CSE325 Principles of Operating Systems Virtual Memory David P. Duggan dduggan@sandia.gov March 7, 2013
Reading Assignment 9 Chapters 10 & 11 File Systems, due 3/21 3/7/13 CSE325 - Virtual Memory 2
Outline Paging Why virtual memory? Virtual memory organization Demand paging, handle page faults Page replacement Random Replacement Belady s Optimal Algorithm Least Recently Used (LRU) Least Frequently Used (LFU) First In First Out (FIFO) 3/7/13 CSE325 - Virtual Memory 3
Paging Fundamentals Memory management scheme Allows use of non-contiguous physical memory by a process Requires a backing store Requires hardware support 3/7/13 CSE325 - Virtual Memory 4
Basic Method Physical memory is structured into frames Logical memory is structured into pages Size of frames and pages are the same Backing store is structured into fixed-size blocks the same size as frames Addresses are divided into 2 parts Page number (p) [index into page table] Page offset [index into page] 3/7/13 CSE325 - Virtual Memory 5
Basic Method, Cont. Hardware support Could be implemented in registers Page-table base register (PTBR) Translation look-aside buffer (TLB) Small, usually 64 to 1024 entries Protection Pages can be set as read-only, read-write Page-table length register (PTLR) Page table has valid-invalid designators 3/7/13 CSE325 - Virtual Memory 6
Shared Pages Some code is pure, or reentrant, code Many processes can share those pages What types of programs might have reentrant code? 3/7/13 CSE325 - Virtual Memory 7
Page Tables Large memory space can make large page tables Can use a two-level page table structure Section, page, offset Hashed page tables Inverted page tables 3/7/13 CSE325 - Virtual Memory 8
Motivation for Virtual Memory Virtual memory separation of user logical memory from physical memory. Only part of the program needs to be in memory for execution. Physical address space can be non-contiguous. Logical address space can therefore be much larger than physical address space. Allows address spaces to be shared by several processes. Allows for more efficient process creation. Virtual memory can be implemented via: Demand paging Demand segmentation 3/7/13 CSE325 - Virtual Memory 9
Virtual Memory Organization " Virtual Memory That is Larger Than Physical Memory 3/7/13 CSE325 - Virtual Memory 10
<1% 15% 35% 20% <1% 30% Why is virtual memory is a feasible idea? Address Space for P i Execution time Locality Address space is logically partitioned Text, data, stack Initialization, main, error handle Different parts have different reference patterns: Initialization code (used once) Code for Φ1 Code for Φ2 Code for Φ3 Code for error 1 Code for error 2 Code for error 3 Data & stack Is there any correlation between the size of a part of the address space and the amount of time that a process will execute in that locality? 11
Virtual Memory Every process has code and data locality Code tends to execute in a few fragments at one time Tend to reference same set of data structures Dynamically load/unload currently-used address space fragments as the process executes Uses dynamic address relocation/binding Generalization of base-limit registers Physical address corresponding to a compiletime address is not bound until run time 3/7/13 CSE325 - Virtual Memory 12
Virtual Memory (Cont.) Since binding changes with time, use a dynamic virtual address map, B t Virtual Address Space B t 3/7/13 CSE325 - Virtual Memory 13
Fragmented Virtual Address Space Secondary Memory Physical Address Space 0 Virtual Address Space for P i Virtual Address Space for P j Virtual Address Space for P k Complete virtual address space is stored in secondary memory Each address space is fragmented Fragments of the virtual address space are dynamically loaded into primary memory at any given time n-1 Primary Memory 14
Size of Blocks of Memory Virtual memory system transfers blocks of the address space to/from primary memory Fixed size blocks: System-defined pages are moved back and forth between primary and secondary memory Variable size blocks: Programmer-defined segments corresponding to logical fragments are the unit of movement Paging is the commercially dominant form of virtual memory today 3/7/13 CSE325 - Virtual Memory 15
Demand Paging Bring a page into memory only when it is needed (benefits?) Less I/O needed Less physical memory needed Faster response More users Page is needed? invalid reference abort reference to it not-in-memory bring to memory 3/7/13 CSE325 - Virtual Memory 16
