CS370: Operating Systems [Spring 2017] Dept. Of Computer Science, Colorado State University
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1 Frequently asked questions from the previous class survey CS 370: OPERATING SYSTEMS [MEMORY MANAGEMENT] Shrideep Pallickara Computer Science Colorado State University MS-DOS.COM? How does performing fast I/O fit into all of this? What if you have compile-time binding, and you wait before running it and that memory segment was taken? Why not compress memory contents? Wouldn t that would save memory space requirements? ISA only for registers and main-memory? No instructions for the cache? Contiguous vs. non-contiguous Other questions: TLB, pages, etc. L20.1 L20.2 Topics covered in this lecture Contiguous memory allocations Fragmentations External and Internal Paging Hardware support for paging Dynamic Storage Allocation Problem Satisfying a request of size n from the set of available spaces First fit Best fit Worst fit L20.3 L20.4 First fit Scan list of segments until you find a memory-hole that is big enough Hole is broken up into two pieces One for the process The other is unused memory Best Fit Scan the entire list from beginning to the end Pick the smallest memory-hole that is adequate to host the process L20.5 L20.6 SLIDES CREATED BY: SHRIDEEP PALLICKARA L20.1
2 Comparing Best Fit and First Fit Best fit is slower than first fit Surprisingly, it also results in more wasted memory than first fit Tends to fill up memory with tiny, useless holes Worst fit How about going to the other extreme? Always take the largest available memory-hole Perhaps, the new memory-hole would be useful Simulations have shown that worst fit is not a good idea either L20.7 L20.8 Contiguous Memory Allocation: Fragmentation As processes are loaded/removed from memory Free memory space is broken into small pieces External fragmentation Enough space to satisfy request; BUT Available spaces are not contiguous FRAGMENTATION L20.9 L20.10 Fragmentation: Example Fragmentation can be internal as well P 3 P 4 P 5 P 1 Process P 5 cannot be loaded because memory space is fragmented Memory allocated to process may be slightly larger than requested Internal fragmentation Unused memory is internal to blocks P 2 L20.11 L20.12 SLIDES CREATED BY: SHRIDEEP PALLICKARA L20.2
3 Compaction: Solution to external fragmentation Shuffle memory contents Objective: Place free memory into large block Not possible if relocation is static Load time Approach involves moving: 1 Processes towards one end 2 Gaps towards the other end Compaction: Example P 3 P P 5 4 P 1 P 2 L20.13 L20.14 Memory compaction is time intensive and is usually not done Let s consider a machine with 1 GB of RAM The machine can copy 4 bytes in 20 nanoseconds Time to compact all the memory? 10 9 x (20x10-9 /4) = 5 seconds (approximately) Note: 1 GB is approximately 10 9 bytes. Summarizing the pure Swapping based approach Bring in each process, in its entirety, into memory Run process for a while before eviction due to: Space being needed for another process Process becomes idle n Idle processes should not take up space in memory L20.15 L20.16 OVERVIEW OF THE MAPPING PROCESS L20.17 L20.18 SLIDES CREATED BY: SHRIDEEP PALLICKARA L20.3
4 Overview of how mapping of logical and physical addresses is performed Virtual address CPU Memory Management Unit (MMU) Translation Lookaside Buffer (TLB) Physical Memory Noncontiguous memory management Physical address MMU may access Physical Memory to perform translations {PageTable may be stored there} PAGING L20.19 L20.20 The Paging memory management scheme Physical address space of process can be noncontiguous Solves problem of fitting variable-sized memory chunks to backing store Backing store has fragmentation problem n Compaction is impossible Basic method for implementing paging Break memory into fixed-sized blocks Physical memory: frames Same size Logical memory: pages Backing store is also divided the same way L20.21 L20.22 What will seem odd, and perhaps cool, about paging [1/2] While a program thinks of its memory as linear It is usually scattered throughout physical memory in a kind of abstract mosaic The processor will execute one instruction after another using virtual addresses The virtual addresses are still linear However, an instruction located at the end of a page will be located in a completely different region of physical memory from the next instruction at start of another page What will seem odd, and perhaps cool, about paging [2/2] Data structures appear to be contiguous using virtual addresses But a large matrix is scattered across many physical page frames L20.23 L20.24 SLIDES CREATED BY: SHRIDEEP PALLICKARA L20.4
