Outline

1. Outline • Midterm results summary • Distributed file systems – continued • Distributed shared memory

2. Midterm Results Summary • The distribution • The average is 83.20 COP 5611 - Operating Systems

3. Distributed Shared Memory • Distributed computing is mainly based on the message passing model • Client/server model • Remote procedure calls • Distributed shared memory • is a resource management component that implements the shared memory model in distributed systems, where there is no physically shared memory COP 5611 - Operating Systems

4. Memory Hierarchies COP 5611 - Operating Systems

5. Virtual memory COP 5611 - Operating Systems

6. Page Table Mapping COP 5611 - Operating Systems

7. Distributed Shared Memory – cont. • This is a further extension of the virtual memory management on a single computer • When a process accesses data in the shared address space, a mapping manager maps the shared memory address to the physical memory, which can be local or remote COP 5611 - Operating Systems

8. Distributed Shared Memory – cont. COP 5611 - Operating Systems

9. Distributed Shared Memory – cont. A mapping manager is a layer of software to map addresses in user space into the physical memory addresses COP 5611 - Operating Systems

10. Distributed Shared Memory – cont. • With DSM, application programs can access data in the shared space as they access data in traditional virtual memory • Mapping manager can move data among the local main memory, local disk, and another nodes COP 5611 - Operating Systems

11. Distributed Shared Memory – cont. • Advantages of DSM • It is easier to design and develop algorithms with DSM than message passing models • DSM allows complex structures to be passed by reference, simplifying the distributed application development • DSM can cut down the overhead of communication by exploiting locality in programs • DSM can overcome some the architectural limitations of shared memory machines COP 5611 - Operating Systems

12. Distributed Shared Memory – cont. • Central implementation issues in DSM • How to keep track of the location of remote data • How to overcome the communication delays and high overhead associated with communication protocols • How to improve the system performance COP 5611 - Operating Systems

13. The Central-Server Algorithm • A central server maintains all the shared data • It serves the read requests from other nodes or clients by returning the data items to them • It updates the data on write requests by clients COP 5611 - Operating Systems

14. The Central-Server Algorithm– cont. COP 5611 - Operating Systems

15. The Migration Algorithm • In contrast to the central-server algorithm, data in the migration algorithm is shipped to the location of data access request • Subsequent accesses can then be performed locally • Thrashing can be a problem COP 5611 - Operating Systems

16. The Migration Algorithm – cont. COP 5611 - Operating Systems

17. The Read-Replication Algorithm • The read-replication algorithm allows multiple node to have read access or one node to have read-write access COP 5611 - Operating Systems

18. The Read-Replication Algorithm– cont. COP 5611 - Operating Systems

19. The Full-Replication Algorithm • This is a further extension of the read-replication algorithm • It allows multiple nodes to have both read and write access to shared data blocks • Data consistency issue COP 5611 - Operating Systems

20. The Full-Replication Algorithm– cont. COP 5611 - Operating Systems

21. Memory Coherence • Memory consistency models • Sequential consistency • General consistency • Processor consistency • Weak consistency • Release consistency COP 5611 - Operating Systems

22. Memory Coherence – cont. • Coherence protocols • A protocol to keep replicas coherent • Write-invalidate protocol • A write to a shared data causes the invalidation of all copies except the one where the write occurs • Write-update protocol • A write to a shared data causes all copies of that data to be updated COP 5611 - Operating Systems

23. Design Issues • Granularity • The size of the shared memory unit • Advantages of large sizes • A page size that is a multiple of the size provided by the underlying memory management system allows for the integration of DSM and the memory management system • Better utilization of locality of reference • Disadvantages • The greater the chance for contention • False sharing COP 5611 - Operating Systems

24. False Sharing COP 5611 - Operating Systems

25. Design Issues – cont. • Page replacement • Traditional methods such as LRU can not be used directly • Page access modes must be taken into consideration • For example, private pages may be replaced before shared pages COP 5611 - Operating Systems

26. Case Studies • IVY • Integrated Shared Virtual Memory at Yale • The granularity is a page • The address space is divided into shared virtual memory address space and private space • The coherence protocol • Write fault handling • Read fault handing COP 5611 - Operating Systems

27. IVY – cont. • Coherence protocol • The centralized manager scheme • Fixed distributed manager scheme • Dynamic distributed manager scheme COP 5611 - Operating Systems

28. IVY – cont. COP 5611 - Operating Systems

29. IVY – cont. • Double fault • Memory allocation • Process synchronization COP 5611 - Operating Systems

30. Case Studies – cont. • Mirage • Thrashing control • Clouds • The RA kernel • Distributed shared memory controller COP 5611 - Operating Systems

31. Summary • Distributed shared memory tries to provide an easy-to-use interface for distributed applications • Distributed programs access data in the shared address space as they access data in local memory • However, the performance is a critical issue • Replication is used to reduce the high cost of communications • Memory coherence needs to be taken of COP 5611 - Operating Systems