What is the difference between write-through and write-back cache?

1 Write-through cache

Write-through cache is a method where every write operation to the cache is also performed on the main memory. This means that the cache and the main memory are always consistent and have the same data. The benefit of this approach is that it avoids data loss and corruption in case of power failure or system crash, since the main memory is always up to date. The drawback is that it increases the write latency and the bus traffic, since every write to the cache requires a write to the main memory as well.

2 Write-back cache

Write-back cache is a method where write operations to the cache are not immediately reflected on the main memory. Instead, the cache marks the modified data as dirty and updates the main memory only when the cache line is replaced or evicted. This means that the cache and the main memory may have different data at some point. The benefit of this approach is that it reduces the write latency and the bus traffic, since writes to the cache are faster and less frequent than writes to the main memory. The drawback is that it increases the risk of data loss and corruption in case of power failure or system crash, since the main memory may not have the latest data.

3 Cache coherence

Cache coherence is a property that ensures that multiple caches that share the same main memory have a consistent view of the data. This is important when there are multiple processors or cores that have their own caches and access the same memory locations. If one processor modifies a data item in its cache, the other processors need to be aware of that change and invalidate or update their own caches accordingly. Otherwise, they may operate on stale or inconsistent data, leading to errors or bugs.

6 How to choose between write-through and write-back cache

There is no definitive answer to which cache method is better, as it depends on the trade-off between performance and reliability. Write-through cache may be preferable for applications that require high data integrity and low write frequency, such as databases or file systems. Write-back cache may be preferable for applications that require high write performance and low data sensitivity, such as video games or image processing. The choice of cache method may also depend on the availability and cost of other components, such as battery backup, error correction, or cache coherence hardware.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?