重要的是如何设置的最大的fd上选择？ [英] how important is setting max fd on select?

问题描述

在一个无限循环中，我使用选择听100+文件描述符。如果FD具有一定的包准备好被读取，我通知分配给该文件描述符的包处理器线程，我没有为下一轮该文件描述符的比特，直到我收到来自数据处理器线程说是做的通知。我不知道我的code将如何低效的，如果我将不计算最大。 FD的选择，每次我清楚/套起订的文件描述符。我期待文件描述符是几乎连续的，数据到达率是几千字节每秒每个FD。

Within an infinite loop, I am listening 100+ file descriptors using select. If fd has some packets ready to be read, I notify the packet processor thread assigned to this file descriptor and I don't set the bit for this file descriptor for the next round until I receive a notification from data processor thread saying it is done. I wonder how inefficient my code would be if I won't calculate the max. fd for select everytime I clear/set a file descriptor from the set. I am expecting file descriptors to be nearly contiguous, data arrival rate to be a few thousands bytes every second for each fd.

推荐答案

您真的应该使用调查而不是选择。两者都是标准的，但调查更容易使用，不把一个上限，您可以检查文件描述符的数量（而选择限制你的编译时间常数 FD_SETSIZE ），更高效。如果你使用选择，你可以随时通过 FD_SETSIZE 的第一个参数，但是这当然会给予最差情况下的性能，因为内核必须扫描整个 FD_SET ;通过实际的MAX + 1允许较短的搜索，但仍然没有有效的数组传递给调查。

You should really use poll instead of select. Both are standard, but poll is easier to use, does not place a limit on the number of file descriptors you can check (whereas select limits you to the compile-time constant FD_SETSIZE), and more efficient. If you do use select, you can always pass FD_SETSIZE for the first argument, but this will of course give worst-case performance since the kernel has to scan the whole fd_set; passing the actual max+1 allows a shorter search, but still not as efficient as the array passed to poll.

有关它的价值，这几天好像时尚使用非标准的Linux 的epoll 或任何BSD当量。这些接口可具有一定的优势，如果你有一个庞大的数字长寿命（至少几个往返）连接的（数万的量级），但在其他性能不会noticably更好（并且，在下端，可能会更糟），而且这些接口当然是不可移植的，在我看来，更难比平原正确使用，便携调查。

For what it's worth, these days it seems stylish to use the nonstandard Linux epoll or whatever the BSD equivalent is. These interfaces may have some advantages if you have a huge number (on the order of tens of thousands) of long-lived (at least several round trips) connections, but otherwise performance will not be noticably better (and, at the lower end, may be worse), and these interfaces are of course non-portable, and in my opinion, harder to use correctly than the plain, portable poll.

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