fork/join 中处于等待中的任务实际上不阻止线程。
一旦所有25个线程都被阻塞的时候，等待中的任务就无法分配到线程了。
Once all 25 threads were blocked, the tasks they are waiting on cannot be attached to a thread.
红色的任务是等待中的，意味着它们还没有被执行。
Tasks in red are pending, meaning they haven't been executed yet.
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