Chapter 7 Memory Management Basic requirements of Memory Management Memory Partitioning Paging Segmentation

Memory Management A program must be loaded into main memory to be executed. The principal operation of memory management is to bring processes into main memory for execution by the processor.

Memory Management Memory needs to be allocated to ensure a reasonable supply of ready processes to consume available processor time Otherwise, for much of the time all of the processes will be waiting for I/O and the processor will be idle.

The need for memory management Memory is cheap today, and getting cheaper But applications are demanding more and more memory, there is never enough! Memory Management involves swapping blocks of data from secondary storage. Memory I/O is slow compared to CPU The OS must cleverly time the swapping to maximise the CPU’s efficiency

Memory Management Requirements Relocation Protection Sharing Logical organisation Physical organisation

Relocation The programmer does not know where the program will be placed in memory when it is executed, it may be swapped to disk and return to main memory at a different location (relocated) But, OS knows because it is managing memory and is responsible for bringing this process into main memory

Relocation Addressing The processor and OS must be able to translate the memory references found in the code of the program into actual physical memory addresses (to be discussed)

Protection Processes should not be able to reference memory locations in another process without permission. Impossible to check absolute addresses at compile time because the location of a program in main memory is unpredictable. Must be checked at run time by the processor.

Sharing Allow several processes to access the same portion of memory Better to allow each process executing the same program access to the same copy of the program rather than have their own separate copy Processes that are cooperating on some task may need to share access to the same data structure

Logical Organization Memory is organized linearly (usually) In contrast, programs are organized into modules Modules can be written and compiled independently Different degrees of protection can be given to different modules (read-only, execute-only) Modules can be shared among processes Segmentation helps here

Physical Organization Cannot leave the programmer with the responsibility to manage memory Memory available for a program plus its data may be insufficient Programmer does not know how much space will be available or where the space will be The task of moving information between different levels of memory should be a system responsibility

Partitioning An early method of managing memory Pre-virtual memory Not used much now But, it will clarify the later discussion of virtual memory if we look first at partitioning Virtual Memory has evolved from the partitioning methods

Types of Partitioning Fixed Partitioning Dynamic Partitioning Simple Paging Simple Segmentation Virtual Memory Paging Virtual Memory Segmentation

Fixed Partitioning Partition memory into regions with fixed boundaries Equal-size partitions Any process whose size is less than or equal to the partition size can be loaded into an available partition

Fixed Partitioning Problems A program may be too big to fit into a partition Main memory use is inefficient. Any program, no matter how small, occupies an entire partition. This is results in internal fragmentation (wasted space internal to a partition).

Fixed Partitioning Solution – Unequal Size Partitions Lessen both problems But doesn’t solve completely Larger programs can be accommodated Smaller programs can be placed in smaller partitions, reducing internal fragmentation

Fixed Partitioning Placement Algorithm Equal-size Placement is trivial: a process can be loaded into any available partition Unequal-size Can assign each process to the smallest partition within which it will fit Queue for each partition  Processes are assigned in such a way as to minimize wasted memory within a partition

Fixed Partitioning Placement Algorithm It is possible that a partition is unused even though some smaller process could have been assigned to it Select the smallest available partition that will hold the process

Fixed Partitioning Remaining Problems The number of active processes is limited by the system i.e., limited by the pre-determined number of partitions Partition sizes are preset at system generation time, small jobs will not use space efficiently

Dynamic Partitioning Dynamic partitioning can overcome some of the difficulties with fixed partitioning Partitions are of variable length and number Process is allocated exactly as much memory as required

Dynamic Partitioning Example External Fragmentation Memory external to all processes is fragmented and memory utilization declines Can resolve using compaction OS moves processes so that they are contiguous  Time consuming and wastes CPU time OS (8M) P2 (14M) P1 (20M) Empty (6M) P4(8M) P2 (14M) Empty (56M) Empty (6M) P3 (18M) Empty (4M)

Dynamic Partitioning Operating system must decide which free block to allocate to a process Best-fit algorithm Chooses the block that is closest in size to the request  Worst performer overall Since smallest block is found for a process, the fragment left behind is too small to satisfy other requests Memory compaction must be done more often

