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VM Design Issues

VM Design Issues. Vivek Pai / Kai Li Princeton University. Mini-Gedankenexperimenten. What’s the refresh rate of your monitor? What is the access time of a hard drive? What response time determines sluggishness or speediness? What’s the relation?

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VM Design Issues

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  1. VM Design Issues Vivek Pai / Kai Li Princeton University

  2. Mini-Gedankenexperimenten • What’s the refresh rate of your monitor? • What is the access time of a hard drive? • What response time determines sluggishness or speediness? What’s the relation? • What determines the running speed of a program that’s paging heavily? • If you have a program that pages heavily, what are your options to improve the situation?

  3. Mechanics • Let’s finish off last lecture • Memory mapping, Unified VM next time • No assigned reading yet, may not exist • Mid-term on track • Covers everything before it • Open Q&A session? • Is there interest? • If so, when?

  4. Where We Left Off Last Time • Various approaches to evicting pages • Some discussion about why doing even “well” is hard to implement • Belady’s algorithm for off-line analysis • We just finished variations on FIFO • In particular, enhanced FIFO with 2nd chance

  5. Lessons From Enhanced FIFO • Observation: it’s easier to evict a clean page than a dirty page • 2nd observation: sometimes the disk and CPU are idle • Optimization: when system’s free, write dirty pages back to disk, but don’t evict • Called flushing – often falls to pager daemon

  6. Least Recently Used (LRU) • Algorithm • Replace page that hasn’t been used for the longest time • Question • What hardware mechanisms required to implement LRU?

  7. Implementing LRU • Perfect • Use a timestamp on each reference • Keep a list of pages ordered by time of reference Least recently used Mostly recently used 5 3 4 7 9 11 2 1 15

  8. Approximate LRU Most recently used Least recently used LRU N categories pages in order of last reference Crude LRU 2 categories pages referenced since the last page fault pages not referenced since the last page fault 8-bit count . . . 256 categories 0 1 2 3 254 255

  9. Aging: Not Frequently Used (NFU) 00000000 00000000 10000000 01000000 10100000 00000000 10000000 01000000 10100000 01010000 10000000 11000000 11100000 01110000 00111000 • Algorithm • Shift reference bits into counters • Pick the page with the smallest counter • Main difference between NFU and LRU? • NFU has a short history (counter length) • How many bits are enough? • In practice 8 bits are quite good • Pros: Require one reference bit • Cons: Require looking at all counters 00000000 00000000 00000000 10000000 01000000

  10. Where Do We Get Storage? • 32 bit VA to 32 bit PA – no space, right? • Offset within page is the same • No need to store offset • 4KB page = 12 bits of offset • Those 12 bits are “free” in PTE • Page # + other info <= 32 bits • Makes storing info easy

  11. Valid Writable Owner (user/kernel) Write-through Cache disabled Accessed (referenced) Dirty PDE maps 4MB Global x86 Page Table Entry Page frame number U P Cw Gl L D A Cd Wt O W V 12 31 Reserved

  12. What Happens on Diagonal Lines • My screen is 1024*768 pixels • 256 colors = 1 byte per pixel = .75MB • 64K colors = 2 bytes/pixel = 1.5MB • Page size is 4KB • Screen is 192 or 384 pages • 1 page = several horizontal lines • Diagonal/vertical lines = TLB badness • “Superpages” to the rescue

  13. The Big Picture • We’ve talked about single evictions • Most computers are multiprogrammed • Single eviction decision still needed • New concern – allocating resources • How to be “fair enough” and achieve good overall throughput • This is a competitive world – local and global resource allocation decisions

  14. Program Behaviors • 80/20 rule • > 80% memory references are made by < 20% of code • Locality • Spatial and temporal • Working set • Keep a set of pages in memory would avoid a lot of page faults Working set # page faults # pages in memory

  15. Observations re Working Set • Working set isn’t static • There often isn’t a single “working set” • Multiple plateaus in previous curve • Program coding style affects working set • Working set is hard to gauge • What’s the working set of an interactive program?

