Integrate the Glib main loop into the libev event loop (C++)

[Marc Lehmann]

On Wed, Jul 07, 2021 at 12:06:37AM +0200, Oleksandr Kozlov <aleks.a.kozlov at gmail.com> wrote:
> from the EV::Glib Perl module [3].

Note that this is likely going to be slow, as glib does not have the
necessary hooks to efficiently be embedded into another event loop.

>   ctx.poll_fds.clear();
>   int timeout = 0;
>   ctx.context->query(ctx.priority, timeout, ctx.poll_fds);
>   for (Glib::PollFD &poll_fd : ctx.poll_fds) {
>     int fd = poll_fd.get_fd();
>     ctx.ios.try_emplace(fd, fd, to_ev_events(poll_fd.get_events()));

This looks as if you cache io watchers by fd, and could be the reason for
the problem - Glib doesn't tell you when the fd changes, so the fd is not
a unique enough primary key for your map (which really should be an array,
too).

What EV::Glib is doing is create I/O watchers in a prepare call and stop
them in the check call - you could simply push them onto a vector and clear
it out in check.

You could also try forcing libev to use select or poll as backend, which,
if Glib uses a lot of watchers, is likely more efficient, and this is a
quick way of checking whether this is your real problem - note that using
these backends for these side effects is not really supported by libev,
so you should definitely fix the c++ side, possibly followed by using
select/poll for efficiency.

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