5 Paging: Analogy Paging: Logical and Physical Memory Shuffling several decks of cards together A single process in its virtual address page sees the cards of a single deck in order A different process sees a completely different deck, but it will also be in order In physical memory, however, the decks of all processes currently running will be shuffled together, apparently at random Page tables are the magician s assistant in locating cards from the shuffled decks L20.25 Page 0 Page 1 Page 2 Page 3 Logical Memory Page Table Page 0 Page 2 Page 1 Page 3 Physical Memory L20.26 Paging Hardware: Performing address translation Page size Page number Logical Address CPU p d Page offset Physical Address f f d Frame f A power of 2 Typical sizes: 512 bytes 16 MB Size of logical address: 2 m Page size: 2 n p f f Page number m - n Page offset n m bits Logical address Page Table L20.27 L20.28 Paging and Fragmentation No external fragmentation Free frame available for allocation to other processes Internal fragmentation possible Last frame may not be full If process size is independent of page size n Internal fragmentation = ½ page per process Page sizes Processes, data sets, and memory have all grown over time Page sizes have also increased Some CPUs/kernels support multiple page sizes L20.29 L20.30 SLIDES CREATED BY: SHRIDEEP PALLICKARA L20.5
6 Paging: User program views memory as a single space Program is scattered throughout memory User view and physical memory reconciled by Address-translation hardware Process has no way of addressing memory outside of its page table OS manages the physical memory Maintains frame-table; one entry per frame Free or allocated? If allocated: Which page of which process Maintains a page table for each process Used by CPU dispatcher to define hardware page table when process is CPU-bound n Paging increases context switching time L20.31 L20.32 Example: 32-bit address space Page size = 4K Logical address = 0x23FA427 What s the offset within the page? 0x427 What s the page number? 0x23FA Page table entry maps 0x23FA to frame 0x12345 what is the physical memory address for the logical address? 0x Example: 32-bit address space Page size = 1K Logical address = 0x23FA427 What s the offset within the page? What s the page number? L20.33 L20.34 The purpose of the page table is to map virtual pages onto physical frames Think of the page table as a function Takes virtual page number as an argument Produces physical frame number as result All accesses to memory must go through a map. Efficiency is important. HARDWARE SUPPORT FOR PAGING Virtual page field in virtual address replaced by frame field Physical memory address L20.35 L20.36 SLIDES CREATED BY: SHRIDEEP PALLICKARA L20.6
7 Two major issues facing page tables Can be extremely large With a 4 KB page size, a 32-bit address space has 1 million pages Also, each process has its own page table The mapping must be fast Virtual-to-physical mapping must be done on every memory reference Page table lookup should not be a bottleneck Implementing the page table: Dedicated registers When a process is assigned the CPU, the dispatcher reloads these registers Feasible if the page table is small However, for most contemporary systems entries are greater than 10 6 L20.37 L20.38 Implementing the page table in memory Page table base register (PTBR) points to page table 2 memory accesses for each access One for the page-table entry One for the byte Observation Most programs make a large number of references to a small number of pages Not the other way around Only a small fraction of the page table entries are heavily read Others are barely used at all L20.39 L20.40 Translation look-aside buffer Small, fast-lookup hardware cache Number of TLB entries is small (64 ~ 1024) Contains few page-table entries Each entry of the TLB consists of 2 parts A key and a value When the associative memory is presented with an item Item is compared with all keys simultaneously Using the TLB with page tables (1) TLB contains only a few page table entries When a logical address is generated by the CPU, the page number is presented to the TLB When frame number is found, use to access memory Usually just 10-20% longer than an unmapped memory reference L20.41 L20.42 SLIDES CREATED BY: SHRIDEEP PALLICKARA L20.7
8 Using the TLB with page tables (2) What if there is a TLB miss? Memory reference to page table is made Replacement policies for the entries Some TLBs allow certain entries to be wired down TLB entries for kernel code are wired down TLB and Address Space Identifiers (ASIDs) ASID uniquely identifies each process Allows TLB to contain addresses from several different processes simultaneously When resolving page numbers TLB ensures that ASIDs match If not, it is treated as a TLB miss L20.43 L20.44 Without ASIDs TLB must be flushed with every context switch Each process has its own page table Without flushing or ASIDs, TLB could include old entries Valid virtual addresses But incorrect or invalid physical addresses n From previous process The contents of this slide-set are based on the following references Avi Silberschatz, Peter Galvin, Greg Gagne. Operating Systems Concepts, 9 th edition. John Wiley & Sons, Inc. ISBN-13: [Chapter 8] Andrew S Tanenbaum and Herbert Bos. Modern Operating Systems. 4 th Edition, Prentice Hall. ISBN: X/ [Chapter 3] Thomas Anderson and Michael Dahlin. Operating Systems Principles and Practice. 2nd Edition. Recursive Books. ISBN: [Chapter 8] L20.45 L20.46 SLIDES CREATED BY: SHRIDEEP PALLICKARA L20.8
Frequently asked questions from the previous class survey
CS 370: OPERATING SYSTEMS [MEMORY MANAGEMENT] Shrideep Pallickara Computer Science Colorado State University L20.1 Frequently asked questions from the previous class survey Virtual addresses L20.2 SLIDES
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