libev: new experimental Linux AIO backend available for testing

[Marc Lehmann]

Hi!

I am pleased to announce that the current CVS version of libev has
an experimental linux aio backend that can take advantage of the new
functionality added to linux 4.18 (https://lwn.net/Articles/743714/).

It is exciting because it can batch both requests and events (and mightr
be much faster than epoll in common use cases), and does not have most
of the downsides and design issues of epoll.

Of course, this wouldn't be linux if it would be usable out of the box:
as of 4.19, only very few file types are supported, namely, from my
testing, pipes, sockets, files (surprise! beats epoll!) and a few devices
(/dev/null), but not e.g. ttys (dangit!), so it is not yet working as a
generic epoll replacement (basically, it's like a worse kqueue). And it's
not embeddable either of course, so you can't even combine it with another
more generic mechanism. D'oh.

Currently, the code is not very optimized and probably buggy, but ready
for experimentation (being a few hours old by now :). Feedback would be
very much appreciated, even if you don't want to use it, as the backend
should currently just be silently compiled in by default without being
used, so even reports about compilation errors would be helpful.

If you want to try it out, you need a CVS checkout of libev:

   cvs -z3 -d :pserver:anonymous at cvs.schmorp.de/schmorpforge co libev

If you use the libev.m4 autoconf macros then support for
EVBACKEND_LINUXAIO aio should be automatically enabled when your headers
allow it (e.g. SYS_io_setup and so on are defined and linux/aio_abi.h is
available).

If not, you try to force compilation by specifying -DEV_USE_LINUXAIO when
compiling, e.g. for the EV perl module:

   EV_EXTRA_DEFS=-DEV_USE_LINUXAIO perl Makefile.PL; make; make install

It is also currently always disabled at runtime by default, so if it
compiles, nothing should change until you explicitgly request it, e.g.
using the LIBEV_FLAGS environment variable:

   LIBEV_FLAGS=64 ./my_libev_program

again, feedback at this stage is very appreciated, although I wouldn't
expect it to work flawlessly or conduct serious performance tests.

Thanks in advamce and have fun!

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