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R. Rashid, A. Tevanian, M. Young, D. Golub, R. Baron, D. Black, W. Bolosky and J. Chew

MACHINE-INDEPENDENT VIRTUAL MEMORY MANAGEMENT FOR PAGED UNIPROCESSOR AND MULTIPROCESSOR ARCHITECTURES. R. Rashid, A. Tevanian, M. Young, D. Golub, R. Baron, D. Black, W. Bolosky and J. Chew Carnegie-Mellon University IEEE Trans. on Computers ,1988. THE PAPER.

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R. Rashid, A. Tevanian, M. Young, D. Golub, R. Baron, D. Black, W. Bolosky and J. Chew

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  1. MACHINE-INDEPENDENT VIRTUAL MEMORY MANAGEMENT FOR PAGED UNIPROCESSOR AND MULTIPROCESSOR ARCHITECTURES R. Rashid, A. Tevanian, M. Young, D. Golub, R. Baron, D. Black, W. Bolosky and J. Chew Carnegie-Mellon University IEEE Trans. on Computers,1988

  2. THE PAPER • Presents the Mach virtual memory system • Three most important issues: • Use of external pagers to support mapped files • Concept of inheritance • Copy on write • Shortened version of A. Tevanian’s dissertation

  3. GENERAL OBJECTIVES • To be as portable as the UNIX virtual memory system while supporting more functionality: • Mapped files • Threads through page inheritance • To support multiprocessing, distributed systems and large address spaces

  4. Virtual Memory and I/O Buffering (I) • Current situation: Process in main memory System calls Virtual Memory I/O buffer Swap area Disk Drive

  5. Virtual Memory and I/O Buffering (II) • In a VM system, we have • Implicit transfers of data between main memory and swap area (page faults, etc.) • Implicit transfers of information between the disk drive and the system I/O buffer • Explicit transfers of information between the I/O buffer and the process address space

  6. Virtual Memory and I/O Buffering (III) • I/O buffering greatly reduces number of disk accesses • Each I/O request must still be serviced by the OS: • Two context switches per I/O request • A better solution consists of mapping files in the process virtual address space

  7. Mapped files (I) Process in main memory Usual VM Pager “External” Pager Swap area Disk Drive

  8. Mapped files (II) • When a process opens a file, the whole file is mapped into the process virtual address space • No data transfer takes place • File blocks are brought in memoryon demand • File contents are accessed using regular program instructions (or library functions) • Shared files are in shared memory segments

  9. Mach implementation Process virtual address space Usual VM Pager “External” Pager Swap area FileSystem

  10. Comments • Solution requires very large address spaces • Most programs will continue to access files through calls to read() and write() • Function calls instead of system calls • Two major problems • Harder to know the exact size of a file • Much harder to emulate the UNIX consistency model in a distributed file system • How can we have atomic writes?

  11. Threads • Also known as lightweight processes • Share the address space of their parent • Can be • Kernel-supported • Implemented at user level • Kernel-supported threads are essential in multiprocessor architectures

  12. Mach VM user interface • Consistent on all machines supporting Mach: including the features that cannot be efficiently implemented on a specific hardware • Full support for multiprocessing: thread support, efficient data sharing mechanisms, etc.. • Modular paging: external pagers are allowed to implement file mapping or recoverable virtual memory (for transaction management).

  13. VM IMPLEMENTATION • Main implementation problem was hardware incompatibilities • BSD VM implementation was tailored to VAX hardware (and its lack of a page-referenced bit) • Mach designers wanted a design that would be architecture neutral • Many competing microprocessor architectures were then available

  14. Data structures • Resident page table: keeps track of Mach pages residing in main memory • Memory object: a unit of backing storage such as a disk file or a swap area • Address map: a doubly linked list of map entries each of which maps a range of virtual addresses to a region of a memory object • P-map: the memory-mapping data structure used by the hardware

  15. First Current Last VM VM From From To To Object Object Offset Offset Protection Protection Inheritance Inheritance Previous Previous Next Next The address map First could map code segment (inheritance = share) could map stack segment (inheritance = copy)

  16. Inheritance (I) • After a regular UNIX fork() • code segment is shared between parent and child • child inherits a copy of data segment of parent • Mach inheritance attribute specifies if pages in a given range of addresses are to be shared, copied or ignored

  17. Inheritance (II) • Pages of a mapped file are always shared between parent and child to preserve file sharing semantics • Pages in the data segment can either be • copied to maintain UNIX fork() semantics • shared if we want to create a thread instead of a regular UNIX process

  18. Lazy evaluation • Mach VM system postpones execution of tasks whenever possible • Approach is based on the belief that task is likely to become unnecessary • copying whole data segment of parent process in a fork() that is very likely to be followed by an exec() • Mach uses copy-on-write

  19. Copy on write (I) • Already present in Accent • Best solution for efficient implementation of UNIX fork() • When Mach is told to copy a range of pages, it lets processes share the same copy of each page but traps write accesses • Only pages that are modified are copied

  20. Copy on write (II) Process A and B share a range of pages X COW creates new copy Process B tries to modify shared page

  21. Page replacement policy (I) Global pool of pages FIFO Expelled pages Reclaimed pages Global Queue Disk

  22. Page replacement policy (II) • Similar to that of VAX VMS • Requires little hardware support • Major change is global FIFO pool replacing resident sets of all programs • Much easier to tune • Does not support real-time processes • Can use external pagers

  23. Locks and deadlocks • Mach VM algorithms rely on locks to achieve exclusive access to kernel data structures • Price to pay for a parallel kernel • To prevent deadlocks, all algorithms gain locks using the same linear ordering • Well known deadlock prevention technique

  24. Miscellanea • Total size of the machine-dependent part of Mach VM implementation is about 16 Kbytes. • Copy-on-write is used to implement efficient message passing : • Messages are shared by sender and receiver until either of them modifies the data. • Shared librariesare supported through the mapped file interface

  25. Problem with inverted page table • IBM RT had a single inverted page table for its whole memory • One page table entry per page frame • A page frame could not belong to two processes at the same time • Cannot implement shared pages in an efficient fashion • Mach still offers the feature

  26. FINAL COMMENTS • Paper is hard to read but covers a lot of ground • You should at least understand • mapped files • external pagers and memory objects • the concept of inheritance • copy-on-write • the Mach page replacement policy

  27. More about Mach • Mach provides UNIX emulation through either • a UNIX emulator in the kernel • a UNIX emulation server in user space • Even tried to emulate UNIX through a set of specific servers, all in user space • GNU’s HURD

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