1. Outline Announcement Distributed scheduling – continued
Quiz at the end of today’s class

2. COP 5611 - Operating Systems
Announcement
Schedule for the rest of the semester
4/10: Recovery
4/15: Fault tolerance
4/17: Class evaluation
Protection and security
4/22: Protection and security – continued
Quiz #3
4/24: Existing distributed systems and review
Final exam
5:30-7:30PM, April 29, 2003
Cumulative
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COP Operating Systems

3. COP 5611 - Operating Systems
Motivations
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COP Operating Systems

4. COP 5611 - Operating Systems
Motivations – cont.
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COP Operating Systems

5. Distributed Scheduling
A distributed scheduler is a resource management component of a distributed operating system that focuses on judiciously and transparently redistributing the load of the system among the computers to maximize the overall performance
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COP Operating Systems

6. Components of a Load Distributing Algorithm
Four components
Transfer policy
Determines when a node needs to send tasks to other nodes or can receive tasks from other nodes
Selection policy
Determines which task(s) to transfer
Location policy
Find suitable nodes for load sharing
Information policy
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COP Operating Systems

7. COP 5611 - Operating Systems
Stability
The queuing-theoretic perspective
The CPU queues grow without bound if arrival rate is greater than the rate at which the system can perform work
A load distributing algorithm is effective under a given set of conditions if it improves the performance relative to that of a system not using load distribution
Algorithmic stability
An algorithm is unstable if it can perform fruitless actions indefinitely with finite probability
Processor thrashing
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COP Operating Systems

8. Sender-Initiated Algorithms
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9. Receiver-Initiated Algorithms
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COP Operating Systems

10. COP 5611 - Operating Systems
Empirical Comparison of Sender-Initiated and Receiver-Initiated Algorithms
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COP Operating Systems

11. Symmetrically Initiated Algorithms
Sender-initiated component
A sender broadcasts a TooHigh message, sets a TooHigh timeout alarm, and listens for an Accept
A receiver that receives a TooHigh message cancels its TooLow timeout, sends an Accept message to the sender, and increases its load value
On receiving an Accept message, if the site is still a sender, choose the best task to transfer and transfer it
If no Accept has been received before the timeout, it broadcasts a ChangeAverage message to increase the average load estimates at the other nodes
February 23, 2019
COP Operating Systems

12. Symmetrically Initiated Algorithms – cont.
Receiver-initiated component
It broadcasts a TooLow message, set a TooLow timeout alarm, and starts listening for a TooHigh message
If TooHigh message is received, it cancels its TooLow timeout, sends an Accept message to the sender, and increases its load value
If no TooHigh message is received before the timeout, the receiver broadcasts a ChangeAverage message to decrease the average at other nodes
February 23, 2019
COP Operating Systems

13. COP 5611 - Operating Systems
Comparison
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14. COP 5611 - Operating Systems
Adaptive Algorithms
A stable symmetrically initiated algorithm
Each node keeps of a senders list, a receivers list, and an OK list
By classifying the nodes in the system as Sender/overloaded, Receiver/underloaded, or OK using the information gathered through polling
February 23, 2019
COP Operating Systems

15. A Stable Symmetrically Initiated Algorithm – cont.
Sender-initiated component
The sender polls the node at the head of the receiver
The polled node moves the sender to the head of its sender list and sends a message indicating it is a receiver, sender, or OK node
The sender updates the polled node based on the reply
If the polled node is a receiver, it transfers a task
The polling process stops if its receiver’s list becomes empty, or the number of polls reaches a PollLimit
February 23, 2019
COP Operating Systems

16. A Stable Symmetrically Initiated Algorithm – cont.
Receiver-initiated component
The nodes polled in the following order
Head to tail of its senders list
Tail to head in the OK list
Tail to head in the receivers list
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COP Operating Systems

17. A Stable Sender-Initiated Algorithm
This algorithm uses the sender-initiated algorithm of the stable symmetrically initiated algorithm
Each node is augmented by an array called the statevector
It keeps track of its status at all the other nodes in the system
It is updated based on the information at the polling stage
The receiver-initiated component is replaced by the following protocol
When a node becomes a receiver, it informs all the nodes that are misinformed
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COP Operating Systems

18. COP 5611 - Operating Systems
Comparison
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COP Operating Systems

19. Performance Under Heterogeneous Workloads
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20. Selecting a Suitable Load Sharing Algorithm
The best algorithm depends on the system under consideration
For example, if the system never attains high loads, sender-initiated algorithms will give an improved algorithm
Stable scheduling algorithms should be used for systems that can reach high loads
For systems with heterogeneous work loads, adaptive stable algorithms are preferable
February 23, 2019
COP Operating Systems

21. Other Requirements of Load Distributing
Scalability
The algorithm should work well in large distributed systems
Location transparency
Determinism
Preemption
Heterogeneity
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COP Operating Systems

22. COP 5611 - Operating Systems
Case Studies
The V-System
The Sprite system
Condor system
The Stealth distributed scheduler
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COP Operating Systems

23. Task Placement vs. Task Migration
Task placement refers to the transfer of a task that is yet to begin execution to a new location and starts its execution there
Task migration refers to the transfer of task that has already begun execution to a new location and continuing its execution there
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COP Operating Systems

24. COP 5611 - Operating Systems
Task Migration
State transfer
The task’s state includes the content of registers, the task stack, the task’s status, virtual memory address space, file descriptors, any temporary files and buffered messages
In addition, current working directory, signal masks and handlers, resource usage statistics, and references to children and parent processes
Unfreeze
The task is installed at the new machine and is put in the ready queue
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COP Operating Systems

25. Issues in Task Migration
State transfer
The cost to support remote execution, including delays due to freezing the tasks, obtaining and transferring the state, and unfreezing the task
Residual dependencies
Transferring pages in the virtual memory space
Redirection of messages
Location-dependent system calls
Residual dependencies are undesirable
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COP Operating Systems

26. State Transfer Mechanisms
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COP Operating Systems

27. Location Transparency
Location transparency is essential to support task migration
Task migration should hide the locations of tasks
Remote execution of tasks should not require any special provisions in programs
These require names be independent of their locations
Addresses are maintained as hints
An object can be accessed through pointers
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COP Operating Systems

28. Task Migration Performance
Cost of process migration in Sprite
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COP Operating Systems

29. Task Migration Performance – cont.
Cost of process migration in Charlotte
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COP Operating Systems

30. COP 5611 - Operating Systems
Summary
Load distributed algorithms try to improve the overall system performance by transferring load from heavily loaded nodes to lightly loaded or idle nodes
There are different load distributed algorithms developed
To be effective, these algorithms must be able to collect the necessary information efficiently and minimize the overhead of task transferring and delays to due to task transferring
February 23, 2019
COP Operating Systems