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Memory Management and Virtual Memory

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  1. Memory Management and Virtual Memory Fred Kuhns (, Applied Research Laboratory Department of Computer Science and Engineering Washington University in St. Louis

  2. Memory Management • Central Component of any operating system • Hierarchical layering • registers • cache • primary (main) memory • secondary (backing store, local disk) memory • file servers (networked storage) • Policies related to memory requirements of processes (i.e. all or part resident) • goals of admitting new processes (long term), memory allocation (medium term) and processor scheduling (short term) must be considered together. • Common goal is to optimize the number of runnable process resident in memory CS523 – Operating Systems

  3. UNIX Memory Management • UNIX uses a demand paged virtual memory architecture • anticipatory paging is where the OS proactively pages • Memory is managed at the page level • page frame or simply frame == physical page • virtual page or simply page • Basic memory allocation responsibility of the page-level allocator • has two principle clients: paging system and kernel memory allocator CS523 – Operating Systems

  4. proc struct Stack Stack Stack kernel stack/u area kernel stack/u area kernel stack/u area Data Data Data Text (shared) Text (shared) Text (shared) The Virtual Address Space kernel memory CS523 – Operating Systems

  5. Kernel stack stack Process address space Data Text (shared) 0x00000000 Process Address Space (one approach) 0xffffffff Kernel address space 0x7fffffff CS523 – Operating Systems

  6. Virtual Memory Goals • Run applications larger than physical memory. • Run partially loaded programs. • Multiprogramming: > one program simultaneously reside in memory. • Allow relocatable programs – anywhere, anytime • Application Portability: • Applications should not have to manage memory resources • Write machine independent code – program should not depend on memory architecture. • Permit sharing of memory segments or regions. For example, read-only code segments should be shared between program instances. CS523 – Operating Systems

  7. Virtual Memory Costs • Space: Translation tables and other data used by VM system reduce available memory to programs • Time: Address translation time is added to the cost (execution time) of each instruction. • Async: Also page fault handling may result in page I/O operations increasing latency and possibly affecting unrelated processes. • Overhead: Memory management operations have been measured to consume up to 10% of the CPU time on a busy system. • Efficiency: Allocating memory in pages may result in fragmentation CS523 – Operating Systems

  8. Processes and Memory • Process runs on a virtual machine as defined by the underlying hardware. • Focus is on Hardware support for a virtual address space • virtual addresses independent of physical memory • Key hardware component is the Memory Management Unit (MMU) • address translation: virtual to physical memory • simplifies context switching • ensures virtual address space protection CS523 – Operating Systems

  9. Memory allocation physical page Page-level allocator Kernel memory Allocator Paging system Network buffers Data structures temp storage process Buffer cache CS523 – Operating Systems

  10. Page-level Allocation • Kernel maintains a list of free page frames (physical memory) • Since kernel and user space programs use virtual memory addresses, the physical location of a page is not important • Pages are allocated from the free list • Two principal clients: • paging system • kernel memory allocator CS523 – Operating Systems

  11. The Paging System • Responsible for allocating and managing the address space of processes • Primary goal is to allow processes to run in a virtual address space and to perform address translations transparently • In demand-paged system the page is the basic unit of memory allocation, protection and address translation. Virtual address is converted to physical page (frame) number and offset. CS523 – Operating Systems

  12. The Paging System • Requirements: • Address space management • Address Translation – translation maps used by MMU, may result in a page fault exception • Physical memory management – physical memory used as a cache for useful data. • Memory Protection – HW support exploited • Memory Sharing • Monitoring system load • Other facilities – for example memory mapped files or shared libraries CS523 – Operating Systems

  13. page0 page0 page1 page2 page3 pagen page1 page2 page3 page4 page5 page6 page7 Paged Virtual Memory Working set Physical address space P1 virtual address space P2 virtual address space page0 page1 page2 page3 pagen pagen Address Translation resident Page frames Non-resident CS523 – Operating Systems

  14. The Virtual Address Space • Address space along with processes register context reflects the current state • exec causes kernel to build new process image: • memory “regions”: text, initialized data, uninitialized data, modified data, stack, heap, shared memory and shared libraries. • These regions may differ in protection, initialization and sharing. Protections usually set at page level when allocated. • Process may start running before any of its pages are resident in memory. CS523 – Operating Systems

