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watcher reads a byte it will write that byte to a random other "server".

The effect is that there will be a lot of I/O watchers, only part of which
are active at any one point (so there is a constant number of active
fds for each loop iteration, but which fds these are is random). The
timeout is reset each time something is read because that reflects how
most timeouts work (and puts extra pressure on the event loops).

In this benchmark, we use 10000 socket pairs (20000 sockets), of which 100
(1%) are active. This mirrors the activity of large servers with many
connections, most of which are idle at any one point in time.

Source code for this benchmark is found as F<eg/bench2> in the AnyEvent
distribution. It uses the L<AE> interface, which makes a real difference
for the EV and Perl backends only.

=head3 Explanation of the columns

I<sockets> is the number of sockets, and twice the number of "servers" (as
each server has a read and write socket end).

I<create> is the time it takes to create a socket pair (which is
nontrivial) and two watchers: an I/O watcher and a timeout watcher.

I<request>, the most important value, is the time it takes to handle a
single "request", that is, reading the token from the pipe and forwarding
it to another server. This includes deleting the old timeout and creating
a new one that moves the timeout into the future.

=head3 Results

     name sockets create  request 
       EV   20000  62.66     7.99 
     Perl   20000  68.32    32.64 
  IOAsync   20000 174.06   101.15 epoll
  IOAsync   20000 174.67   610.84 poll
    Event   20000 202.69   242.91 
     Glib   20000 557.01  1689.52 
      POE   20000 341.54 12086.32 uses POE::Loop::Event

=head3 Discussion

This benchmark I<does> measure scalability and overall performance of the
particular event loop.

EV is again fastest. Since it is using epoll on my system, the setup time
is relatively high, though.

Perl surprisingly comes second. It is much faster than the C-based event
loops Event and Glib.

IO::Async performs very well when using its epoll backend, and still quite
good compared to Glib when using its pure perl backend.

Event suffers from high setup time as well (look at its code and you will
understand why). Callback invocation also has a high overhead compared to
the C<< $_->() for .. >>-style loop that the Perl event loop uses. Event
uses select or poll in basically all documented configurations.

Glib is hit hard by its quadratic behaviour w.r.t. many watchers. It
clearly fails to perform with many filehandles or in busy servers.

POE is still completely out of the picture, taking over 1000 times as long
as EV, and over 100 times as long as the Perl implementation, even though
it uses a C-based event loop in this case.

=head3 Summary

=over 4

=item * The pure perl implementation performs extremely well.

=item * Avoid Glib or POE in large projects where performance matters.

=back

=head2 BENCHMARKING SMALL SERVERS

While event loops should scale (and select-based ones do not...) even to
large servers, most programs we (or I :) actually write have only a few
I/O watchers.

In this benchmark, I use the same benchmark program as in the large server
case, but it uses only eight "servers", of which three are active at any
one time. This should reflect performance for a small server relatively
well.

The columns are identical to the previous table.

=head3 Results

    name sockets create request 
      EV      16  20.00    6.54 
    Perl      16  25.75   12.62 
   Event      16  81.27   35.86 
    Glib      16  32.63   15.48 
     POE      16 261.87  276.28 uses POE::Loop::Event

=head3 Discussion

The benchmark tries to test the performance of a typical small
server. While knowing how various event loops perform is interesting, keep
in mind that their overhead in this case is usually not as important, due
to the small absolute number of watchers (that is, you need efficiency and
speed most when you have lots of watchers, not when you only have a few of
them).

EV is again fastest.

Perl again comes second. It is noticeably faster than the C-based event
loops Event and Glib, although the difference is too small to really
matter.

POE also performs much better in this case, but is is still far behind the
others.

=head3 Summary

=over 4

=item * C-based event loops perform very well with small number of