  16. Working Set • Main idea • Keep the working set in memory • An algorithm • On a page fault, scan through all pages of the process • If the reference bit is 1, record the current time for the page • If the reference bit is 0, check the “last use time” • If the page has not been used within d, replace the page • Otherwise, go to the next • Add the faulting page to the working set

  17. WSClock Paging Algorithm • Follow the clock hand • If the reference bit is 1, set reference bit to 0, set the current time for the page and go to the next • If the reference bit is 0, check “last use time” • If page has been used within d, go to the next • If page hasn’t been used within d and modify bit is 1 • Schedule the page for page out and go to the next • If page hasn’t been used within d and modified bit is 0 • Replace this page

  18. Simulating Modify Bit with Access Bits • Set pages read-only if they are read-write • Use a reserved bit to remember if the page is really read-only • On a read fault • If it is not really read-only, then record a modify in the data structure and change it to read-write • Restart the instruction

  19. Implementing LRU without Reference Bit • Some machines have no reference bit • VAX, for example • Use the valid bit or access bit to simulate • Invalidate all valid bits (even they are valid) • Use a reserved bit to remember if a page is really valid • On a page fault • If it is a valid reference, set the valid bit and place the page in the LRU list • If it is a invalid reference, do the page replacement • Restart the faulting instruction

  20. Demand Paging • Pure demand paging relies only on faults to bring in pages • Problems? • Possibly lots of faults at startup • Ignores spatial locality • Remedies • Loading groups of pages per fault • Prefetching/preloading

  21. Speed and Sluggishness • Slow is > .1 seconds (100 ms) • Speedy is << .1 seconds • Monitors tend to be 60+ Hz = <16.7ms between screen paints • Disks have seek + rotational delay • Seek is somewhere between 7-16 ms • At 7200rpm, one rotation = 1/120 sec = 8ms. Half-rotation is 4ms • Conclusion? One disk access OK, six are bad

  22. Disk Address • Use physical memory as a cache for disk • Where to find a page on a page fault? • PPage# field is a disk address Virtual address space Physical memory invalid

  23. Imagine a Global LRU • Global – across all processes • Idea – when a page is needed, pick the oldest page in the system • Problems? Process mixes? • Interactive processes • Active large-memory sweep processes • Mitigating damage?

  24. Amdahl’s Law • Gene Amdahl (IBM, then Amdahl) • Noticed the bottlenecks to speedup • Assume speedup affects one component • New time = (1-not affected) + affected/speedup • In other words, diminishing returns

  25. NT x86 Virtual Address Space Layouts 00000000 Application code Globals Per-thread stacks DLL code 3-GB user space 7FFFFFFF 80000000 Kernel & exec HAL Boot drivers C0000000 C0800000 Process page tables Hyperspace BFFFFFFF C0000000 System cache Paged pool Nonpaged pool 1-GB system space FFFFFFFF FFFFFFFF

  26. Virtual Address Space in Win95 and Win98 00000000 User accessible Unique per process (per application), user mode 7FFFFFFF 80000000 Shared, process-writable (DLLs, shared memory, Win16 applications) Systemwide user mode C0000000 Win95 and Win98 Systemwide kernel mode Operating system (Ring 0 components) FFFFFFFF

  27. Details with VM Management • Create a process’s virtual address space • Allocate page table entries (reserve in NT) • Allocate backing store space (commit in NT) • Put related info into PCB • Destroy a virtual address space • Deallocate all disk pages (decommit in NT) • Deallocate all page table entries (release in NT) • Deallocate all page frames

  28. Page States (NT) • Active: Part of a working set and a PTE points to it • Transition: I/O in progress (not in any working sets) • Standby: Was in a working set, but removed. A PTE points to it, not modified and invalid. • Modified: Was in a working set, but removed. A PTE points to it, modified and invalid. • Modified no write: Same as modified but no write back • Free: Free with non-zero content • Zeroed: Free with zero content • Bad: hardware errors

  29. Dynamics in NT VM Demand zero fault Page in or allocation Standby list Free list Zero list Bad list Process working set Modified writer Zero thread “Soft” faults Modified list Working set replacement

  30. Shared Memory • How to destroy a virtual address space? • Link all PTEs • Reference count • How to swap out/in? • Link all PTEs • Operation on all entries • How to pin/unpin? • Link all PTEs • Reference count w . . . . . . Page table . . . Process 1 w Physical pages . . . . . . Page table Process 2

  31. Child’s virtual address space uses the same page mapping as parent’s Make all pages read-only Make child process ready On a read, nothing happens On a write, generates an access fault map to a new page frame copy the page over restart the instruction Copy-On-Write r r . . . . . . Page table . . . Parent process r r Physical pages . . . . . . Page table Child process

  32. Issues of Copy-On-Write • How to destroy an address space • Same as shared memory case? • How to swap in/out? • Same as shared memory • How to pin/unpin • Same as shared memory

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