OS Fall’02 Virtual Memory Operating Systems Fall 2002

OS Fall’02 Paging and Virtual Memory  Paging makes virtual memory possible Logical to physical address mapping is dynamic Processes can be broken to a number of pages that need not be mapped into a contiguous region of the main memory => It is not necessary that all of the process pages be in main memory during execution

OS Fall’02 How does this work?  CPU can execute a process as long as some portion of its address space is mapped onto the physical memory E.g., next instruction and data addresses are mapped  Once a reference to an unmapped page is generated (page fault): Put the process into blocking state Read the page from disk into the memory Resume the process

OS Fall’02 Benefits  More processes may be maintained in the main memory Better system utilization  The process size is not restricted by the physical memory size: the process memory is virtual But what is the limit anyway?

OS Fall’02 Why is this practical?  Observation: Program branching and data access patterns are not random  Principle of locality: program and data references tend to cluster => Only a fraction of the process virtual address space need to be resident to allow the process to execute for sufficiently long

OS Fall’02 Virtual memory implementation  Efficient run-time address translation Hardware support, control data structures  Fetch policy Demand paging: page is brought into the memory only when page-fault occurs Pre-paging: pages are brought in advance  Page replacement policy Which page to evict when a page fault occurs?

OS Fall’02 Thrashing  A condition when the system is engaged in moving pages back and forth between memory and disk most of the time  Bad page replacement policy may result in thrashing  Programs with non-local behavior

OS Fall’02 Address translation  Virtual address is divided into page number and offset  Process page table maintains mappings of virtual pages onto physical frames Each process has its own unique page table Virtual Address Page NumberOffset

OS Fall’02 Forward-mapped page tables (FMPT)  Page table entry (PTE) structure  Page table is an array of the above Index is the virtual page number PM Frame Number Other Control Bits Page Table Frame # Page # P: present bit M: modified bit

OS Fall’02 Address Translation using FMPT ProgramPagingMain Memory Virtual address Register Page Table Page Frame Offset P# Frame # Page Table Ptr Page #OffsetFrame #Offset +

OS Fall’02 Handling large address spaces  One level FMPT is not suitable for large virtual address spaces 32 bit addresses, 4K (2 12 ) page size, 2 32 / 2 12 = 2 20 entries ~4 bytes each => 4Mbytes resident page table per process! What about 64 bit architectures??  Solutions: multi-level FMPT Inverted page tables (IPT)

OS Fall’02 Multilevel FMPT  Use bits of the virtual address to index a hierarchy of page tables  The leaf is a regular PTE  Only the root is required to stay resident in main memory Other portions of the hierarchy are subject to paging as regular process pages

OS Fall’02 Two-level FMPT page number page offset pipi p2p2 d 10 12

OS Fall’02 Two-level FMPT

OS Fall’02 Inverted page table (IPT)  A single table with one entry per physical page  Each entry contains the virtual address currently mapped to a physical page (plus control bits)  Different processes may reference the same virtual address values Address space identifier (ASID) uniquely identifies the process address space

OS Fall’02 Address translation with IPT  Virtual address is first indexed into the hash anchor table (HAT)  The HAT provides a pointer to a linked list of potential page table entries  The list is searched sequentially for the virtual address (and ASID) match  If no match is found -> page fault

OS Fall’02 Address translation with IPT Virtual address page number offset hash + HAT base register ASID register page number ASID Frame# IPT + IPT base register frame number HAT

OS Fall’02 Translation Lookaside Buffer (TLB)  With VM accessing a memory location involves at least two intermediate memory accesses Page table access + memory access  TLB caches recent virtual to physical address mappings ASID or TLB flash is used to enforce protection

OS Fall’02 TLB internals  TLB is associative, high speed memory Each entry is a pair (tag,value) When presented with an item it is compared to all keys simultaneously If found, the value is returned; otherwise, it is a TLB miss Expensive: number of typical TLB entries: Do not confuse with memory cache!

OS Fall’02 Address translation with TLB

OS Fall’02 Bits in the PTE: Present (valid)  Present (valid) bit Indicates whether the page is assigned to frame or not Invalid page can be not a part of any memory segment A reference to an invalid page generates page fault which is handled by the operating system

OS Fall’02 Bits in PTE: modified, used  Modified (dirty) bit Indicates whether the page has been modified Unmodified pages need not be written back to the disk when evicted  Used bit Indicates whether the page has been accessed recently Used by the page replacement algorithm

OS Fall’02 Bits in PTE  Access permissions bit indicates whether the page is read-only or read-write  UNIX copy-on-write bit Set whether more than one process shares a page If one of the processes writes into the page, a separate copy must first be made for all other processes sharing the page Useful for optimizing fork()

OS Fall’02 Protection with VM  Preventing processes from accessing other process pages  Simple with FMPT Load the process page table base address into a register upon context switch  ASID with IPT

OS Fall’02 Segmentation with paging  Segmentation simplifies protection and sharing, enforce modularity, but prone to external fragmentation  Paging transparent, eliminates ext. fragmentation, allows for sophisticated memory management  Segmentation and paging can be combined

OS Fall’02 Address translation Main Memory Page Frame Offset Paging Page Table P# + Frame #Offset Seg Table Ptr + S # SegmentationProgram Segment Table Seg #Page #Offset

OS Fall’02 Page size considerations  Small page size better approximates locality large page tables inefficient disk transfer  Large page size internal fragmentation  Most modern architectures support a number of different page sizes  a configurable system parameter

OS Fall’02 Next: Page replacement