The third line of T1: while (flag == 0); is an example of "busy waiting". In bad cases, the "stuff independent of X" that T1 executes could finish much sooner than the code to produce X. If this happens, T1 will waste a bunch of processor cycles by checking the while loop over and over again, instead of allowing other threads to do something useful with said cycles, thus potentially affecting performance non-negligibly.
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kayvonf
@dyc: You probably were taught that busy waiting was a poor use of resources: since the thread that is busy waiting is tying up the processor, preventing threads that might set the flag variable to 1 from executing. But this assumes that all relevant threads are sharing the same processing resources.
One thing to consider is whether busy waiting is so bad on a multi-processor system. If you only have two threads in your program, and are running on a machine with two processors, it might be an okay idea -- and even preferable -- to busy-wait! There's no other thread that needs to run that the processor should be released for!
We'll discuss implications (good and bad) of busy-waiting on a parallel system in detail later in the course. It's useful to learn about cache coherence before before discussing the issue in detail.
The third line of T1:
while (flag == 0);is an example of "busy waiting". In bad cases, the "stuff independent of X" that T1 executes could finish much sooner than the code to produce X. If this happens, T1 will waste a bunch of processor cycles by checking the while loop over and over again, instead of allowing other threads to do something useful with said cycles, thus potentially affecting performance non-negligibly.This comment was marked helpful 0 times.
@dyc: You probably were taught that busy waiting was a poor use of resources: since the thread that is busy waiting is tying up the processor, preventing threads that might set the
flagvariable to 1 from executing. But this assumes that all relevant threads are sharing the same processing resources.One thing to consider is whether busy waiting is so bad on a multi-processor system. If you only have two threads in your program, and are running on a machine with two processors, it might be an okay idea -- and even preferable -- to busy-wait! There's no other thread that needs to run that the processor should be released for!
We'll discuss implications (good and bad) of busy-waiting on a parallel system in detail later in the course. It's useful to learn about cache coherence before before discussing the issue in detail.
This comment was marked helpful 0 times.