Task scheduling

1
Task scheduling

• What are the goals of a modern operating system
  scheduler, and how does Linux achieve them?

2
Types of scheduling

• Stallings identifies three distinct kinds of
  process-scheduling decisions
• Long-term which tasks will the system admit?
  When? And in what order?
• Medium-term which tasks will be temporarily
  swapped out to disk? And when will they be
  swapped back in to main memory?
• Short-term which of the ready-to-run tasks will
  next gain control of the processor?

3
When are decisions made?

• Linux makes its short-term scheduling decisions
• When a timer interrupt occurs
• When an I/O request completes
• When a system-call is invoked
• When a signal is sent
• Linux makes its longer-term scheduling decisions
• When a task exits
• When CPUs idle-time exceeds a given threshold

4
Goals

• Specific scheduling policies are chosen to
  support desired system behaviors
• There are multiple goals -- and sometimes they
  may even appear to be contradictory
• So scheduling policies are a compromise

5
User-oriented goals

• Rapid response-time
• Short turnaround-times
• Assured deadlines
• Predictable performance

6
System-oriented goals

• Optimum throughput
• Maximum CPU utilization
• Balanced resource allocation
• Enforce priorities
• Assure Fairness

7
Fairness algorithms

• One fairness principle is known as FCFS
  (First-Come, First-Served), though it may not
  provide optimal throughput
• Another way to implement fairness is
  round-robin scheduling (timeslicing) in which
  every task gets allocated an equal-size slice
  of the CPUs available time, and all tasks take
  their turn at executing

8
Task types

• Some tasks are CPU bound
• They regularly consume the entire amount of
  processor time that they are allotted
• Some tasks are I/O bound
• They seldom use up their entire timeslice, but
  instead sleep while awaiting an I/O request

9
How does kernel distinguish?

• If the scheduler repeatedly gets invoked because
  a task has used up its timeslice, the kernel
  treats that task as CPU bound
• If the scheduler repeatedly gets invoked because
  a task is going to sleep (i.e., its awaiting
  completion of an I/O request), then that task is
  treated as I/O bound

10
Responsiveness

• To achieve improved responsiveness in interactive
  applications, an OS kernel can assign a higher
  priority to I/O bound tasks
• Tasks that have higher priority will always get
  scheduled before any tasks that have lower
  priority get scheduled
• However this could result in starvation

11
Dynamic priorities

• Linux combines priority-based scheduling with
  round-robin scheduling
• Linux allows priorities to be dynamically
  recomputed
• Separate queues are used for tasks with differing
  priorities, while tasks that have equal priority
  are scheduled round-robin
• Tasks can migrate between priorities

12
In-class exercises

• Exercise 1 write an application program which
  exhibits CPU bound behavior
• Exercise 2 write an application program which
  exhibits I/O bound behavior
• Exercise 3 write a program which would exhibit
  alternating behavior (becoming an I/O bound task
  for awhile, then becoming a CPU bound task for
  awhile)