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

2. Midterm Results Summary
The distribution
The average is 83.20
 70
 75 4
 80 6
 85 1
 90 3
 95
 100
> 100 2
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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
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4. COP 5611 - Operating Systems
Memory Hierarchies
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5. COP 5611 - Operating Systems
Virtual memory
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6. COP 5611 - Operating Systems
Page Table Mapping
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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
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8. Distributed Shared Memory – cont.
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9. Distributed Shared Memory – cont.
A mapping manager is a layer of software to map addresses in user space into the physical memory addresses
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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
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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
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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
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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
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14. The Central-Server Algorithm – cont.
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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
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16. The Migration Algorithm – cont.
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17. The Read-Replication Algorithm
The read-replication algorithm allows multiple node to have read access or one node to have read-write access
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18. The Read-Replication Algorithm – cont.
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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
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20. The Full-Replication Algorithm – cont.
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21. COP 5611 - Operating Systems
Memory Coherence
Memory consistency models
Sequential consistency
General consistency
Processor consistency
Weak consistency
Release consistency
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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
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23. COP 5611 - Operating Systems
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
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24. COP 5611 - Operating Systems
False Sharing
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25. COP 5611 - Operating Systems
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
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26. COP 5611 - Operating Systems
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
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27. COP 5611 - Operating Systems
IVY – cont.
Coherence protocol
The centralized manager scheme
Fixed distributed manager scheme
Dynamic distributed manager scheme
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28. COP 5611 - Operating Systems
IVY – cont.
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29. COP 5611 - Operating Systems
IVY – cont.
Double fault
Memory allocation
Process synchronization
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30. COP 5611 - Operating Systems
Case Studies – cont.
Mirage
Thrashing control
Clouds
The RA kernel
Distributed shared memory controller
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31. COP 5611 - Operating Systems
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
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