MEMORY MANAGEMENT
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Operating System - memory Management Part 6 - Notes, Study notes of Operating Systems
In this document description about Memory Management, Buddy system, File-System Interface, Access Methods, File Attributes, File Operations.
Typology: Study notes
2010/2011
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MEMORY MANAGEMENT
Buddy system
- (^) The "buddy system" allocates memory from a fixed-size segment consisting of physically contiguous pages.
- (^) Memory is allocated from this segment using a power- of-2 allocator, which satisfies requests in units sized as a power of 2 (4 KB, 8 KB, 16 KB, and so forth).
- (^) A request in units not appropriately sized is rounded up to the next highest power of 2.
- (^) For example, if a request for 11 KB is made, it is satisfied with a 16-KB segment.
Buddy system
- (^) However, the next-highest power of 2 from 21 KB is 32 KB so either BL or BR is again divided into two 32-KB buddies, CL and CR.
- (^) One of these buddies is used to satisfy the 21-KB request.
Buddy system
- (^) The obvious drawback to the buddy system is that rounding up to the next highest power of 2 is very likely to cause fragmentation within allocated segments.
- (^) For example, a 33-KB request can only be satisfied with a 64-KB segment. In fact, we cannot guarantee that less than 50 percent of the allocated unit will be wasted due to internal fragmentation.
FILE MANAGEMENT
Objectives
- (^) To explain the function of file systems
- (^) To describe the interfaces to file systems
- (^) To discuss file-system design tradeoffs, including access methods, file sharing, file locking, and directory structures
- (^) To explore file-system protection
File Concept
- (^) Contiguous logical address space
- (^) Types:
- (^) Data
- (^) numeric
- (^) character
- (^) binary
- (^) Program
- (^) Data
File Attributes
- (^) Name – only information kept in human-readable form
- (^) Identifier – unique tag (number) identifies file within file system
- (^) Type – needed for systems that support different types
- (^) Location – pointer to file location on device
- (^) Size – current file size
- (^) Protection – controls who can do reading, writing, executing
- (^) Time, date, and user identification – data for protection, security, and usage monitoring
- (^) Information about files are kept in the directory structure, which is maintained on the disk
File Operations
- (^) File is an abstract data type
- (^) Create
- (^) Write
- (^) Read
- (^) Reposition within file
- (^) Delete
- (^) Truncate
- Open(Fi) – search the directory structure on disk for entry Fi , and move the content of entry to memory
- Close (Fi) – move the content of entry Fi in memory to directory structure on disk
Open File Locking
- (^) Provided by some operating systems and file systems
- (^) Mediates access to a file
- (^) Mandatory or advisory:
- (^) Mandatory – access is denied depending on locks held and requested
- (^) Advisory – processes can find status of locks and decide what to do