�ݺ�ߣ

EVENT LOOPS
Everything 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 ISTHIS 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")

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

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.”

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
...

A Gentle Introduction to Event Loops

while (!mExiting)
NS_ProcessNextEvent(thread);

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)

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

�ݺ�ߣ

A Gentle Introduction to Event Loops

Recommended

More Related Content

What's hot (20)

Similar to A Gentle Introduction to Event Loops (20)

Recently uploaded (20)

A Gentle Introduction to Event Loops