pages
  • Also pre-zero unused pages in idle loop
    • Need 0-filled pages for stack, heap, anonymously mmapped memory
    • Zeroing them only on demand is slower
    • Hence, many OSes zero freed pages while CPU is idle

Selecting physical pages

  • May need to eject some pages
    • More on eviction policy in two slides
  • May also have a choice of physical pages
  • Direct-mapped physical caches
    • Virtual -> Physical mapping can affect performance
    • In old days: Physical adddredd A conflicts with $kC+A$ (where k is any integer, C is cache size)
    • Applications can conflict with each other or themselves
    • Scientific application benefit if consecutive virtual pages do not confilict in the cache
    • Many other applications do better with random mapping
    • There days: CPUs more sophisticated than kC + A

Superpages

  • How should OS make use of “large” mappings
    • x86 has 24 MB pages that might be useful
    • Alpha has even more choices: 8KB, 64KB, 512KB, 4MB
  • Sometimes more pages in L2 cache than TLB entries
    • Don’t want costly TLB misses going to main memory
  • Or have two-level TLBs
    • Want to maximize hit rate in faster L1 TLB
  • OS can transparently support superpages
    • “reserve” appropriate physical pages if possible
    • Promote contiguous pages to superpages
    • Does complicate evicting (esp. dirty pages) - demote

Eviction policies

Starw man: FIFO eviction

  • Evict oldest fetched page in system
  • Example-reference string 1,2,3,4,1,2,5,1,2,3,4,5
  • 3 physical pages: 9 page faults

Thrashing

Details of paging

The user-level perspective

Case study: 4.4 BSD

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