The nice idea of this implementation is that no thread can set the b->flag twice until other threads leaving the previous round. It's not based on the b->counter, so prevent one thread from modifying the b->flag twice.
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tcz
This works by effectively assigning each barrier location in the code a different target flag value, alternating between 0 and 1. Then, the flag variable will not match the target value until everybody has left the old barrier location. When all locations have the same target value, as in slide 23, then there is no distinguishing which barrier a processor is waiting at.
A more obvious and equivalent solution is to simply assign each location an incrementally increasing target value, such as barrier 0, barrier 1, barrier 2, etc. Then, when the last thread reaches barrier location x, it updates the flag to x. However, the solution in the slide is, I think, more elegant.
The nice idea of this implementation is that no thread can set the
b->flagtwice until other threads leaving the previous round. It's not based on theb->counter, so prevent one thread from modifying theb->flagtwice.This comment was marked helpful 0 times.
This works by effectively assigning each barrier location in the code a different target flag value, alternating between 0 and 1. Then, the flag variable will not match the target value until everybody has left the old barrier location. When all locations have the same target value, as in slide 23, then there is no distinguishing which barrier a processor is waiting at.
A more obvious and equivalent solution is to simply assign each location an incrementally increasing target value, such as barrier 0, barrier 1, barrier 2, etc. Then, when the last thread reaches barrier location x, it updates the flag to x. However, the solution in the slide is, I think, more elegant.
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This is so much better than the leaving counter!
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