A Gentle Introduction to Event Loops

EVENT LOOPSEverything you wanted to know

but couldn't find on StackOverflow

ABOUT ME

• I've misunderstood event loops for over a decade

• 2–3 years ago I set out on a quest to really understand them

• I failed

• but learned a few things

WE ALL START SOMEWHERE

<?php echo "im in ur pagez, writin php!!1!" ?>

alert("Hi mom!");

WHY IS THIS SO SLOW?…for my $host (@hosts) { say $host . " is " . (ping($host) ? "up" : "down"); }

for host in hosts: print host + " is " + ("up" if ping(host) else "down")

…BECAUSE NETWORKS

FORKING

<?php

function ping($host) { ... }

foreach ($hosts as $host) { $pid = pcntl_fork(); if ($pid) { pcntl_wait($status); } else { ping($host); } } ?>

THREADING

Some people, when confronted with a problem, think, “I know, I'll use threads”—

now two they hav erpoblesm.

Knock knock.Race condition.Who's there?

–Apple, "Threaded Programming Guide: Thread Management"

“Another cost to consider when writing threaded code is the production costs. Designing a threaded application can sometimes require fundamental changes to the way you organize your application’s data structures. Making those changes might be necessary to avoid the use of synchronization, which can itself impose a tremendous performance penalty on poorly designed applications. Designing those data structures, and debugging problems in threaded code, can increase the time it takes to develop a threaded application. Avoiding those costs can create bigger problems at runtime, however, if your threads spend too much time waiting on locks or doing nothing.”

NON-BLOCKING IO

LET’S ORDER BREAKFAST!

pancakes

soft-boiled egg

orange juice

BLOCKING CHEF

mix pancakes heat skillet cook

pancakes boil water cook eggs cut juice

NON-BLOCKING CHEF

mix pancakes

heat skillet cook pancakes

boil water cook eggs

cut juice

for host in hosts: clientsocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM) clientsocket.setblocking(0) clientsocket.connect((host, port))

while 1: reads, writes, fails = select.select(in, out, [], wait) # do something with these handles ...

EVENT LOOPS

while (!mExiting) NS_ProcessNextEvent(thread);

DEVICES & GAMES

UNITY 3D GAME LOOP

• physics loop: runs until caught up to current frame

• event updates once per loop iteration ("tick")

• network events

• rendering

$.get("some/page.html", function (data) { $("#response").html(data); });

var req = http.request({path: 'some/page.html'}, function(res) { res.on('data', function(data) { console.log(data); }); });

$ua->get('some/page.html', sub { say pop->res->body; });

http.get(url, function(response) { print response.body})

Loop.Run()

reactor (event sources)

http.get(url function(response) { print response.body}

Loop.Run()

message queue (callbacks)

<script type=“text/javascript">

$(document).ready(function() { jQuery.ajax(“http://www.perl.org”, { success: function(data) { alert("loaded!") } }) })

</script>

jQuery.ajax(“http://www.perl.org”, { success: function(data) { alert("loaded!") } })

success: function(data) { alert("loaded!") }

CAVEAT EMPTOR

• event loops can be hard to work with

• don't use callbacks except for simple things

• use Promises, Futures and other abstractions

IMAGE CREDITS

• stopwatch, cloud, database, info by Austin Condiff from the Noun Project

• server by aLf from the Noun Project

• browser by Cindy Hu from the Noun Project

• code by useiconic.com from the Noun Project