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epoll 更高效的原因

1. fd 数据结构
   • select
     • fd bit marks: fd count limit - 1024
   • poll
     • pollfd[]: custom-length array
2. which fds are ready?
   • select, poll dont know
   • epoll know
3. fd copy
   • select, poll copy between user and kernel
   • epoll use shared memory between user and kernel
4. trigger
   • select, poll: level trigger
   • epoll: edge trigger

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