8: Memory Management

Transcription

1 CSC400 - Operating Systems 8: Memory Management J. Sumey Physical Memory as important as the CPU, a computer's physical memory is another important resource that must be carefully & efficiently managed by the OS, regardless of memory size a.k.a. real, main, primary memory main memory is still relatively expensive ex: 1GB $30 3 /MB 1TB $ /MB was more than $1/Kb in early years! CSC400 - Memory Management 2 CSC400 Operating Systems 1

2 Issues modern OSs & apps require larger amounts of main memory memory organization issues single process or multiple? if multiple, same size partitions or variable? contiguous or non-contiguous partitions? single chunk for entire program or multiple nonadjacent chunks (can use gaps) time limits for allocation? CSC400 - Memory Management 3 Memory Manager that part of the OS that is concerned with memory organization and management strategies means by which the memory manager performs its functions allocates/deallocates memory to/from processes, swaps between primary & secondary memories also controls memory controller hardware CSC400 - Memory Management 4 CSC400 Operating Systems 2

3 Memory Hierarchy multi-level model of memory based on speeds & costs of different memories hierarchy: 1. cache small, very fast, expensive, volatile; typically on-board the CPU 2. primary (RAM) medium-speed, medium-price, volatile; the "working" or "main" memory 3. secondary (hard drive) large, slower, cheap, non-volatile 4. tertiary/archival (tapes, optical) massive, slowest, cheapest CSC400 - Memory Management 5 Memory Management Strategies goal: optimize use of main memory strategy types: fetch when to load next part of program or data from hard drive to RAM demand or anticipatory placement where in RAM to put it ex: best-fit, worst-fit, first-fit replacement what RAM contents to remove when memory becomes full (discussed later) CSC400 - Memory Management 6 CSC400 Operating Systems 3

4 Monoprogramming a.k.a. single-user contiguous allocation runs only 1 program at a time for 1 user (in addition to the OS) ex: MS-DOS operation prompt, load program, execute to completion, prompt, programs limited by size of main memory, unless programmer uses overlays division of program into logical sections which can overlay each other at run time messy & difficult to incorporate into programs CSC400 - Memory Management 7 Multiprogramming - Fixed Partitions supports CPU switching among n loaded processes by dividing system memory into n fixed-sized partitions incoming jobs are moved thru input queue(s) until allocated to a partition, then runs to completion increases CPU utilization & system thruput wastes memory unless process space matches partition size (fragmentation) needed relocating assemblers/compilers CSC400 - Memory Management 8 CSC400 Operating Systems 4

5 Protection how can the OS be protected against errant user programs? which could actually destroy the OS! monoprogramming systems need only a boundary register keeps user-mode program on one side can only modify in supervisor-mode multiprogramming systems use register pair base: sets physical address of beginning of process limit: defines length of usable memory space thus, user program is confined to memory range CSC400 - Memory Management 9 Memory Allocation via Bit Maps memory is divided into fixed-size allocation units (AUs); the status of each AU is tracked by a corresponding bit in a "AU bit map" 1=used, 0=free sizeof(bitmap) = sizeof(memory) #AUs design issue: size of AU? waste in AUs vs. waste for bitmap drawback: searching bitmap for a k-unit chunk of free memory CSC400 - Memory Management 10 CSC400 Operating Systems 5

6 Variable-Partition Multiprogramming sets partition size according to process needs allows more jobs to be loaded in main memory than in fixed partition method, increasing level of multiprogramming doesn't exhibit internal fragmentation, but does have external fragmentation as holes are left when processes finish must track holes in a free memory list, coalescing and compacting as necessary CSC400 - Memory Management 11 Memory Allocation via Linked-Lists maintains a "picture" of memory usage in a linked list of allocated & free memory segments sorted by memory address each node tracks used/free status, starting address of segment, length of segment list maintenance is fairly straightforward: deallocation 4 possibilities are AXB, AXf, fxb, fxf allocation first fit, next fit, best fit, worst fit CSC400 - Memory Management 12 CSC400 Operating Systems 6

7 Memory Placement Strategies determines which memory hole an incoming job is placed first-fit find 1 st large enough available hole, quick next-fit: resume at end of previous search best-fit smallest available hole to hold job leaves many small holes; sorting overhead worst-fit always pick largest available hole, still leaving a large hole; also has sorting overhead CSC400 - Memory Management 13 Swapping concept of moving entire process between RAM (partitions) & disk as processes are scheduled, blocked, etc. swap out writing a process's memory contents to secondary storage (disk) swap in bringing the process image back to main memory may happen many times before process completion typically used in more sophisticated virtual memory systems CSC400 - Memory Management 14 CSC400 Operating Systems 7

8 Linking & Loading of executables appx. 7A in Stallings CSC400 - Memory Management 15 CSC400 Operating Systems 8