  15. Initial Access to Pages • Text and initialized data are read in from executable file. • Uninitialized data are zero-filled pages • Shared libraries from library file • The u area and stacks are setup during process creation (copied from parent). CS523 – Operating Systems

  16. Swap Area: VM Backing Store • Swap Area: Pages are copied to the swap device to free up space for running programs. • Swapping plus paging for two-tiered scheme • Requires a swap map to locate swapped out pages • MMU set dirty bit for page if it has been modified • Text pages need not be backed by swap CS523 – Operating Systems

  17. Translation Maps • Hardware Translation Tables – each access to memory must have the virtual address translated to a physical memory location • Page tables provide the MMU with this mapping • MMU uses TLB to cache recent translations • Other maps used by the OS: • Address space map – describes a virtual address space for a process or kernel • Physical memory map – kernel uses to perform reverse maps and to describe a pages ownership, references and protections. • Backing store map – used to locate non-resident pages CS523 – Operating Systems

  18. Replacement Algorithms • Deciding when to reclaim a page: Defined in terms of criteria used for selecting pages to reclaim • Reference string: pages referenced over time • fault rate: page faults for some length of a reference string (i.e. over a period of time) • Algorithms evaluated based on effectiveness on collected (real) reference strings • Implementations usually require sample reference strings • Local versus global policies: • Most UNIX implementation use a global replacement policy but guarantee a minimum CS523 – Operating Systems

  19. Working set Model • Assumes a slowing changing locality of reference • processes tend to localize references to a small set of pages • implies that if a page was recently accessed then it will be accessed again in the “near” future • if working set is in memory then few page faults • A simple model is a least recently used (LRU) policy: • if a page has been accessed “recently” then assume it will be need again else assume it will not be needed • else free pages not accessed “recently” • Implement using an approximate set: • number of pages held versus fault rate. • Set high and low water marks • Most kernels implement a scheme whereby pages are periodically freed and placed on a free pool. • Prepaging: working set resident before scheduling process CS523 – Operating Systems

  20. Working set model • Reference pattern nonuniform but clustered • w(k,t) = pages (set) corresponding to the last k memory references, at time t. • w(1,t)  w(2,t)  …  w(n,t) • |w(k,t)| = size of w is monotonically increasing with k • Practical considerations lead to using a processes virtual time rather than k recent references • consider page references in past  virtual time units. • clear R bit every clock tick, it is set when a page is referenced • on page fault if R is set then update virtual time of last useelse set age = current_vt – last_use_vt. If age >  then reclaim page else it is in the working set. continue to scan all entriesif no entries found then reclaim oldest. If all had R bit set then randomly select page to reclaim. CS523 – Operating Systems

  21. WSClock Algorithm • Similar to clock algorithm with circular list and clock hand. • scans list and if R = 1 then it is cleared and the current virtual time is written. • advance hand • if R = 0 then check timestamp > T then replace (if dirty schedule for write else put on free list) • Alternatively, can use two clock hands CS523 – Operating Systems

  22. Text (shared) Initialized Data Unitialized Data Heap (Dynamic) stack (dynamic) Environment Example Paging System CPU Unitialized data Stack and heap DRAM Allocated virtual pages Low Address (0x00000000) Swap app1 Address space Disk UFS High Address (0x7fffffff) Text and initialized data app1 CS523 – Operating Systems

  23. Hardware Requirements • Protection: Prevent process from changing own memory maps • Residency: CPU distinguishes between resident and non-resident pages • Loading: Load pages and restart interrupted program instructions • Dirty: Determine if pages have been modified CS523 – Operating Systems

  24. Memory Management Unit • Translates Virtual Addresses • page tables • Translation Lookaside Buffer (TLB) • Page tables • One for kernel addresses • one or more for user space processes • Page Table Entry (PTE) one per virtual page • 32 bits - page frame, protection, valid, modified, referenced CS523 – Operating Systems

  25. Translation • Virtual address: • virtual page number + offset • Finds PTE for virtual page • Extract physical page and adds offset • Fail (MMU raises an exception - page fault): • bounds error - outside address range • validation error - non-resident page • protection error - not permitted access CS523 – Operating Systems

