C.4.1 Invalidate Queues
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C.4.1 Invalidate Queues

One reason that invalidate acknowledge messages can take so long is that they must ensure that the corresponding cache line is actually invalidated, and this invalidation can be delayed if the cache is busy, for example, if the CPU is intensively loading and storing data, all of which resides in the cache. In addition, if a large number of invalidate messages arrive in a short time period, a given CPU might fall behind in processing them, thus possibly stalling all the other CPUs.

However, the CPU need not actually invalidate the cache line before sending the acknowledgement. It could instead queue the invalidate message with the understanding that the message will be processed before the CPU sends any further messages regarding that cache line.