Cache Coherence

Definition of Cache Coherence

Cache coherence refers to a consistency mechanism in multiprocessor systems, ensuring that all processors access the same up-to-date copy of shared data stored in their local caches. It helps to maintain data consistency across multiple cache memories by utilizing specific protocols. This concept prevents unexpected program behavior caused by inconsistencies between cached copies of data and guarantees data accuracy and integrity while improving system performance.


The phonetics of the keyword “Cache Coherence” is:/ˈkæʃ kəˈhɪrəns/

Key Takeaways

  1. Cache coherence ensures that multiple processors in a shared-memory multiprocessor system are able to maintain a single, globally consistent view of memory, by keeping all cached copies of shared data in sync.
  2. Cache coherence protocols, such as MESI and MOESI, are used to manage the state of cached data and control the actions that can be performed on data in the cache to maintain consistency across the system.
  3. Implementing cache coherence improves the performance and predictability of multiprocessor systems but adds complexity to the cache subsystem due to the need for communication and coordination between caches.

Importance of Cache Coherence

Cache coherence is an essential concept in the world of computer systems, specifically in multi-processor systems, as it ensures consistency and accuracy of accessed and shared data among processors.

It is important because, without cache coherence, different processors may access outdated or invalid copies of data in their respective private caches, leading to unexpected and undesirable behavior.

Maintaining cache coherence is crucial for the correct execution of parallel programs, and it directly impacts the performance and reliability of multi-processor systems.

Various cache coherence protocols are designed to ensure that all the caches have a consistent and updated view of the underlying memory, enabling smooth and efficient inter-processor communication and preventing potential loss or corruption of data during parallel processing operations.


Cache coherence is a crucial mechanism in multiprocessor systems to ensure that all processors in a system maintain a consistent view of the shared memory. It plays a vital role in maintaining the integrity, consistency, and performance of data stored in caches.

As multiple processors attempt to read and write data simultaneously, cache coherence manages the synchronization among various cache memory levels to prevent discrepancies or outdated data from being accessed. This mechanism prevents errors and ensures that the system behaves predictably and accurately as it executes programs and processes data.

Cache coherence protocols facilitate the communication and coordination among various caches to ensure that they provide the most recent data to the processors. Examples of such protocols are the MESI (Modified, Exclusive, Shared, Invalid) and the MOESI (Modified, Owner, Exclusive, Shared, Invalid) protocols.

By addressing systems’ needs through these protocols, cache coherence optimizes the overall system performance and prevents unexpected behaviors or errors caused by outdated or inaccessible memory data. As a result, cache coherence plays an essential role in maintaining the accuracy and efficiency of multiprocessor systems while enabling effective utilization of cache memory in parallel computing environments.

Examples of Cache Coherence

Cache coherence is a crucial aspect of computer architecture that ensures the consistency of data in multiprocessor systems. It ensures that any read operation on a memory location returns the most up-to-date value. Here are three real-world examples that involve cache coherence:

Multicore Processors: In computer systems with multicore processors, each core has a private cache to store frequently accessed data. Cache coherence plays an essential role in keeping all caches consistent across cores when one core modifies a shared memory location. This way, all processors are always working with the most consistent data, significantly improving the system’s performance.

Database Systems: In distributed database systems, a cache mechanism is employed to minimize the latency of accessing frequently used data. Cache coherence becomes crucial in such scenarios when multiple users are interacting with the same data simultaneously. To maintain consistency, cache coherence protocols ensure that any changes to the data are reflected across all cache copies, avoiding conflicting or out-of-date information.

Cloud Computing: In cloud computing environments, virtual machines and applications share resources across multiple physical servers and systems. Cache coherence becomes important in situations where there is shared data being accessed or modified by multiple entities within the cloud infrastructure. By implementing cache coherence mechanisms, cloud providers can ensure that all virtual machines and applications access consistent data, maintaining data integrity and enhancing the overall performance of the cloud infrastructure.

Cache Coherence FAQ

What is Cache Coherence?

Cache coherence refers to the consistency of shared data in a multiprocessor system, where each processor has its own cache. It ensures that a read operation by a processor returns the most recent write value made by any processor in the system. The coherence is maintained by managing the propagation and visibility of writes across the caches in the system.

Why is Cache Coherence important?

Cache coherence is important to ensure that all processors in a multiprocessor system operate on the most recent data and avoid working with inconsistent data. It plays a crucial role in maintaining the correctness and performance of parallel applications by allowing consistent data sharing between processors and avoiding unnecessary synchronization overhead.

What are the basic requirements for Cache Coherence?

There are three basic requirements for cache coherence:

  1. Write Propagation: Any write operation to a shared location by a processor must become visible to all processors eventually.
  2. Transaction Serialization: A read operation by a processor to a shared location, following a write by the same processor, must return the value of the most recent write.
  3. Coherence Order: If two writes to the same location are ordered by their processors, their order must be observed by all other processors in the system.

What are the common Cache Coherence Protocols?

There are several cache coherence protocols, but the two most common ones are the MESI Protocol and the MOESI Protocol:

  1. MESI (Modified, Exclusive, Shared, and Invalid) Protocol: This protocol uses four cache states to manage the coherence of cached data. It reduces the bus traffic by only broadcasting the modified data when necessary.
  2. MOESI (Modified, Owner, Exclusive, Shared, and Invalid) Protocol: This protocol is an extension of MESI and includes an additional “Owner” state. This state helps in optimizing data sharing by allowing multiple shared copies while a single cache block takes responsibility for handling write requests.

What factors affect Cache Coherence performance?

Some factors that may affect cache coherence performance include:

  1. Protocol implementation: The choice of coherence protocol and its implementation can affect the performance, traffic, and latency in the system.
  2. Cache hierarchy: The deeper the cache hierarchy, the longer it takes to maintain coherence between caches, resulting in increased latency.
  3. Sharing patterns: The frequency of shared data and the rate of updates can impact the performance of maintaining cache coherence.
  4. System size: Larger systems with more processors and caches can increase complexity and have a higher risk of performance degradation due to increased communication overhead.

Related Technology Terms

  • Cache Coherence Protocol
  • Write Invalidate Policy
  • Write Update Policy
  • MESI Protocol (Modified, Exclusive, Shared, Invalid)
  • MOESI Protocol (Modified, Owner, Exclusive, Shared, Invalid)

Sources for More Information


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