Demand Paging A page is a fixed size, 2 h, block of virtual addresses A page frame is a fixed size, 2 h, block of physical memory (the same size as a page) When a virtual address, x, in page i is referenced by the CPU If page i is loaded at page frame j, the virtual address is relocated to page frame j If page is not loaded, the OS interrupts the process and loads the page into a page frame 3/7/13 CSE325 - Virtual Memory 17
Valid-Invalid Bit With each page table entry a valid invalid bit is associated (1 in-memory, 0 not-in-memory) Initially valid invalid bit is set to 0 on all entries Example of a page table snapshot: Frame #" " page table" valid-invalid bit" 1" 1" 1" 1" 0" 0" 0" During address translation, if valid invalid bit in page table entry is 0 page fault 18
Page Table When Some Pages Are Not in Main Memory Important! 3/7/13 CSE325 - Virtual Memory 19
Page Fault If there is ever a reference to a page, first reference will trap to OS page fault OS looks at another table to decide: Invalid reference abort. Just not in memory. Find an empty frame. Swap page into frame. Reset tables, validation bit = 1. Restart instruction 3/7/13 CSE325 - Virtual Memory 20
Handling a Page Fault 3/7/13 CSE325 - Virtual Memory 21
Demand Paging Algorithm 1. Page fault occurs 2. Process with missing page is interrupted 3. Memory manager locates the missing page 4. Page frame is unloaded (replacement policy) 5. Page is loaded in the vacated page frame 6. Page table is updated 7. Process is continued with instruction that caused page fault 3/7/13 CSE325 - Virtual Memory 22
Addresses Suppose there are G= 2 g 2 h =2 g+h virtual addresses and H=2 j+h physical addresses assigned to a process Each page/page frame is 2 h addresses There are 2 g pages in the virtual address space 2 j page frames are allocated to the process Rather than map individual addresses B t maps the 2 g pages to the 2 j page frames That is, page_frame j = B t (page i ) Address k in page i corresponds to address k in page_frame j 3/7/13 CSE325 - Virtual Memory 23
Page-Based Address Translation Let N = {d 0, d 1, d n-1 } be the pages Let M = {b 0, b 1,, b m-1 } be page frames Virtual address, i, satisfies 0 i<g= 2 g+h Physical address, k = U2 h +V (0 V<G= 2 h ) U is page frame number V is the line number within the page (offset) B t :[0:G-1] <U, V> {Ω} Since every page is size c=2 h page number = U = i/c line number = V = i mod c 3/7/13 CSE325 - Virtual Memory 24
What happens if there is no free frame? Page replacement find some page in memory, but not really in use, swap it out algorithm performance want an algorithm which will result in minimum number of page faults Same page may be brought into memory several times 3/7/13 CSE325 - Virtual Memory 25
Performance of Demand Paging Page Fault Rate 0 p 1.0 if p = 0 no page faults if p = 1, every reference is a fault Page fault overhead is: Time to swap page out (not always needed) Time to swap page in Time to restart process Effective Access Time (EAT) EAT = (1 p) x memory access time + p x page fault overhead 3/7/13 CSE325 - Virtual Memory 26
Demand Paging Example Memory access time = 1 microsecond 50% of the time the page that is being replaced has been modified and therefore needs to be swapped out Swap Page Time = 10 msec = 10,000 µsec EAT = (1 p) x 1 + p(15000) = 1 + 14999p (in µsec) 3/7/13 CSE325 - Virtual Memory 27
Need For Page Replacement 3/7/13 CSE325 - Virtual Memory 28
Page Replacement Prevent over-allocation of memory by modifying page-fault service routine to include page replacement Use modify (dirty) bit to reduce overhead of page transfers only modified pages are written to disk Page replacement completes separation between logical memory and physical memory large virtual memory can be provided on a smaller physical memory 3/7/13 CSE325 - Virtual Memory 29
Basic Page Replacement 1. Find the location of the desired page on disk 2. Find a free frame: - If there is a free frame, use it - If there is no free frame, use a page replacement algorithm to select a victim frame 3. Read the desired page into the (newly) free frame. Update the page and frame tables. 4. Continue the process from instruction that caused the page fault 30
Page Replacement 3/7/13 CSE325 - Virtual Memory 31