Dynamic Partitioning First-fit algorithm Scans memory from the beginning and chooses the first available block that is large enough  Simplest and fastest  May have many process loaded in the front end of memory such that small free partitions must be searched over on each subsequent pass

Dynamic Partitioning Next-fit Scans memory from the location of the last placement  More often allocate a free block at the end of memory where the largest block is found The largest block of free memory is quickly broken up into smaller blocks Compaction is required more frequently

Dynamic Partitioning Allocation

Buddy System A reasonable compromise to overcome the disadvantages of both the fixed and dynamic partitioning schemes Entire space available is treated as a single block of 2U If a request of size s where 2U-1 < s  2U entire block is allocated Otherwise block is split into two equal buddies Process continues until the smallest block greater than or equal to s is generated

Buddy System Example

Buddy System Tree Representation A binary tree representation of the buddy allocation immediately after the Release B request The leaf nodes represent the current partitioning the memory

Relocation The actual (absolute) memory locations are determined when program is loaded into memory A process may occupy different partitions which means different absolute memory locations during execution Swapping Compaction

Addresses Logical Physical or Absolute Reference to a memory location independent of the current assignment of data to memory A relative address is expressed as a location relative to some known point (typically the program origin) Physical or Absolute The absolute address or actual location in main memory A translation must be made from a logical address to a physical address before memory access can be achieved

Relocation Hardware Support 1. Add the value in the base register to the relative address to produce an absolute address 0. Initialize base and bounds registers when a process is assigned to the Running state 2. Compare the resulting address to the value in the bounds register

Relocation Registers Used during Execution Base register Starting address for the process Bounds register Ending location of the process These values are set when the process is loaded or when the process is swapped in

Relocation Registers Used during Execution The value of the base register is added to a relative address to produce an absolute address The resulting address is compared with the value in the bounds register If the address is within bounds, instruction execution may proceed Otherwise, an interrupt indicating error is generated to OS

Paging Partition memory into small equal fixed-size chunks (frames) and divide each process into the same size chunks (pages) Pages of process could be assigned to available frames of memory No external fragmentation but little internal fragmentation consisting of only a fraction of the last page of a process

Paging Processes and Frames OS finds free frames and loads the pages of Process A, B & C Process B is suspended and is swapped out of main memory All of the processes in main memory are blocked, and OS brings in Process D A.1 A.2 A.3 B.0 D.0 B.1 D.1 B.2 D.2 C.0 C.1 C.2 C.3 D.3 D.4

Paging Operating system maintains a page table for each process which contains the frame location for each page in the process Given a logical address (page number, offset), the processor uses the page table to produce a physical address

Paging Page Table page number frame number

Paging Logical Addresses Using a page size that is a power of 2, a logical address (page no., offset) is identical to its relative address Example 16-bit address 210 =1024-byte page 10-bit offset 6-bit page number A maximum of 26 =64 pages

Paging Logical to Physical Address Translation Consider an address of n+m bits, where the leftmost n bits are the page no. and the rightmost m bits are the offset 1. Extract the page no. 2. Use the page no. as an index into the process page table to find the frame number, k 3. The physical address is constructed by appending the offset to k

Segmentation A program can be subdivided into segments Segments may vary in length There is a maximum segment length Segmentation is similar to dynamic partitioning But, a program may occupy more than one partition No internal fragmentation but suffers from external fragmentation (as does dynamic partitioning)

Segmentation A logical address consists of two parts a segment number and an offset There is a segment table for each process and a list of free blocks of main memory. Each segment table entry would have to give the starting address in main memory of the corresponding segment. the length of the segment, to assure that invalid addresses are not used.

Segmentation Logical Addresses There is no simple relationship between a logical address (segment no., offset) and the physical address Example 16-bit address 12-bit offset 4-bit segment number maximum segment size= 212=4096

Segmentation Logical to Physical Address Translation Consider an address of n + m bits, where the leftmost n bits are the segment no. and the rightmost m bits are the offset 3. If the offset  the length of the segment, the address is invalid 1.Extract the segment no. 4.The physical address is the sum of the starting physical address of the segment plus the offset 2.Use the segment no. as an index into the process segment table to find the starting physical address of the segment