线程切换代价测试程序 Latency of thread context-switch
线程切换可以在单个进程内部发生，也可以跨进程发生。
Thread switches can occur either inside of a single process or across processes.
争用同步之所以慢，是因为它涉及多个线程切换和系统调用。
Contended synchronizations are slow because they involve multiple thread switches and system calls.
当一个线程向另一个线程请求信息时，就可能引起线程切换。
One thread asking another for information can cause a thread switch, or one thread preempting another can cause a thread switch.
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