Re: [Boost-users] [asio] Using io_service for synchronous tcp accept + custom tasks

On 4/02/2015 20:38, Aaron Levy wrote:

04.02.2015, 06:22, "Gavin Lambert" :

...

Also, I could be wrong about this, but I think if you eg. perform a blocking read inside your sock job it will tie up a whole thread for the duration, which means that you will quickly run out if you get multiple connections. Using async code should avoid this.

Consider an alternative to my example while still using sync I/O.

io_service svc;

endpoint ep(ip::tcp::v4(), port); acceptor acceptor(svc, ep);

boost::thread_group group;

while (true) { shared_ptr<socket> sock(new socket(svc)); acceptor.accept(*sock);

group.create_thread([sock]() { /* do some processing */ }); } group.join_all(); // we never reach here

The problem with the first example would manifest itself with increasing rate of connections - connections would take longer to be accepted and be ready for read / write. With this one, connections would not remain hung up for too long, but subsequent I/O could because there could be too many threads vying for the processors. I'm wondering which one would be preferable.

Using a thread per connection is a server antipattern. You can usually get away with it for a surprisingly large number of connections, but it's still highly vulnerable to a DDoS (either as an actual attack or just from becoming too popular).

Are there any specific advantages of sync I/O over async I/O other than simple code?

No, the only advantage of sync I/O is that it's easier for humans to follow. (Well, that's not strictly true; if you do bind a single thread per connection then you can make use of TLS, and a few other esoteric bits like that.) Async code in general performs better (if written properly, which can be trickier than sync code) because threads only wait for work when there is no work to do, not blocking because one particular connection has nothing to do. So you need less threads overall and there are less context switches required when multiple items of work are ready. But the code is a bit more disjointed (although lambdas can help with this). Coroutines are a good compromise between the two -- they read almost like synchronous code but have performance closer to async code.