From: Lars Christensen <larschbelunktumdk@...>
Date: 2009-08-11T00:07:00+09:00
Subject: Re: Soft realtime with EventMachine and timer resolution

On Sat, Aug 8, 2009 at 7:40 AM, Macario Ortega<macarui@gmail.com> wrote:
>                loop do
>                    @next += @interval
>                    @tick += 1
>                    Thread.new do
>                        tick
>                    end
>                    sleep @sleep_time until Time.now.to_f >= @next

This seem to be a recurrent mistake when implementing periodic events.
"Sleep X" does not ensure invokation every X seconds. It merely ensure
that there will be "at least X" seconds of delay between each
invokation. It could be X + 0.1, X + 1.0 or even 2 * X - there is
simply no guarantee. The code in the loop apart from 'sleep' also
takes time to execute (or the Garbage Collecter may run, or it can be
suspended by the OS while not sleeping). This time spent here may be
negilible most of the time, but OS's are not deterministic on this
scale, so it may randomly take 10ms, 100ms or more if other processes
on your computer are run at the same time. </rant>

To get any sort of accuracy, you should always sleep "until the next
event should occur" rather than a fixed time. For example:

next = Time.now
interval = 0.05
loop do
  invoke_periodic_event
  next += interval
  time_to_wait = next - Time.now
  sleep time_to_wait if time_to_wait > 0.0
end

This will not avoid the jitter, but will compensate when it occurs. If
jitter is extreme, you may want to add checks to reset the 'next'
variable to something resonable if it gets too far behind.

In music, jitter on a single beat may even add flavor to your music,
but if you shift the time scale all time the time, you are confusing
your listeners :-)

EM's add_periodic_timer is also "flawed" in this way that it doesn't
event attempt to invoke your method at fixed intervals but rather
"sleeps" a fixed interval between each invokation.