Directory

Definition

In computer architecture, a directory is a data structure that tracks the status of cache lines in a multiprocessor system to maintain cache coherence. It helps ensure that multiple processors accessing shared data see consistent values by keeping track of which caches have copies of each memory block and their states. This coordination minimizes the chances of stale or inconsistent data across different caches, crucial for performance in multiprocessor environments.

5 Must Know Facts For Your Next Test

1. A directory keeps track of the ownership and state of cache lines in each processor's cache, which is essential for maintaining coherence in systems with multiple caches.
2. Directories can either be centralized, where one structure manages all cache coherence information, or distributed, where each cache maintains its own portion of the directory.
3. When a processor wants to read or write to a shared memory location, it first checks the directory to determine which caches contain a copy of the data and its state.
4. Using a directory-based approach can reduce the amount of traffic on the system bus compared to snooping protocols, especially in systems with many processors.
5. Directory-based protocols typically support finer granularity than bus-based methods, allowing for more efficient use of bandwidth and reduced latency in accessing shared data.

Review Questions

• How does a directory help maintain cache coherence in a multiprocessor system?
  • A directory maintains cache coherence by tracking which caches hold copies of each memory block and their respective states. When a processor needs to access shared data, it consults the directory to see if other caches have valid copies. This process ensures that any updates to shared data are consistently reflected across all caches, preventing issues like stale reads and writes.
• Compare centralized and distributed directory architectures regarding their advantages and disadvantages.
  • Centralized directories offer simplicity and easier management since all coherence information is stored in one location. However, they can become bottlenecks as the number of processors increases. Distributed directories spread the information across multiple locations, reducing bottlenecks and potentially improving scalability but introducing complexity in maintaining consistency across directories.
• Evaluate the impact of directory-based cache coherence on system performance compared to traditional snooping protocols.
  • Directory-based cache coherence can significantly improve system performance over traditional snooping protocols by reducing bus traffic and contention among processors. Since the directory directly manages which caches have access to specific memory blocks, it minimizes unnecessary invalidation messages and allows for more efficient data retrieval. This method is particularly beneficial in large-scale multiprocessor systems where many caches are involved, leading to lower latency and higher throughput for memory operations.

Related terms

Cache Coherence: The mechanism that ensures multiple copies of data in different caches reflect the most recent write, preventing inconsistencies.

Snooping Protocols:

Protocols used to maintain cache coherence by monitoring communication on a shared bus to update or invalidate cached data.

MESI Protocol: A widely used cache coherence protocol that stands for Modified, Exclusive, Shared, and Invalid, representing the possible states of a cache line.