When scheduling tasks on a processor core, why does simply summing task durations incorrectly estimate total runtime if some tasks access shared memory resources?

💡 Explanation

Summing task durations overcounts the true runtime because the mutual exclusion mechanism of critical sections enforces serialization on shared resources, therefore actual runtime includes waiting, rather than only execution, unlike the isolated case. Cache misses or branch prediction issues are performance factors but do not directly invalidate simple summation.

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