Update based cache coherence looks a lot better here because updating prevents cache misses by ensuring that the most recent version of the data is in the cache. However, it can cause a lot of unnecessary traffic when a cache line is read from once and never touched by a processor again. If another processor keeps writing to the same line, the untouched line will be repeatedly updated even though it's never needed again.
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In other words, the "update based" strategy suffers from the fact that cache will always try to keep itself full. Users have no explicit control to evict cache. That's why an "update" operation might need to be constantly communicated across processors, and become expensive.
Update based cache coherence looks a lot better here because updating prevents cache misses by ensuring that the most recent version of the data is in the cache. However, it can cause a lot of unnecessary traffic when a cache line is read from once and never touched by a processor again. If another processor keeps writing to the same line, the untouched line will be repeatedly updated even though it's never needed again.
In other words, the "update based" strategy suffers from the fact that cache will always try to keep itself full. Users have no explicit control to evict cache. That's why an "update" operation might need to be constantly communicated across processors, and become expensive.