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Questions and Answers
What is the main issue that cache coherence protocols aim to address?
What is the main issue that cache coherence protocols aim to address?
- Maintaining a uniform state for each cached block of data (correct)
- Preventing multiple processors from operating asynchronously
- Ensuring that main memory always has the most up-to-date data
- Eliminating the need for caches in a multiprocessing environment
Why does the cache inconsistency problem occur in a multiprocessing environment?
Why does the cache inconsistency problem occur in a multiprocessing environment?
- Because main memory and cache cannot communicate effectively
- As a result of insufficient hardware support for synchronization
- To encourage private caches to store more data
- Due to the asynchronous and independent operation of multiple processors (correct)
What is one of the identified sources of the cache inconsistency problem?
What is one of the identified sources of the cache inconsistency problem?
- Hardware not supporting synchronization
- Storing of write data (correct)
- Excessive process migration
- Main memory being faster than cache
How do cache coherence schemes help in preventing the cache coherence problem?
How do cache coherence schemes help in preventing the cache coherence problem?
What role does software-implemented synchronization play in addressing the cache coherence problem?
What role does software-implemented synchronization play in addressing the cache coherence problem?
Why does the cache inconsistency problem only occur when private caches are used?
Why does the cache inconsistency problem only occur when private caches are used?
What is a possible consequence of using a write-back cache policy when updating shared data in a multiprocessor system?
What is a possible consequence of using a write-back cache policy when updating shared data in a multiprocessor system?
In a multiprocessor system with private caches, what may happen if a data element X is updated in one processor's cache but not immediately reflected in the shared memory?
In a multiprocessor system with private caches, what may happen if a data element X is updated in one processor's cache but not immediately reflected in the shared memory?
Which scenario may lead to inconsistency between cache copies labeled X and X' when a process migrates between processors in a multiprocessor system?
Which scenario may lead to inconsistency between cache copies labeled X and X' when a process migrates between processors in a multiprocessor system?
What is one possible drawback of attaching H0 processors to private caches in order to address the U0 inconsistency problem?
What is one possible drawback of attaching H0 processors to private caches in order to address the U0 inconsistency problem?
What issue may arise when outputting data directly from shared memory, bypassing the caches in a multiprocessor system?
What issue may arise when outputting data directly from shared memory, bypassing the caches in a multiprocessor system?
Why must special precautions be taken to avoid inconsistencies when a process migrates between processors in a multiprocessor system?
Why must special precautions be taken to avoid inconsistencies when a process migrates between processors in a multiprocessor system?
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Study Notes
Cache Coherence Protocols
- Cache coherence protocols are used to cope with the multicache inconsistency problem in multiprocessor systems.
- There are two types of cache coherence protocols: snoopy protocols for bus-connected systems and directory-based protocols for network-connected systems.
The Cache Coherence Problem
- The cache coherence problem occurs when multiple processors maintain locally cached copies of a unique shared-memory location, resulting in a globally inconsistent view of memory.
- The problem arises due to data sharing, process migration, or I/O operations.
- Three sources of cache inconsistencies are:
- Storing of write data
- Process migration
- I/O operations
Inconsistency due to Data Sharing
- Cache inconsistency occurs when multiple private caches are used and share a common memory location.
- Consider a multiprocessor system with two processors, each using a private cache and sharing the main memory.
- If one processor updates a shared data element, the caches may become inconsistent.
Process Migration and I/O
- Process migration can cause inconsistency between cache copies when a process containing a shared variable migrates from one processor to another.
- I/O operations can also cause inconsistency when they bypass the caches and update the main memory directly.
- Attaching I/O processors to private caches can help maintain consistency, but may result in higher cache perturbations and poor locality of I/O data.
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