1. Networks and Operating Systems: Exercise Session 2
Salvatore di Girolamo (TA)
Networks and Operating Systems: Exercise Session 2

2. Difference between user- and kernel-level threads?
Kernel-level threads: implemented in kernel-space, the OS sees them
User-level threads: managed by user-level libraries, much cheaper than kernel threads
Advantages/disadvantages
Good for frequently blocking applications
Scheduling can be inefficient
Can block the entire process
Significant memory overhead in the kernel
Fast switching
CPU-time can be assigned according to the their number
Slow management operations
No OS support required

3. Threads: user- to kernel- level threads mapping
One-to-one
Many-to-one
Many-to-many
Kernel
User
CPU 0
CPU 1
Kernel
User
CPU 0
CPU 1
Kernel
User
CPU 0
CPU 1
Text
Data
BSS
Thread 1 stack
Thread 2 stack
Thread 3 stack
Text
BSS
Thread 1 stack
Thread 2 stack
Thread 3 stack
Data

4. Scheduling
Life of a scheduler: What to schedule? When to schedule? For how long?
Scheduler needs CPU to decide what to schedule
Any time spent in scheduler is “wasted” time
Want to minimize overhead of decisions
To maximize utilization of CPU
But low overhead is no good if your scheduler picks the “wrong” things to run!
Trade-off between:
scheduler complexity/overhead and
quality of resulting schedule

5. When to schedule? When: A running process blocks (or calls yield())
e.g., initiates blocking I/O or waits on a child
A blocked process unblocks
I/O completes
A running or waiting process terminates
An interrupt occurs
I/O or timer
2 or 4 can involve preemption

6. Scheduling: Preemption
Difference between preemptive and non-preemptive scheduling?
Preemptive: Processes dispatched and descheduled without warning
How?
timer interrupt, page fault, etc.
Non-preemptive: Each process explicitly gives up the scheduler
How?
Start I/O, executes a “yield()” call, etc.
What could be the problem with non-preemptive scheduling?
Which one would you use in a real time system?

7. Round-robin Simplest interactive algorithm
Run all runnable tasks for fixed quantum in turn
Advantages:
It’s easy to implement
It’s easy to understand, and analyze
Higher turnaround time than SJF, but better response
Disadvantages:
It’s rarely what you want
Treats all tasks the same

8. Priority Very general class of scheduling algorithms
Assign every task a priority
Dispatch highest priority runnable task
Priorities can be dynamically changed
Schedule processes with same priority using
Round Robin
FCFS
etc.
Priority 100
Priority 4
Priority 3
Priority 2
Priority 1 T
Runnable tasks
Priority …

9. Scheduling: First-come first-served
Simplest algorithm!
Task
Execution time A 24 B 3 C
What about the waiting time?
Waiting times: 0, 24, 27
Avg. = ( )/3 = 17 A B C 24 27 30

10. Scheduling: First-come first-served
Different arrival order
Task
Execution time A 24 B 3 C
What about the waiting time?
Waiting times: 6, 0, 3
Avg. = (0+3+6)/3 = 3
Much better 
But unpredictable  B C A 3 6 30

11. Scheduling: Shortest-job first Task Execution time A 6 B 8 C 7 D 3
Always run process with the shortest execution time.
Optimal: minimizes waiting time (and hence turnaround time)
Task
Execution time A 6 B 8 C 7 D 3
What to do if new jobs arrive (interactive load)?
Shortest Remaining Time First: new, short jobs may preempt longer jobs already running D A C B

12. Scheduling: Round Robin
Simplest interactive algorithm
Run all runnable tasks for fixed quantum in turn
Advantages:
It’s easy to implement
It’s easy to understand, and analyze
Higher turnaround time than SJF, but better response
Disadvantages:
It’s rarely what you want
Treats all tasks the same

13. RT Scheduling: Earliest Deadline First
Schedule task with earliest deadline first (duh..)
Dynamic, online.
Tasks don’t actually have to be periodic…
More complex - O(n) – for scheduling decisions
EDF will find a feasible schedule if:
Which is very handy. Assuming zero context switch time…

14. Scheduling: exercise Process Burst time Priority P1 10 1 2 5 3 1 4 2
Notes:
SJF: equal burst length processes are scheduled in FCFS
Priority: non-preemptive priority, small priority number means high priority, equal priority processes are scheduled in FCFS)
RR: quantum=1. P2 P3 P4 P5
Time: 1 2 3 4 5 6 7 8 10 9 11 12 13 19 16 14 15 17 18
FCFS: P1 P1 P1 P1 P1 P1 P1 P1 P1 P1 P2 P3 P3 P4 P5 P5 P5 P5 P5
SJF: P2 P4 P3 P3 P5 P5 P5 P5 P5 P1 P1 P1 P1 P1 P1 P1 P1 P1 P1
RR: P1 P2 P3 P4 P5 P1 P3 P5 P1 P5 P1 P5 P1 P5 P1 P1 P1 P1 P1
Source: