Review: Process Communication

1
Review Process Communication

• Sequential Communication
• Result of P1 becomes at termination the input to
  P2
• Tools Redirect (gt,gtgt) and Pipe ()
• Concurrent Communication
• between 2 running processes via shared resources
• Resource Sharing
• Each resource has a Boolean variable associated
• A process can request a resource and set the lock
  1
• To free a resource lock 0
• Usually you only lock a resource if you want to
  change it

2
Problems of Sharing Resources

• Inconsistency
• Unpredictable and wrong behavior (example of
  printer queue with lost job)
• Deadlock
• Processes are waiting for resources held by
  another
• Race conditions
• Conditions under which sharing can lead to
  inconsistencies and deadlock

3
Solutions

• Inconsistency can be avoided by ensuring
  uninterrupted blocks of instructions
• Preventive
• Test and Set Instruction
• Semaphores
• Critical Block
• Detect and resolve

4
The Dining Philosophers
F3
Plate with food
P3
P4
F2
F4
P2
P5
P1
F1
Fork
F5
5
Detection for the Philosophers

• 5 Processes (Philosophers) P1, P2, P3, P4, P5
• 5 Resources (Forks) F1, F2, F3, F4, F5
• Implementation of P
• think
• grab_left_fork
• grab_right_fork
• eat
• free_right_fork
• free_left_fork
• Task draw the dependency graph

6
Dependency Graph
F1
F5
F2
F4
F3
7
Example for Dependency Graph

• Notation R(resource) request/lock
• F(resource) free resource
• P1 R(a),R(b),R(c),F(c),R(d),F(b),F(d),F(a,)
• P2 R(e),R(d),R(c),F(c),F(d),F(e)
• P3 R(a),R(d),R(e),F(a),F(d),F(e)
• P4 R(c),R(k),R(a),F(a),F(k),F(c)
• Question Which process (P2,P3,P4) can you let
  run without danger of deadlock

8
Solution

• Please copy the graph from the board!
• You cant run the following combinations
• P2,P3
• P1,P4

9
Use of Dependency Graph

• Prevention the OS does not allow a process to
  start if it could create a deadlock
• this is in some cases a overly conservative and
  in some sense unfair policy
• the implementation of this policy is
  computationally very expensive!
• Detection wait for deadlock to happen
• If a number of processes seem to be waiting
  beyond a certain timelimit, build the graph
  including the ownership of resources and select a
  process such that killing it will break the circle

10
Process Scheduling

• Scheduler decides
• Which process in ready queue is next to run
• When to stop a running process

11
Scheduling

• Main Constraint Every switch has overhead
• Scheduling objectives depend on environment
• Batch systems High performance computing, Server
• Throughput maximize jobs per time
• Turnaround time minimize time between creation
  and termination
• User interactive systems Eureka or Sales
• Response Time
• Fairness

12
Batch Systems

• Processes are executed sequentially to minimize
  switching overhead (no timeout arrow)
• When to stop a running process Only if it is
  waiting for I/O
• Which process to run next
• First comes first serves
• Fair, but can lead to long waiting of minor jobs
• Shortest job next
• assumes runtime is known, penalizes long jobs

13
Batch Example

• Processes arriving in the following order
• A(20s) at t10 ,B(14000s) at t20 ,C(10s) at
  t25, D(5s) at t 30
• CPU currently runs E and terminates at t18
• Execution Order for First-comes first-serves
• A, B, C, D
• Execution Order for Shortest First
• A (terminates at 28), C (terminates at 38), D
  (terminates at 43), B (terminates at 14043)

14
Interactive Systems Multiprogramming

• Each process is allowed to run for a certain time
  (quantum) after which it is interrupted
• Parameter quantum length (e.g., 25ms)
• Too small big overhead due to context
  switch(e.g., 1ms)
• Too big low response time (wait to see character
  you typed)
• Scheduling Schemes
• Round Robin Scheduling
• Priority Scheduling
• Lottery Scheduling

15
Round Robin

• Simplest
• Scheduler has ready queue with first in first out
  
• When interrupted after timeout (quantum is over)
• Which next next in queue

Ready Queue
Running
T1
A
T2
B
16
Priority Scheduling

• Disadvantage of Round Robin
• Even very important jobs wait in line
• Alternative multiple ready queues for different
  priority classes
• Round robin for jobs in highest priority queue
• Only if empty it picks from the next lower
  priority
• Works well if there are few priority classes and
  few high priority jobs
• Can starve low priority jobs

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Example Priority

• Quantum 10 sec, Switch cost 1 sec
• Priority Levels 0, 1, 2 where 2 is highest
• CPU started at t0 to run A (p0, t30)
• at t5 arrives B (p1, t20)
• at t15 arrives C (p1, t20)
• at t25 arrives D (p2, t5)
• Task draw timeline of execution
• Solution A, B, C, D, B, C, A, A

18
Lottery Scheduling

• Similar to Priority Scheduling
• Multiple queue for different priorities
• Every process that is next in his queue gets a
  lottery ticket
• High priority jobs get more lottery tickets
• OS picks a ticket to determine the next job
• Higher priority has a higher probability to be
  executed next but you dont starve low priority
  jobs

19
What you should be able to do

• Draw dependency graph from process specification
  and determine whether there is a chance of
  deadlock and which processes you can run safe
  together
• Determine execution order for batch (first comes
  first serves, shortest first), Round Robin and
  Priority Scheduling