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 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 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 Virtual Memory and I O Buffering I Current situation Process in main memory System calls Virtual Memory I O buffer Swap area Disk Drive 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 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 Mapped files I Process in main memory Usual VM Pager External Pager Swap area Disk Drive 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 memory on demand File contents are accessed using regular program instructions or library functions Shared files are in shared memory segments Mach implementation Process virtual address space Usual VM Pager Swap area External Pager File System 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 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 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 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 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 The address map First Current Last VM From To Object Offset Protection Inheritance Previous Next could map code segment inheritance share VM From To Object Offset Protection Inheritance Previous Next could map stack segment inheritance copy 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 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 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 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 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 Page replacement policy I Global pool of pages FIFO Expelled pages Reclaimed pages Global Queue Disk 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 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 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 libraries are supported through the mapped file interface 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 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 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