Semester Review Chris Gill CSE 422S - Operating Systems Organization Washington University in St. Louis St. Louis, MO 63130

CSE 522S – Advanced Operating Systems Semester Exam 8-10 questions, which will test for comprehension, some key terms and details, and a bit of analysis Studios are useful for analysis & concepts (and familiarity) Details from slides & readings matter as well Will cover material throughout the semester You will be allowed two 8.5 inch by 11 inch pages of notes (with information on one or both sides of each) Should be readable with your normally corrected vision Pens, pencils, erasers, water bottles also allowed No other materials or sources of information (phones off) 80 minutes total, 8:40-10am on Wed Nov 29th Try to arrive early, in Cupples II L001 (note the location!) Will distribute exams, start promptly at 8:40am CSE 522S – Advanced Operating Systems

Core Concepts (Pre-Midterm) Kernel execution Timing System calls Time sources (clocks) Interrupt handlers Timers Kernel threads Timer granularity Benchmarking Kernel programming Tracing Kernel modules Core kernel image Kernel tracing System call tracing CSE 522S – Advanced Operating Systems

Core Concepts (Pre-Midterm) Processes Interrupt handling Process creation Interrupt context Process address space Process context Process family tree Top half processing Bottom half processing Scheduling IRQs Tasklets O(1) Scheduler Work queues CFS Scheduler Real-Time Scheduler Scheduler tensions CSE 522S – Advanced Operating Systems

CSE 522S – Advanced Operating Systems Example Question #1 Imagine you are implementing a new OS feature that communicates with user space. User programs request an action from the kernel, and the kernel returns some data. Consider two options for implementing this feature: syscall vs. kernel module. Give two advantages of implementing this by creating a new system call (4 points): Give two advantages of implementing this by creating a new kernel module that reads and writes files (4 points): If you use the kernel module approach, what must user programs do to pass requests to it after it has been loaded (2 points)? CSE 522S – Advanced Operating Systems

CSE 522S – Advanced Operating Systems Example Question #2 You have a periodic system where the Linux real-time scheduler runs every ms, the round-robin scheduling interval is 100ms, and there are five tasks, which will consume 300ms, 300ms, 400ms, 500ms, and 700ms of processor time respectively each time they run. If your system schedules tasks in FIFO order on a single core, what is the longest a task might wait before it runs (3 points)? If it instead schedules tasks in round-robin order on a single core, what is the longest a task might wait before it runs (3 points)? Rank the round-robin and FIFO schedulers as as either “HIGH” or “LOW” for each of: response time, overhead, fairness (3 points). CSE 522S – Advanced Operating Systems

CSE 522S – Advanced Operating Systems Example Question #3 In the blank next to each term below, please write the letter for the text that best matches it and that it best matches (4 points). ___ task structure a. Non-root node of process family tree ___ init process b. Implements process family tree node ___ child process c. Extends process family tree ___ fork call d. Root of the process family tree CSE 522S – Advanced Operating Systems

Core Concepts (Post-Midterm) Kernel synch Process address spaces Mutexes Virtual memory areas Memory mapping and unmapping Spinlocks Deadlock Race Conditions Shared memory Kernel memory Only use kernel to map initially and unmap after done Page level management Use same physical memory Object level management Map to (different) virtual addresses in the processes Slab allocation User space memory CSE 522S – Advanced Operating Systems

Core Concepts (Post-Midterm) VFS layer Superblocks, filesystems, paths, namespaces, inodes, directory entries Page cache and writeback Mapping files into memory Directory entry hierarchy Read/write caching Eviction strategies Block I/O layer Flusher threads, writebacks Character vs. block devices New bio and bio_vec structures I/O scheduling CSE 522S – Advanced Operating Systems

CSE 522S – Advanced Operating Systems Example Question #4 Briefly, why are disk I/O schedulers sometimes called “elevators?” (2 points) Briefly, why do most I/O schedulers merge requests that target adjacent data blocks? (3 points) Briefly, why does Linux cache write requests in memory, rather than always writing them directly back to disk? (3 points) CSE 522S – Advanced Operating Systems

CSE 522S – Advanced Operating Systems Example Question #5 User-space programs may be linked either statically or dynamically (or a combination of the two). Briefly, when is static linking performed? (2 points) Briefly, when is dynamic linking performed? (2 points) Briefly, what is one advantage of static linking over dynamic linking? (2 points) Briefly, what is one advantage of dynamic linking over static linking? (2 points) CSE 522S – Advanced Operating Systems

CSE 522S – Advanced Operating Systems Example Question #6 Please circle the letter next to each of the following descriptions that illustrates the idea of “lock data, not code” (4 points). Put locks around a sequence of reads and writes to same memory location Make critical sections as long as possible Lock a data structure when accessing or removing elements Use spin lock instead of mutex CSE 522S – Advanced Operating Systems

CSE 522S – Advanced Operating Systems Semester Exam 8-10 questions, which will test for comprehension, some key terms and details, and a bit of analysis Studios are useful for analysis & concepts (and familiarity) Details from slides & readings matter as well Will cover material throughout the semester You will be allowed two 8.5 inch by 11 inch pages of notes (with information on one or both sides of each) Should be readable with your normally corrected vision Pens, pencils, erasers, water bottles also allowed No other materials or sources of information (phones off) 80 minutes total, 8:40-10am on Wed Nov 29th Try to arrive early, in Cupples II L001 (note the location!) Will distribute exams, start promptly at 8:40am CSE 522S – Advanced Operating Systems