  26. Some details • Limit Page Table size: • segments • page the page table (multi-level page table) • MMU has registers which point to the current page table(s) • kernel and MMU can modify page tables and registers • Problem: • Page tables require perhaps multiple memory access per instruction • Solution: • rely on HW caching (virtual address cache) • cache the translations themselves - TLB CS523 – Operating Systems

  27. Translation Lookaside Buffer • Associative cache of address translations • Entries may contain a tag identifying the process as well as the virtual address. • Why is this important? • MMU typically manages the TLB • Kernel may need to invalidate entries, • Would the kernel ever need to invalidate entries? • Contains page table entries that have been most recently used • Functions same way as a memory cache • Given a virtual address, processor examines the TLB • If present (a hit), the frame number is retrieved and the real address is formed • If not found (a miss), page number is used to index the process page table CS523 – Operating Systems

  28. Address Translation - General CPU virtual address cache MMU Physical address data Global memory CS523 – Operating Systems

  29. context table pointer context Address Translation Overview MMU Virtual address CPU physical address cache TLB Page tables CS523 – Operating Systems

  30. Cache/Main-Memory Structure Slot Number Memory Address Tag Block 0 0 1 1 2 Block (k words) 2 3 C - 1 Block Length (k words) (b) Cache Block 2n - 1 Word Length (a) Main Memory CS523 – Operating Systems

  31. offset in page virtual page number Page Table Entry • Resident bit indicates if page is in memory • Modify bit to indicate if page has been altered since loaded into main memory • Other control bits • frame number, this is the physical frame address. Y bits X bits Virtual address Page Table Entry (PTE) M R control bits frame number Z bits CS523 – Operating Systems

  32. offset in page virtual page number Example 1-level address Translation Virtual address DRAM Frames 12 bits 20 bits Frame X X offset add PTE control bits R M frame number (Process) Page Table current page table register CS523 – Operating Systems

  33. SuperSPARC Reference MMU Physical address Physical page offset Context Tbl Ptr register Context Tbl 12 Bits 24 Bits PTD Level 1 Level 2 PTD Level 2 PTD Context register 12 bit PTE 6 bits 8 bits 6 bits 12 bits Virtual address 4096 index 1 index 2 index 3 offset virtual page • 12 bit index for 4096 entries • 8 bit index for 256 entries • 6 bit index for 64 entries • Virtual page number has 20 bits for 1M pages • Physical frame number has 24 bits with a 12 bit offset,permitting 16M frames. CS523 – Operating Systems

  34. Page Table Descriptor/Entry Page Table Descriptor type Page Table Pointer 2 1 0 Page Table Entry ACC M R C type Phy Page Number 8 7 6 5 4 2 1 0 Type = PTD, PTE, Invalid C - Cacheable M - Modify R - Reference ACC - Access permissions CS523 – Operating Systems

  35. SVR4 VM Architecture • File Mapping – Two interpretations • Used as a Fundamental Organizational scheme. Entire Address Space viewed as a collection of mappings to different objects (such as files) • Applications map a file into their address space • Types of Mappings: Shared and Private • Memory Object: represents mapping from region a of memory to backing store (swap, local/remote file, frame buffer) • VM provides common framework, Memory objects provide the specific implementation • operations such as fetching and flushing page to backing store CS523 – Operating Systems

  36. VM • Address space is a set of mappings to data objects. • An address is only valid if it is mapped to an existing object • File system provides the name space and mechanisms to access data. • Uses the vnode layer to interact with the file system. • Each named memory object is associated with a vnode (but a vnode may map to many objects) • Unnamed objects represented by anonymous objects • Physical memory is treated as a cache for the data objects • Page is the smallest unit of allocation, protection, address translation and mapping. • Address space can be thought of as an array of pages CS523 – Operating Systems

  37. VM Approach Traditional Approach Process P1 Process P2 process process File Mapping Versus read/write mmap(): Address space read/write: Copy Copy Virtual Memory System Buffer Cache P1 pages Copy Copy CS523 – Operating Systems

