1. ICS 143 Principles of Operating Systems
Discussion Section Week 7

2. Today Announcements Programming Assignment Homework #3 Lecture Review
Pop Quiz #2
11/17/2018

3. Announcements Programming Assignment Homework #2 scores out now
Due Wed, 10 June 2015, 11:55pm via EEE Dropbox
Homework #2 scores out now
Homework #1 scores will follow shortly
Homework #3:
Out now on class website
Due Thu, 21 May 2015, 11:55pm via EEE Dropbox
11/17/2018

4. Programming Assignment
Assignment description has moved to GitHub:
Scroll down to see README.md
Check repo history frequently for updates:
s/master
11/17/2018

5. Programming Assignment (2)
Input: text file
One incoming process per line
<process-id> <burst-time> <arrival-time> <share>
<process-id> <arrival-time> <burst-time> <share>
Output: text file
One scheduled process per line
<process-id> <finish-time> <wait-time> <turnaround- time>
Last line:
<average-wait-time> <average-turnaround-time>
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6. Programming Assignment (3)
Other updates:
New assumption for Proportional Share (PS) scheduler:
If PID 3 with share 60 and PID4 with share 10 are in ready queue at the same time, then PID 3 must be scheduled first.
Please take a look at `ps5.input` and `ps5.expected` for a complete example.
Time limit:
Each test must finish within **2.1 seconds**.
After that time, timeout will occur and that test will be assumed to have failed.
All updates will appear in repo history
11/17/2018

7. Deadlocks A process is deadlocked if
it is waiting for an event that will never occur
A process is indefinitely postponed if
it is delayed repeatedly over a long period of time while the attention of the system is given to other processes
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8. Resources Resource: commodity required by a process to execute
Serially reusable resources
CPU cycles, memory space, I/O devices, files
Acquire => use => release
Consumable resources
Messages, buffers of information, interrupts
Create => acquire => use
Resource ceases to exist after it has been used
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9. Deadlock Conditions
Necessary and sufficient conditions for deadlock to occur (must hold simultaneously):
Mutual exclusion
Only one process at a time can use the resource
Hold and wait
Processes hold resources already allocated to them while waiting for other resources
No preemption
Resources are released by processes holding them only after that process has completed its task
Circular wait
A circular chain of processes exists in which each process waits for one or more resources held by the next process in the chain.
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10. Resource Allocation Graph
Process:
Resource type with 4 instances:
Pi requests instance of Rj:
Pi is holding instance of Rj:
Deadlock??
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11. Deadlocks in Resource Allocation Graphs
If graph contains no cycles:
NO DEADLOCK
If graph contains a cycle:
If only one instance per resource type, then deadlock
If several instances per resource type, possibility of a deadlock
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12. Deadlock Management Deadlock detection Deadlock avoidance
Allow possibility of a deadlock, determine if deadlock has occurred and which processes and resources are involved
Deadlock avoidance
Allow possibility of a deadlock, but sidestep it as it occurs
Deadlock recovery
Allow possibility of a deadlock, detect it; then clear the problem, allowing processes to complete and resources to be reused
Deadlock prevention
Design the system in such a way that deadlocks can never occur
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13. Deadlock Prevention How to prevent deadlock?
If any of the conditions for deadlock is denied, deadlock is impossible
Mutual exclusion: Allow multiple processes to access shared resources at the same time
Hold and wait: Guarantee that when a process requests a resource, it does not hold other resources
No preemption: Release resources instead of waiting for process completion
Circular wait: Require that processes request resources in increasing order: if process holds resource of type N, can only request types > N
Price to be paid: reduced performance (overhead), allow data corruption, etc.
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14. Deadlock Avoidance Banker’s algorithm (Dijkstra):
Set of resources, set of customers, banker
Each customer tells banker maximum number of resources it needs
Customer borrows resources from banker
Customer returns resources to banker
Customer eventually pays back loan
Banker only lends resources if the system will be in a safe state after the loan
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15. Safe State
System is in a safe state if there exists a safe sequence of all processes
Sequence <P1, P2, …, Pn> is safe if for each Pi: the resources that Pi can still request can be satisfied with currently available resources and resources held by Pj with j<I
If system is in a safe state: NO DEADLOCK
If system is in an unsafe state: possibility of deadlock
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16. Banker’s Algorithm
Allocation: How many instances of each resource type a process is currently holding
Max: The maximum number of resource instances (per resource type) a process may request
Available: How many instances of each resource type are currently available
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17. Banker’s Algorithm (2)
Need: How many more instances of each resource type a process needs to complete
Need[i,j] = Max[i,j] – Allocation[i,j]
For process i and resource type j
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18. Banker’s Algorithm (3) ? ≤
Consider P1 requesting (1,0,2). Can the request be granted?
Need to make sure Request ≤ Available
Request
A B C ≤ ?
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19. Banker’s Algorithm (4)
Is the system in a safe state? If yes, give a safe sequence.
Can I run P0? P1? P2? P3? P4?
E.g.: Run P1:
etc. …
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20. Deadlock Detection Allow system to enter deadlock state
Detection Algorithm:
Periodically invoke an algorithm that searches for a cycle in the graph, but O(n^2)!
Detect unsafe sequence via Banker’s algorithm
How often to check?
Every time a request for allocation cannot be granted immediately
Every hour or whenever CPU utilization drops
etc.
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21. Deadlock Recovery How to recover from deadlock:
Abort all deadlocked processes
Abort one process at a time until deadlock cycle is eliminated
In which order should we choose to abort?
Select a victim: minimize cost
Priority of the process
Resources used by process
Resources needed by process to complete
How many processes will need to be terminated
Rollback: return to some safe state, restart process from that state
Starvation possible?
Yes, same process may always be picked as victim.
Fix: Include number of rollback in cost factor
11/17/2018

22. ? Memory Management First-fit: Best-fit: Worst-fit:
Allocate the first hole that is big enough
Best-fit:
Allocate the smallest hole that is big enough
Must search entire list, unless ordered in size
Worst-fit:
Allocate the largest hole
Must search entire list
First-fit and best-fit are better than worst-fit in terms of speed and storage utilization
P0 requests 200KB
Main Memory
500KB free ?
780KB free
250KB free
150KB free
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23. Pop Quiz #2
11/17/2018