Memory management techniques in operating system pdf

in Printable by

It determines how memory is allocated among competing processes, deciding which gets memory, when they receive it, and how much they are allowed. When memory is allocated it determines which memory locations will be assigned. All the computer’s memory, usually memory management techniques in operating system pdf the exception of a small portion reserved for the operating system, is available to the single application.

Memory management consists of allocating a partition to a job when it starts and unallocating it when the job ends. Partitioned allocation usually requires some hardware support to prevent the jobs from interfering with one another or with the operating system. Compaction moves “in-use” areas of memory to eliminate “holes” or unused areas of memory caused by process termination in order to create larger contiguous free areas. The physical memory can be allocated on a page basis while the address space appears contiguous. Usually, with paged memory management, each job runs in its own address space.

Segmentation allows better access protection than other schemes because memory references are relative to a specific segment and the hardware will not permit the application to reference memory not defined for that segment. It is possible to implement segmentation with or without paging. Without paging support the segment is the physical unit swapped in and out of memory if required. With paging support the pages are usually the unit of swapping and segmentation only adds an additional level of security. Addresses in a segmented system usually consist of the segment id and an offset relative to the segment base address, defined to be offset zero. 16,383 segments of up to 4GiB each. A process could have up to 4046 segments.

This page was last edited on 23 November 2017, at 19:47. This article is about memory management at the application level. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when no longer needed. Several methods have been devised that increase the effectiveness of memory management. The quality of the virtual memory manager can have an extensive effect on overall system performance.

The task of fulfilling an allocation request consists of locating a block of unused memory of sufficient size. The specific dynamic memory allocation algorithm implemented can impact performance significantly. If a smaller size is requested than is available, the smallest available size is selected and split. One of the resulting parts is selected, and the process repeats until the request is complete.

When a block is allocated, the allocator will start with the smallest sufficiently large block to avoid needlessly breaking blocks. When a block is freed, it is compared to its buddy. If they are both free, they are combined and placed in the correspondingly larger-sized buddy-block list. Special declarations may allow local variables to retain values between invocations of the procedure, or may allow local variables to be access by other procedures. Garbage collection is a strategy for automatically detecting memory allocated to objects that are no longer usable in a program, and returning that allocated memory to a pool of free memory locations. This method is in contrast to “manual” memory management where a programmer explicitly codes memory requests and memory releases in the program. While automatic garbage has the advantages of reducing programmer workload and preventing certain kinds of memory allocation bugs, garbage collection does require memory resources of its own, and can compete with the application program for processor time.

In this way addition of virtual memory enables granular control over memory systems and methods of access. Even though the memory allocated for specific processes is normally isolated, processes sometimes need to be able to share information. Memory management systems, among other operations, also handle the moving of information between these two levels of memory. 5: Dynamic Storage Allocation, pp. Dynamic storage allocation: A survey and critical review”.