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I think a straightforward question that i cant seem to find any information on. When calling ASIOs io_context.run(), if there is at that moment nothing yet to read/write asynchronously, does asio do busy waiting with that thread or does it do something more clever where the thread can be released and used in other parts of the application or OS?

I looked into the code but its not very clear to me what the answer is. I do see usage of conditional variables in some places so i can only presume that the run call is not busy waiting if it doesnt have to be.

I ask because in our case, we would like to maximise thread efficiency so it was suggested to place a thread sleep inside a recursive async read handler in case asio is busy waiting. We dont get that much network activity for a single thread to be used maximally.

pSquared
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1 Answers1

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It's not busy-waiting. This is documented here: The Proactor Design Pattern: Concurrency Without Threads

It highlights what underlying API's are preferred depending on platforms:

On many platforms, Boost.Asio implements the Proactor design pattern in terms of a Reactor, such as select, epoll or kqueue.

And

On Windows NT, 2000 and XP, Boost.Asio takes advantage of overlapped I/O to provide an efficient implementation of the Proactor design pattern.

Q. it was suggested to place a thread sleep inside a recursive async read handler in case asio is busy waiting

Don't do that. Keeping handlers short will allow you to multiplex all IO on a single service. If you do blocking work, consider posting it to a separate thread (pool).

sehe
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