Re: [Libevent-users] mixing libevent and boost::asio

[Azat Khuzhin <a3at.mail@xxxxxxxxx>]

On Thu, Nov 20, 2014 at 04:21:06PM +0300, Azat Khuzhin wrote:
> On Thu, Nov 20, 2014 at 01:27:14PM +0100, svante karlsson wrote:
> > Thanks,
> > 
> > First of all I made a typo in my own comments - the return parameter from
> > boost::asio::poll is the number of handlers it did run (not one).
> 
> You are right.
> 
> I've just checked io_service.poll():
> $ strace -eepoll_wait ./a.out
> pre poll
> epoll_wait(4, {{EPOLLIN, {u32=11063784, u64=11063784}}, {EPOLLIN, {u32=11063888, u64=11063888}}, {EPOLLIN, {u32=11063796, u64=11063796}}}, 128, 0) = 3
> post poll
> 
> timeout=0 === no blocking.
> 
> > According to spec  it runs handlers that are ready to run, without blocking
> > and I'm pretty sure it does what it says (Not blocking)
> > 
> > Second, it's a bit worrying (for me at least) that the libevent equivalent
> > call does indeed block - Do you have any hints on when it's actually
> > blocking?
> > >(event_base_loop(_event_base, EVLOOP_NONBLOCK);
> 
> I was wrong, sorry, it will not block with EVLOOP_NONBLOCK.
> 
> > 
> > Third, I  also agrees on "and personally I don't think that it is sane to
> > use two event loops in one application", BUT either I rewrite an existing
> > (proprietary) proxy based on asio or an existing db client library
> > (aerospike db) based on libevent.
> > 
> > I'm thinking I might get away with handling the server code from asio and
> > make polling calls to libevent queue in asio callbacks on events - both
> > timer and io related ones but I'm looking for a cleaner solution.

Anyway, to make it work without sleep(), you need fd of event base (for
instance fd that returns epoll_create()).

And neither libevent [1] nor asio [2] exports it.

But even if you will patch both, and you will be able to get this fd,
than you need some logic for other mechanisms like select(), so I guess
appropriate sleep() will be better.

[1] https://github.com/libevent/libevent/blob/master/epoll.C
[2] http://www.boost.org/doc/libs/1_38_0/boost/asio/detail/epoll_reactor.hpp

> > 
> > I've done the same for a libcurl based client in the past and it was
> > possible to get to the sockets before libcurl actually waited for io.
> 
> libcurl is something very different, it is not _aio_ library, but AFAIR it
> allow to multiplexing stuff using custom event loop (i.e. you could get
> internal fd), that's why you could do this using libcurl I suppose.
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