1. Midterm Review Brian Kocoloski
CSE 422S - Operating Systems Organization
Washington University in St. Louis
St. Louis, MO 63130

2. CSE 422S –Operating Systems Organization
Midterm Exam
8-10 questions, which will test for comprehension, some key terms and details, and a bit of analysis
How I would study (in order of importance)
Start with LKD readings
- Review PPT presentations at the same time that you do the readings
Review studios for reinforcement
Types of questions:
True/False
Multiple Choice
Open answer / code snippets
Bring pen/pencil/eraser/water bottle. No other material permitted on your desk
80 minutes total, 8:40-10am on Thu March 1
Will distribute exams, start promptly at 8:40am
CSE 422S –Operating Systems Organization

3. CSE 422S –Operating Systems Organization
Core Concepts
LKD Ch 1
Pg 4: Overview of Operating Systems and Kernels
Figure 1.1: breakdown between user space and kernel space
LKD Ch 2
Pg 16: A Beast of a Different Nature
Understand differences between writing user code and writing kernel code (libraries, header files, etc.)
CSE 422S –Operating Systems Organization

4. CSE 422S –Operating Systems Organization
Core Concepts
LKD Ch 5
What are system calls for?
What are some of the system calls we’ve looked at in the studios? (recall strace from studio 5)
What operations are done in order to invoke a system call?
Why is implementing a new system call not always a great idea?
CSE 422S –Operating Systems Organization

5. CSE 422S –Operating Systems Organization
Core Concepts
LKD Ch 11
What does HZ represent?
How does the kernel keep track of real time?
How does the kernel manage timers? How does it know when a timer has expired?
What happens when a thread invokes schedule()?
Other timing concepts
What are hrtimers useful for? Why can’t the system clock (configured via HZ) be used to implement high resolution times – (i.e., on the order of nanosecond resolution)
CSE 422S –Operating Systems Organization

6. CSE 422S –Operating Systems Organization
Core Concepts
LKD Ch 6
How does the kernel implement “libraries”
Why is “rolling your own” data structures frowned upon?
Be able to perform basic operations on a linked list (I will give you the linked list API if a question asks you to do this)
CSE 422S –Operating Systems Organization

7. CSE 422S –Operating Systems Organization
Core Concepts
LKD Ch 3
What are the data structures used to track processes?
struct task_struct, struct thread_info
What is the difference between task_struct and thread_info?
What are the different states a process can be in?(Figure 3.3 on pg 28)
What is meant by copy-on-write? How does it make it quicker to create new processes?
How are kernel threads different from user threads/processes?
CSE 422S –Operating Systems Organization

8. CSE 422S –Operating Systems Organization
Core Concepts
LKD Ch 4
What are the two questions that drive the behavior of the Linux scheduler?
Which process should run?
For how long should it run?
Differences in requirements between I/O and compute bound processes
Limitations of the O(1) scheduler
How CFS addresses those limitations
vruntime
CSE 422S –Operating Systems Organization

9. CSE 422S –Operating Systems Organization
Core Concepts
Real-time scheduling
Understand the three real-time scheduling classes
Be able to walk through examples of determining which process will execute given different periods, RT priorities, and runtimes (as done in class)
Differences between user and kernel preemption (LKD pg 62) and why kernel preemption is useful from a real-time perspective
CSE 422S –Operating Systems Organization

10. CSE 422S –Operating Systems Organization
Core Concepts
LKD Ch 7 & Ch 8
Interrupt context vs user context
Figure 7.1: path from hardware to kernel interrupt handling
Difference between interrupt context and executing code with interrupts disabled
What is the tension that drove the design of top-half and bottom-half processing?
Be able to explain the differences between the three bottom half interrupt handler types, and to choose which would be best if I give you a set of constraints
CSE 422S –Operating Systems Organization

11. Linux Kernel Familiarity
Be familiar with the main data structures we’ve looked at (you don’t need to memorize the fields, but be aware of what type of information they record)
struct task_struct (include/linux/sched.h)
struct sched_entity (include/linux/sched.h)
If the slides or a studio refers to particular fields of a data structure, understand what those fields refer to
current
current->mm
find_vpid()
pid_task()
CSE 422S –Operating Systems Organization

12. Core Concepts (summary)
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 422S –Operating Systems Organization

13. Core Concepts (summary)
Processes:
Scheduler tensions
Interrupt handling:
Process creation
Process address space
Interrupt context
Process family tree
Process context
Top half processing
Scheduling:
Bottom half processing
O(1) Scheduler
IRQs
CFS Scheduler
Tasklets
Real-Time Scheduler
Work queues
CSE 422S –Operating Systems Organization

14. CSE 422S –Operating Systems Organization
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 422S –Operating Systems Organization

15. CSE 422S –Operating Systems Organization
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 422S –Operating Systems Organization

16. CSE 422S –Operating Systems Organization
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 422S –Operating Systems Organization