Memory Management Organization Memory management depends on: Operating Systems p structure of address field of instruction arguments, Memory Management p position of address field in instruction word, p hardware capabilities of address field transformation. dr. Tomasz Jordan Kruk [email protected] Depending on the length of the address field the address space may be the same in size, smaller or bigger than the scope of operating memory addresses. Institute of Control & Computation Engineering Warsaw University of Technology Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 1/59 Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 3/59 Memory Management Operating System Functions What is expected by programmers from the system memory: Functions of the operating systems from the point of view of memory management: p to be huge, address space management through address translation mechanisms, p to be fast, p memory contents protection, p to be nonvolatile. p p shared areas access control, Memory hierarchy: p effective organization of operating memory management. p small fast expensive memory (e.g. cache), The size of the operating memory for operating system code is usually fixed, p average in size, speed and expense memory (e.g. operating memory), but the allocation process for user processes is usually much more p huge, slow and cheap memory (e.g. disk/tape memory). complicated. Memory Management (MM) type on the level of operating system is dictated by the computer system architecture. Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 2/59 Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 4/59 Memory Allocation Methods M.A.M. With Division Methods of memory allocation to user processes: The goals of the memory division: 1. without division, all the memory allocated to one process. p better utilization of hardware elements of the system, mainly processor Multiprogramming may be implemented by swapping, and memory, 2. memory division, free operating memory divided into blocks allocated p enabling fast context switching. to particular user processes, The following types of systems with memory division may be distinguished: 3. usage of virtual memory, there exist one or more virtual address spaces allocated to user processes and without any direct obvious p systems with static division - either fixed partitions with different mapping to the physical address space. length or blocks with fixed length called frames. p systems with dynamic division - implemented by structures describing free blocks of memory with variable length and by swapping mechanism. Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 5/59 Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 7/59 M.A.M. Without Division Static Division Based on Constant Size Partitions Multiple input queues 0xFFF … 800K Operating Device Partition 4 Partition 4 700K system in drivers in ROM ROM User Partition 3 Single Partition 3 input queue program User program 400K User program Partition 2 Partition 2 Operating Operating 200K system in system in Partition 1 Partition 1 RAM RAM 100K Operating Operating 0 0 0 system system 0 (a) (b) (c) (a) (b) Three simple ways of organizing memory for an operating system with one user process. a. fixed memory partitions with separate input queues for each partition, b. fixed memory partitions with a single input queue. Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 6/59 Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 8/59 Dynamic Division Multiprogramming Modelling For implementation of dynamic memory allocation the following hypothetical 20% I/O wait functions: 100 50% I/O wait p allocate( size, address ) 80 ? choosing from the list of free blocks that one which covers demand, 60 80% I/O wait ? returning the address of the chosen block and removing allocated block from the free block lists as a result of allocation, 40 ? malloc( size ), calloc( n, size ), realloc( ptr, size ). 20 p free( address ) - adding pointed block to the free block list, e.g. free( CPU utilization (in percent) ptr ), 0 1 2 3 4 5 6 7 8 9 10 p msize() - return the size of the biggest currently available free block, Degree of multiprogramming msize() (but: races possible). CPU utilization as a function of number of processes in memory (ignoring operating system overhead). Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 9/59 Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 11/59 MM in Multiprogramming Context Multiprogramming Effectiveness Analisys Multiprogramming has introduced two essential problems to be solved: CPU Arrival minutes # Processes Job time needed 1 2 3 4 1 10:00 4 CPU idle .80 .64 .51 .41 p relocation service - cannot be sure where program will be loaded in 2 10:10 3 CPU busy .20 .36 .49 .59 memory, 3 10:15 2 CPU/process .20 .18 .16 .15 4 10:20 2 p mutual memory protection. (a) (b) 2.0 .9 .8 .3 Job 1 finishes Possible solution: use of base and limit values: 1 Job 2 starts .9 .8 .3 .9 .1 2 .8 .3 .9 p address locations added to base value to map to physical address, 3 .3 .9 .1 .7 p address locations larger than limit value treated as an error. 4 0 0 10 15 20 22 27.6 28.2 31.7 Time (relative to job 1's arrival) (c) a. arrival and work requirements of 4 jobs, b. CPU utilization for 1{4 jobs with 80% I/O wait, c. sequence of events as jobs arrive and finish, numbers show amout of CPU time jobs get in each interval. Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 10/59 Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 12/59 Swapping Allocation Algorithms Time The role of the allocation algorithm is to chose appropriate free block in order to allocate it to some user process. C C C C C Some allocation algorithms: B B B B A p First Fit algorithm, first which matches from the list, A A A D D D p Best Fit algorithm, the optimal in size from the list, Operating Operating Operating Operating Operating Operating Operating system system system system system system system p Worst Fit algorithm, the biggest from the list, (a) (b) (c) (d) (e) (f) (g) p Buddies algorithm, memory (with size 2k) division into two equal in size blocks; dividing by two till obtaining minimal yet big enough block. Memory allocation changes as processes come into memory and leave memory. Shaded regions are unused memory. Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 13/59 Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 15/59 Fragmentation Anti-fragmentation Countermeasure Choosing allocation algorithm the following aspects should be considered: In the system there may be defined value ∆n as a minimal allowed size of allocated block, which makes easier management and improves effectiveness p effectiveness (speed), but may lead into fragmentation. p simplicity, Methods of countermeasure against fragmentation: p fragmentation effect. p deallocation and merging, Internal fragmentation - situation when some pieces of memory although allocated as part of some bigger structure (partition, frame) are not to p rellocation and compaction, be used by user processes. p paging mechanism. External fragmentation - situation when some pieces of memory although unallocated cannot be allocated for some reason (e.g. are too small) for user processes. Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 14/59 Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 16/59 Memory Management - Bitmaps and Lists Virtual Memory Virtual memory memory system consisting of at least two memory types: A B C D E 8 16 24 small and fast (e.g. operating memory) and big but slow (e.g. disk (a) memory), and moreover of additional hardware and software which 1 1 1 1 1 0 0 0 P 0 5 H 5 3 P 8 6 P 14 4 enable automatic moving of memory pieces between those memory 1 1 1 1 1 1 1 1 types. 1 1 0 0 1 1 1 1 H 18 2 P 20 6 P 26 3 H 29 3 X 1 1 1 1 1 0 0 0 Virtual memory should be almost as fast as the faster from above-mentioned Hole Starts Length Process memories and almost as huge as the bigger from the above-mentioned at 18 2 (b) (c) memories. a. part of memory with 5 processes and 3 holes, Methods of virtual memory implementation: ? tick marks show allocation units, p paging, ? shaded regions are free, p segmentation, ? the smaller the allocation unit, the larger the bitmap. p paging with segmentation. b. corresponding bitmap, c. same information as a list. Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 17/59 Faculty of E&IT, Warsaw University of Technology Operating Systems / Memory Management { p. 19/59 Dynamic Allocation Problem Memory Management Unit Role ¡ ¡ B-Stack The CPU sends virtual Room for growth addresses to the MMU Room for growth CPU ¡ ¡ B-Data ¡ package B Actually in use B-Program CPU Memory Disk Memory A-Stack management controller Room for growth unit Room for growth A-Data A Actually in use A-Program Bus Operating Operating system system The MMU sends physical (a) (b) addresses to the memory MMU (ang.