Memory Management. (SGG, Chapter 08) Objectives. Memory Hierarchy. Outline. Instructor: Dr. Tongping Liu. To describe various memory hardware"

Transcription

1 Objectives Memory Management (SGG, hapter 08) To describe various memory hardware To discuss various memory-management techniques, including partitions and swapping Instructor: Dr. Tongping Liu Department of omputer UTS 1 Outline ackground Partition ased llocation Swapping Memory Hierarchy n PU can directly access main memory and registers only n ut programs and data must be brought (from disk) into memory n Memory accesses can be the bottleneck l ache between memory and PU registers n Memory Hierarchy l ache: small, fast, expensive; SRM; l Main memory: medium-speed, not that expensive; DRM l Disk: many gigabytes, slow, cheap, nonvolatile storage PU (Processor/LU) Internal Memory I/O Devices (disks) 1

2 ackground about Memory Think of memory as an array of words containing program instructions and data How do we execute a program? Ø Fetch an instruction à decode à may fetch operands à execute à may store results! Memory hardware sees a stream of DDRESSES How to manage and protect main memory while sharing it among multiple processes? Ø Keeping multiple processes in memory is essential to improve the PU utilization ind Instructions and Data to Memory ddress binding can happen at three different stages Ø ompile time: If memory location is known beforehand, absolute code can be generated; However, must recompile code if starting location changes Ø Execution time: inding delayed until run time if the process can be moved during its execution from one segment to another. Need hardware support for address maps (e.g., base and limit registers) Logical vs. Physical ddress Space Logical address Ø Generated by the PU; also referred to as virtual address! Physical address Ø ddress seen by the memory unit Logical and physical addresses are the same in compile-time address-binding schemes; Logical (virtual) and physical addresses differ in execution-time address-binding scheme! Ø The mapping form logical address to physical address is done by a hardware called memory management unit (MMU).! Ø We will study how this mapping is done and what hardware support is needed Outline ackground Partition ased llocation Swapping 2

3 Simple MMU: Relocation Register Hardware device maps logical (virtual) address to physical address Simple MMU: Relocation Register Dynamic relocation using a relocation register In a simple MMU, the value in the relocation register (base) is added to every address generated by a user process at the time it is sent to memory The user program deals with logical addresses, not real physical addresses Department of omputer UTS 10 Simple MMU: Relocation Register Logical addresses is between 0~max and physical addresses is between R+0 to R+max with the base R The user program deals with logical addresses, by assuming that the process runs in locations 0 to max. Logical addresses must be mapped to physical addresses before they are used Partition ased Memory Management Main memory usually divided into two partitions: Ø Resident operating system, usually held in low memory. Ø User processes then held in high memory. Relocation registers used to protect user processes from each other: Ø ase register contains value of smallest physical address Ø Limit register contains range of logical addresses each logical address must be less than the limit register. Ø MMU maps logical address dynamically. 3

4 ase and Limit Registers pair of base and limit registers define the logic address range of a process. Every memory access is checked by hardware to ensure the correctness Partition-based Memory Management Fixed Partitions Ø Divide memory into fixed size of partitions (not necessarily equal) Ø Each partition for at most one process Ø ase + limit registers for relocation and protection Ø How to determine the partition sizes? Variable Partitions Ø Partition sizes determined dynamically Ø keeps a table of current partitions Ø When a job finishes, leaves a partition hole Ø onsolidate free partitions à compaction Department of omputer UTS 14 Partition-based llocation When a process arrives, it is allocated memory from a hole large enough to accommodate it Operating system maintains information about: Ø a) allocated partitions b) free partitions (hole) D E Department of omputer UTS 15 Dynamic llocation Problem How to satisfy a request of size n from a list of free holes First-fit: llocate the first hole that is big enough est-fit: llocate smallest hole that is big enough; Ø Must search entire list, unless ordered by size Ø Produces the smallest leftover hole Worst-fit: llocate the largest hole; Ø Must also search entire list Ø Produces the largest leftover hole First-fit and best-fit are better than worst-fit in terms of speed and storage utilization. ut all suffer from fragmentation! 4

5 Memory Fragmentation External Fragmentation Ø total memory space exists to satisfy a request, but it is not contiguous Internal Fragmentation Ø allocate memory larger than requested; the size difference is called internal fragmentation (can t be used by others) How can we reduce external fragmentation Ø ompaction: migrate memory segments to place all free chunks together into one larger block Ø ompaction is possible only if relocation is dynamic, and is done at execution time! Outline ackground Partition ased llocation Swapping. Swapping onsider a multi-programming environment: l Each program must be in the memory to be executed l Processes come into memory and l Leave memory when execution is completed D E What should we do, reject it or accept it? Swapping allows you to support more processes, by swapping out an old process to the disk, e.g. here? Swapping Definition process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution Ø acking store: large enough to accommodate copies of all memory images for all users; Ø Roll out, roll in swapping variant used for priority-based scheduling algorithms; lower-priority process is swapped out so higher-priority process can be loaded and executed Swapping can free up memory for additional processes. 5

6 Swapping Major part of swapping time is transfer time; Ø Total transfer time is directly proportional to the amount of memory swapped (e.g., 10M process / 40M per sec = 0.25 sec) Ø May take too much time to be used often Standard swapping requires too much swapping time Modified versions of swapping are found on many systems (i.e., UNIX, Linux, and Windows)! 6