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This example has much less bus traffic than the one on slide 11. Consider the time period when Processor 1 has the lock. As long as Processor 1 has the lock, nothing is being written to lock, so the values in Processor 2 and 3's caches are valid. Since Processor 2 and 3 are spinning in the while(lock != 0) loop, they are only sending read requests (not RdX). Since they only want to read the value and they have a valid value in their cache, they do not need to acquire the bus, reducing the bus traffic in this example.


Here test & test & set has less bus traffic because it repeatedly test if another processor has the lock before doing test & set. Therefore we don't need to use BusRdX since we don't modify the value of the lock. Therefore there might be less bus traffic.