Q21.Marks: +2.0UGC NET Paper 2: Computer Science 26th Nov 2021
The cache coherence problem can be solved
A. by having multiport memory
B. allow only nonshared data to be stored in cache
C. using a snoopy cache controller
D. uisng memory interleaving
Choose the correct answer from the options given below :
1.A and C only
2.B and C only✓ Correct
3.D and C only
4.B and D only
Solution
Explanation:
B. Allow only nonshared data to be stored in cache
Explanation: One way to prevent cache coherence issues is to only allow nonshared data to be cached. In multiprocessor systems, coherence problems arise when multiple processors modify the same data stored in their local caches. By only caching nonshared (private) data, you eliminate the possibility of multiple processors modifying the same cached data, thus avoiding coherence problems entirely. While this method limits caching for shared data, it is an effective solution for avoiding cache coherence issues.
C. Using a snoopy cache controller
Explanation: A snoopy cache controller is one of the most common and efficient ways to handle the cache coherence problem in shared memory multiprocessor systems. In this approach, caches "snoop" on the bus, monitoring memory transactions initiated by other processors. If a cache sees that another processor is trying to read or write to an address it has cached, it takes appropriate action (like updating or invalidating its cache entry) to ensure coherence. This approach is widely used in practice, especially with MESI (Modified, Exclusive, Shared, Invalid) cache coherence protocols.
Why A and D are not correct:
A. Multiport memory: While multiport memory allows multiple processors to access memory simultaneously, it doesn't directly solve the cache coherence problem. It helps with memory access but doesn't coordinate or ensure that cached copies across processors remain consistent.
D. Memory interleaving: Memory interleaving improves memory access speed by splitting memory into sections that can be accessed simultaneously. However, it does not address cache coherence, as it is a technique for optimizing memory access rather than ensuring data consistency between caches.
Conclusion:
B (allowing only nonshared data to be cached) avoids the coherence problem by restricting the types of data that can be cached.
C (snoopy cache controller) actively monitors and ensures that shared data in caches remains consistent, solving the coherence problem efficiently.
Therefore, B and C are correct solutions, and the correct answer is: 2) B and C only.