CMPT 300: Final Review Chapters 8 – 14

2 Memory Management: Ch. 8, 9 Address spaces Logical (virtual): generated by the CPU Physical: seen by the memory unit User program deals with logical addresses; never sees physical addresses Mapping from logical to physical addresses Static: done at compile-time Dynamic: done at load or execution time

3 Memory Allocation Memory is usually divided into two partitions for: Kernel: usually held in low memory User processes: usually held in high memory Typically, the two partitions are managed differently Memory allocation Contiguous  First-, Best-, Worst-fit  Fragmentation: external and internal Paging, segmentation, virtual memory

4 Paging

5 Segmentation user spacephysical memory space

6 Segmentation Hardware

7 Paging and Segmentation Paging issues: Page size  Internal fragmentation and page table size Page table (one table for each process)  Access time: use cache (TLB)  Size: use two- or three-levels page tables use hashed or inverted page tables for > 32 bits architectures Segmentation: variable size External fragmentation Facilitates sharing of code and data (share the whole segment) Combined Segmentation + paging: Example Intel Pentium

8 Virtual Memory Logical address space can be much larger than physical address space Only the needed parts of the program are brought to memory for execution Allows address spaces to be shared by several processes Allows for more efficient process creation Virtual memory can be implemented via Demand paging Demand segmentation

9 Virtual Memory and Page Fault

10 Page Replacement When we need to bring a page from disk and there is no free frame: Find a victim page to evict from memory Objective: minimize number of page faults (they are very costly) Replacement algorithms FIFO, OPT, LRU, second-chance, …

11 Thrashing Thrashing  a process is busy swapping pages in and out more than executing (because OS allocated too few frames to it)

12 Locality Model As a process executes, it moves from locality to locality, where a locality is a set of pages that are actively used together Locality could be: function, module, segment of code, … Locality model helps us to estimate the working set of a process Working set: pages that are actively being used (within a window)

13 Allocating Kernel Memory Treated differently from user memory because Kernel requests memory for structures of varying sizes  Process descriptors (PCB), semaphores, file descriptors, …  Some of them are less than a page Some kernel memory needs to be contiguous  some hardware devices interact directly with physical memory without using virtual memory Virtual memory may just be too expensive for the kernel (cannot afford a page fault)

14 Kernel Memory: Buddy System

15 Kernel Memory: Slab Allocation

16 File Systems: Ch. 10, 11, 12  Interface: file and directory structure  Maintains pointers to logical block addresses Application Programs Logical File System File-organization Module Device Drivers Storage Devices  Implementation: device-specific instructions  Writes specific bit patterns to device controller  Implementation: block allocation, …  Maps logical into physical addresses

17 File System Interface Files The smallest storage unit from the user’s perspective A collection of blocks on the disk from OS’ perspective Directory structure Tree-structured with links (general graphs) Each file has an entry in the directory

18 File System Implementation To implement a file system, we need On-disk structures, e.g.,  directory structure, number of blocks, location of free blocks, boot information, …  (In addition to data blocks, of course) In-memory structures to:  improve performance (caching)  manage file system

19 On-disk Structures Boot block Superblock Directory structure Free block File control block Data block

20 Opening and Reading from a File

21 Block Allocation Allocate free blocks to files to achieve efficient disk space utilization, and fast file access Three common allocation methods Contiguous Linked Indexed OS keeps track of free blocks using Bitmaps Linked lists

22 Combined Scheme: UNIX inode

23 Disk Structure and Scheduling Disk structure cylinders, tracks, sectors, logical blocks Disk operation Move the head to desired track (seek time) Wait for desired sector to rotate under the head (rotational latency time) Transfer the block to a local buffer, then to main memory (transfer time) Disk scheduling To minimize seek time (head movements) Algorithms: FCFC, SSTF, SCAN, LOOK

24 Protection: Ch. 14 Protection: Ensure that each object is accessed correctly and only by those processes that are allowed to do so A process operates within a protection domain Domain = set of access rights (= user-id in UNIX)

25 Access Matrix Protection as a matrix Implemented as Access Control List (columns), or Capability List (rows)

26 Protection in UNIX Default: Coarse-grained protection using 9 bits rwx rwx rwx filename Owner group all_others Optional: fine-grained protection using Access Control List

27 Warp Up: What We Have Covered How to manage resources in a single computer

28 Warp Up: What We Have NOT Covered How to manage resources distributed over multiple computers CMPT 371: Computer Networks  Protocols and algorithms to make computers talk to each other CMPT 401: Operating Systems II  Distributed file systems, distributed synchronization, inter- process communications, consistency and reliability, security Special-purpose operating systems, e.g., Real-time OS OS on small devices (PDAs, sensors,…)

29 That is it with CMPT 300! Good Luck on Your Final