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 Synchronization Mutexes Semaphores Monitors Condition variables Deadlock Scheduling

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

Address Translation

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

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

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)

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

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

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

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:

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

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

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

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

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

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

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

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