如何设置线程池的大小？

For compute intensive tasks, an N_cpu-processor system usually achieves optimum utilization with a thread pool of N_cpu+1 threads. (Even compute intensive threads occasionally take a page fault or pause for some other reason, so an "extra" runnable thread prevents CPU cycles from going unused when this happens.) For tasks that also include I/O or other blocking operations, you want a larger pool, since not all of the threads will be schedulable at all times. In order to size the pool properly, you must estimate the ratio of waiting time to compute time for your tasks; this estimate need not be precise and can be obtained through profiling or instrumentation. Alternatively, the size of the thread pool can be tuned by running the application using several different pool sizes under a benchmark load and observing the level of CPU utilization.

If tasks spend 50 percent of the time being blocked, then the number of threads should be twice the number of available cores. If they spend less time being blocked—that is, they’re computation intensive—then we should have fewer threads but no less than the number of cores. If they spend more time being blocked—that is, they’re IO intensive—then we should have more threads, specifically, several multiples of the number of cores.