CS 551-Memory Management 1

Learning Objectives

• Centralized Memory Management -review

• Simple Memory Model

• Shared Memory Model

• Distributed Shared Memory

• Memory Migration

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Review of Centralized Memory Management

• Virtual memory– Extending the size of available memory beyond

its physical size of RAM– Paging versus segmentation– Internal versus external fragmentation– Segment placement algorithms– Page replacement algorithms

• page faults and thrashing

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Simple Memory Model

• In Simple Memory Model access times for all processors are equal.

• Requires strict control of degree of multi-programming.

• Often does not use virtual memory or caching because of overhead.

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Shared Memory Model

• Used for inter-process communication.

• Multiple processes share memory locations.

• May includes physical RAMs, local cache, and secondary storage.

• Memory access takes place via common bus thus a possibility of a bus contention.

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Bus Contention

• Bus contention occurs when the demand for bus access is excessive.

• Bus contention may cause a bottleneck in a shared memory system.

• Shared memory model usually does implement more than 32 processors on a single bus.

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Shared Memory Support

• UNIX System V supports shared memory via the following function calls:– shmget– shmat– shmdt

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Shared Memory Performance• Performance is an important issue in a

Shared Memory System.

• Important issues:– scalability - ability to accommodate growth

without sacrificing performance– real time needs

• overlap of communication and computation

• prefetching of data (? Answer problem 4.5., p.102)

– non-local memory references are expensive (up to 10:1 ratio)

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Cache Consistency

• Consistency is a term used to describe the function ensuring that all data copies are the same and correct.

• Consistency protocols:– critical section approach– prevention of shared memory caching– snoopy cache (see fig 4.3. p. 89)

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Distributed Shared Memory• Concept introduced in 1989.

• DSM presents a logical shared memory for multi-computer systems.

• DSM maintains communication and data consistency for applications.

• Usually portions of local memory are mapped onto DSM.

• DSM maintains a directory service for all data residing in the system.

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Distributed Shared Memory Management

• DSM Management involves two main decisions:

– How to distribute shared data ?

– How many readers and writers should be allowed for a shared data segment?

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Distributing Shared Data

• Replication– maintaining multiple copies of shared data at

various locations

• Migration– moving the copy of shared data to various

locations (only one copy of shared data is allowed)

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Readers/Writers Protocols for Shared Data

• Single Reader/Single Writer

• Multiple Reader/Single Writer

• Multiple Reader/Multiple Writer

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Single Reader/Single Writer

• Allows one reader or one writer for a shared data set (inhibits concurrency).

• Replication is NOT allowed.

• Both centralized and distributed algorithms are used to control DSM.

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Multiple Reader/Single Writer

• Allows multiple readers but only one writer to a shared data set.

• Consequently it allows for multiple copies of a shared data set.

• If the writer changes the shared data, the readers’ copies are no longer accurate. This requires an invalidation protocol.

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Invalidation protocols for multiple reader/single writer

• Copy Set - is a list of system participants that have a copy of each piece of data.

• Invalidation protocols:– centralized– partially distributed– distributed

• ? Study figures 4.4, 4.5 and 4.6 p.93 and explain them.

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Static and Dynamic Multiple Reader/Single Writer Protocol

• Study Box 4.3 and Box 4.4 page 94.

• ? Compare and contrast the algorithms and state the most important commonalities and differences?

• Homework for 3/1/00

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Centralized Multiple Reader/Single Writer Protocol

• ? Answer question 4.6. p.103?

• ? Write an algorithm for centralized MR/SW problem based on Fig.4.4. and trace your algorithm for request sequence from problem 4.7. p.103?

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Multiple Reader/Multiple Writer

• Allows for multiple copies of the same data and for concurrent writes to multiple copies.

• Not only the data needs to be validated but also it needs coordination for multiple updates.

• Automated solutions are possible for some applications only, and they require a sequencer, i.e. an algorithm for ordering all read and write requests.

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DSM Performance• Issues to be considered:

– Thrashing• How to avoid thrashing, i.e. the situation where data

constantly travels between various locations?

– Selection of a block size• What is the best size for sharable data block?• (page size, packet size, segment size?)

– Location of data• Which is the best original site for shared data location?

– Implementation location for DSM• Where should be DSM implemented? (hardware or software

or both)

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Memory Migration

• Memory Migration requires two fundamental decisions:– When in the migration process will we migrate

memory?– How much memory needs to be migrated?

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Memory Migration Protocols

• Stop-and-copy.

• Concurrent-copy.

• Copy-on-reference– two approaches: data stays at origin, data is

copied to file server.

• ? Study fig. 4.8, 4.9 and 4.10 p.99 and answer problem 4.8 p. 103.