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StreamSelectLoop: Timers with very small intervals do not work correctly #48
Description
Within StreamSelectLoop::run() is the following code:
} elseif ($scheduledAt = $this->timers->getFirst()) { // float
$timeout = $scheduledAt - $this->timers->getTime(); // float - float
if ($timeout < 0) {
$timeout = 0;
} else {
$timeout *= self::MICROSECONDS_PER_SECOND; // float * int
}
}In the case of a periodic timer with an interval equal to Timer::MIN_INTERVAL (0.000001), the above code, in conjunction with the Timers class, essentially does something like the following:
$currentTime = microtime(true);
$ourTimerInterval = 0.000001;
$nextTimerScheduledAt = $currentTime + $ourTimerInterval;
// $timeout is for stream_select()/usleep() call
$timeout = $nextTimerScheduledAt - $currentTime; // This is NOT equal to 0.000001 due to float error [1]
$timeout *= 1000000; // This looks like it should be (int)1, but it is actually approximately (float)0.95$timeout is then passed to stream_select() and usleep(), both of which expect int and so cast (float)0.95 to (int)0, not waiting at all when the timer was intended to delay for 1 microsecond. Furthermore, the fix in #37 is bypassed when this issue occurs as (float)0.95 casts to boolean true.
A quick fix would be to apply rounding to $timeout *= self::MICROSECONDS_PER_SECOND;.
Activity
enumag commentedon Oct 31, 2016
I'm having trouble with periodic timers with period lesser than 1 (1.0 is ok, 0.999 is not) and StreamSelectLoop (didn't try other loops yet). I'm stopping the loop after 1.5 second. With period set to 1 the timer is executed once. With period set to 0.999 it's executed several thousands times.
The suggested fix did not help at all.
kelunik commentedon Nov 16, 2016
Having a timer executed every microsecond is the first mistake. What's the purpose of that timer?
Apart from that I'm glad that we have
intforasync-interop/event-loopinstead of floats.enumag commentedon Nov 16, 2016
I don't want timer executed every microsecond, just every half-second or so - for faster tests execution. But instead of being executed twice in one second it is executed like 3000 times.
Also how can I use the
async-interop/event-looplibrary? There is no non-abstract driver implementation.kelunik commentedon Nov 16, 2016
@enumag Amp will support it in version 2.0.0, currently in development. @WyriHaximus started to build an adapter for React as well.
clue commentedon Nov 16, 2016
@enumag Have you tried the suggested fix in #49 and this did not resolve your issue?
Can you post a gist that helps us reproduce the issue you're seeing?
ivkalita commentedon Mar 20, 2017
@clue, hi! Do you have any idea about how to test this thing? I stuck with the same problem as @joshdifabio in #49 – it's hard to test that this feature is working. We need to run timer with very-very small interval and loop's time cost itself is much more expensive and takes much more time than
usleep(1).jsor commentedon Apr 4, 2017
Fixed in #93