  38. Fundamental Abstractions (data structs) • Page (struct page) • Address Space (struct as) • segment (struct seg) • Hardware Address Translation (struct hat) • Anonymous Page (struct anon) CS523 – Operating Systems

  39. VM Architecture Physical page & offset virtual address Address Translation physical address Proc A fork/exec fault page tables AS layer HAT vnode layer anon layer ... swap layer page layer Persistent storage CS523 – Operating Systems

  40. Physical Memory • Divided into paged and non-paged regions • Paged region described by an array of page structures, each describing one logical page (cluster of hardware pages) • Each physical page (page frame): • described by struct page • mapped to some memory object, with the memory object represented by a vnode • page identity or name = <vnode, offset> CS523 – Operating Systems

  41. Page Struct • page struct stores offset and pointer to corresponding vnode • may sit on several linked lists, has 3 sets of pointers • hash table of vnode and offset • vnode contains list of all object pages currently in memory • free page list or list of pages waiting to be written to backing store • Reference count • synchronization flags (lock, wanted, in-transit) • Copies of modified and referenced bits • HAT field used to locate all translations for this page CS523 – Operating Systems

  42. AS Layer • High-level abstraction describing the virtual address space. • References a linked list of seg (segment) structs that represent non-overlapping page-aligned address regions • Contains the hat structure and a hint to the last segment that had a page fault • Supports two set of operations: those operating on the entire address space and those that affect ranges within the space. CS523 – Operating Systems

  43. Segment Drivers • Segments represent mappings between backing store and address regions • Segment represents an abstract base class with specific drivers being derived classes. • seg struct contains pointer to • a seg_ops vector, these represent the virtual functions. i.e. the type dependent interface to the class. • Methods = {dup, fault, faulta, setprot, checkprot, unmap, swapout, sync} • type-dependent data structure which hold private data • Each segment defines a create routine CS523 – Operating Systems

  44. Segment Drivers • Different types: seg_vn, seg_map, seg_dev, seg_kmem • seg_vn: vnode segment, maps to regular files and anonymous object. • Seg_map: One in system. Use by kernel for transient file mappings for implementing read/write. CS523 – Operating Systems

  45. Process Address Space proc struct struct segvn_data {} struct seg { base size} text struct segvn_data {} struct seg { base size} struct as { segment list hint struct hat {}} data struct segvn_data {} struct seg { base size} stack seg_vn ops struct segu_data {} struct seg { base size} seg_u ops u area CS523 – Operating Systems

  46. Anonymous Pages • Page with no permanent storage, created when process write to a MAP_PRIVATE object • Pages can be discarded with process terminates or unmaps the page. • Swap device used as backing store • Example: initialized data pages when modified become anonymous pages • Related but distinct concept is an anonymous object. • one anonymous object in system represented bu the NULL vnode pointer (/dev/zero) and is the source of all zero-filled pages. • unitialized data and stack regions are MAP_PRIVATE to it • Shared memory regions are MAP_SHARED to it • anonymous object pages are anonymous pages CS523 – Operating Systems

  47. Anonymous vnode layer read and writes pages to swap device one for each swap device Swap device vnode Swap info pointer to anon and freelist anon ref array as anon[] 0 seg free list anon_map segvn_data 0 page 0 one entry for each page in swap vnode page page Per page protect CS523 – Operating Systems

  48. Hardware Address Translation Layer • Isolates all hardware-dependent code from rest of VM • Responsible for all address translations • setup and maintain mappings used by MMU (page tables, directories etc) • each process has it’s own set of translations • uses struct hat which is part of the as struct • Operations • hat layer: hat_alloc, hat_free, hat_dup, hat_swapin, hat_swapout (build/rebuild tables when swapping) • range of pages: hat_chgprot, hat_unload, hat_memload, hat_devload • all translation of a page: hat_pageunload, hat_pagesync (update modified and referenced bits using values in page struct) CS523 – Operating Systems

  49. Misc Topics • Pagedaemon implements page reclamation (replacement) policy. Uses two-handed clock algorithm • Swapping - swapper daemon will swap processes when space gets below a threshold CS523 – Operating Systems

  50. Misc Topics Cont • Non-page-aligned backing store • Virtual swap space in Solaris (swapfs) • includes physical memory • dynamic reallocation CS523 – Operating Systems