Fix busy polling in startup phase

[BenjaminBeichler]

During the startup phase, the socket file descriptors (socket-fds) of the streams are added to the select fd-sets. However, these file descriptors are not read or written, causing the select operation to be non-blocking and leading to busy polling.

To address this issue, a separate fd-set is introduced specifically for the startup phase, which includes only the control socket.

In most scenarios, this results in a short period of increased CPU usage during startup. However, in more uncommon situations, such as simulations that utilize target code, this creates unnecessary load on the system.

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• Version of iperf3 (or development branch, such as master or 3.1-STABLE) to which this pull request applies:

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[BenjaminBeichler]

@bmah888 Are there any problems with that PR?

[bmah888]

@bmah888 Are there any problems with that PR?

Thanks for the PR! I've only given it a quick glimpse, just too many things to do and not enough time. Our main development focus right now is getting the multi-threading functionality tested and ready to merge from the mt branch into the mainline. That will change the behavior of the socket handling code somewhat as each of the sockets used for test data will be serviced by its own thread (the control connection will continue to be serviced by the process's main thread). I think that's going to affect this PR, although I'm not sure in what way exactly.

It's possible there might be some similar busy-waiting issues in how iperf3's application-level pacing (--bitrate option) works in the mt branch.

[BenjaminBeichler]

From my glimpse into the mt branch, it may solve the problematic behavior of the single threaded version. From my understanding, you use blocking reads in the separate threads for each stream and you do not put the stream FDs into the select of the main thread loop.

Nonetheless, the patch would not harm even the mt version, since the logic of the read and write set kept the same. I could easily adapt it for the mt branch. But since the main loop is only working on the control channel, the actual variables could be removed or replaced by only the control channel socket fd.