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Week 10

Week 10. March 10, 2004 Adrienne Noble. Important Dates. Project 4 due tomorrow (Friday) Final Exam on Tuesday, March 16, 2:30-4:20pm. Questions?. Lecture Project 4. Pre-Midterm Topics. Basic OS components and architectural support Processes User versus protected mode Threads

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Week 10

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  1. Week 10 March 10, 2004 Adrienne Noble

  2. Important Dates • Project 4 due tomorrow (Friday) • Final Exam on Tuesday, March 16, 2:30-4:20pm

  3. Questions? • Lecture • Project 4

  4. Pre-Midterm Topics • Basic OS components and architectural support • Processes • User versus protected mode • Threads • Synchronization • Mutexes • Semaphores • Monitors • Condition variables • Deadlock • Scheduling

  5. Memory Management • Multiprogramming brought some problems • Protection • Fast memory access • Consistent way to talk to memory • So we use virtual addresses

  6. Address Translation

  7. Segmentation • Partitions an address space into logical units • Today usually code, stack, heap, etc. • Advantages • Nice to have logical blocks • Disadvantages • Can lead to bad external fragmentation

  8. Paging • Partitions memory into fixed sized chunks • Usually used with segmentation • One page table per segment

  9. Page Faults • OS uses main memory as a cache of disk storage • Just keep accessed pages in main memory • How it happens: • PTE set as invalid when page was evicted • Causes an exception to be thrown when a process tries to access an invalid page • OS runs page fault handler • Uses invalid PTE to locate page on disk (possibly through another data structure) • Gets an empty physical page frame from page replacement algorithm • Reads page into physical page frame • Updates PTE to point to that physical page and be valid • OS restarts faulting process (by putting it on the ready queue)

  10. Page Replacement • Chooses “best” page to evict when more space is needed • Algorithms • Belady’s Algorithm – optimal (page that will be reference furthest in the future) • FIFO – page that was brought in longest ago • LRU – least recently used • LRU clock – approximation of LRU • Hypothetical Working Set Algorithm – try optimize memory given to each process by looking at their working sets • Page Fault Frequency – try to equalize the page fault rate between processes • Thrashing – when the system uses most of it’s time dealing with page faults and very little time doing actual work

  11. Page Tables • One level page table • Simple, but takes lots of space • Two/Three/Four level page tables • Save space • Only need to allocate the blocks that actually contain data • Can page page tables in and out of memory • Slower because more memory accesses are needed • Solution? • TLB • Cache virtual to physical translations • Fully associative (all entries searched in parallel) • Implemented in hardware • >99% hit rate

  12. Disks • Performance depends on • Seek – how fast the disk arm can move • Rotation – how fast the disk can rotate • Transfer – getting data from disk surface to memory • Disk Scheduling • FCFS – ok when load is low • SSTF – minimize seek time, but favors middle blocks and can cause starvation (why?) • SCAN – elevator, favors middle blocks • C-SCAN – one way elevator

  13. File Systems • cse451fs • File names, inodes, writing/reading • Protection • ACLs – list of who has access to file • Capabilities – easy to transfer (like keys) • Disks are divided up:

  14. BSD UNIX Fast File System • Cylinder groups • Blocks for one file must all be located in the same cylinder (along with the file’s inode too) • Seek times are shorter • Increased block size – 4KB • Faster • More internal fragmentation • Aware of hardware • Knows about disk parameters, so it can optimize operations • More complicated code

  15. Berkley Log-Structured File System • Uses disk as a log • Written only at one end • Challenges • Locating inodes • Keep an inode map at fixed location • Free space management • Divide disk into segments, then clean and combine those segments periodically

  16. RAID • Goals - speed and reliability • Level 0 • Striped • Level 1 • Striped and mirrored • Level 2,3,4 • Parity disk • Level 5 • Interleaved distributed parity

  17. Networking • IP (Internet Protocol) • A way of naming hosts and routing packets between hosts • TCP (Transmission Control Protocol) • Makes reliable multi-packet messages • Abstraction above IP

  18. Distributed Systems • Loosely coupled • Network protocols • Autonomous systems • Closely coupled • Single OS • Shares logical and physical resources

  19. RPC (Remote Procedure Call) • Used to call a method on a remote host • Server publishes API • Client does local procedure calls • Calls a stub which manages the details using the RPC runtime system • Transparent to the programmer

  20. Security • Authentication • Username/password • Public/private keys • Authorization • Keep track of user’s read/write access to different objects • Auditing • Record what’s going on

  21. Other Tips • Refresh your knowledge about the projects • Review lecture notes and homework problems • Congratulations on making it through a tough class! 

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