Async/Await

J e f f H a r t
V a n i s h i n g C l o u d s , I n c .
Async/Await for Fun and Profit
Multithreading is just one damn thing after…
…before, or the simultaneously with another.
Scott Meyers and Andrei Alexandrescu
…before
March 7-8, 2015

Why Multithreading?
 “Modern” apps force multithreading
 Desktop/client – avoiding the “toilet bowl”
 Server-side scalability – all about the cores
 Economics – computers not faster since P4 (90nm)
Copyright © Jeff Hart, 2015

Know Your Goal
 Offloading – free the “main” thread
 Still uses another (Thread Pool) thread
 Works for CPU bound (i.e., thread backed)
 Scaling – not using “any” thread
 Still uses IO completion ports
 Only works for IO bound provided by “framework”

Domain: Prime Numbers
 Natural numbers c only divisible by itself and 1
for(int d=2; d<c-1; d++)…
 Only even prime is 2
for(int d=3; d<c-1; d+=2)…
 If c/d = q, then c/q = d; and q or d ≤ SQRT(c)
for(int d=3; d<=Math.Sqrt(c); d+=2)…
 If c/d, then d is a prime or divisible by one<d
So only test against previously found primes
… lots of more powerful sieves

Basic ROT
 await converts Task<T> to <T> (Task to void)
 Using await  changing signature: async Task[<T>]
 Warning on async method w/o using await
 Can’t await in catch/finally or lock
 Can’t make properties async
 Method returns Task<T>, but you return T;
 If you have used await
 Never call async void (event handlers only)

Floor to Ceiling
 Go down to:
 Framework XxxAsync for scaling
 “Creating” asynchronicity
Task.Run( { … } ) way better than worker threads
 Go up to:
 Handler
 MSTest, etc.
 “Eating” asynchronicity
task.ContinueWith( …, TaskContinuationOptions.Only|Not)
task.Result - blocks

We’ve Been At This…
 CLR v1: APM – Async Programming Model
 Simple (mechanical) but limiting
 CLR v2: EAP – Event-based Asynchronous Pattern
 Very flexible but lots of “namespace noise”
 CLR v4: TAP – Task-based Asynchronous Pattern
 V4.5 async/await “complier sugar”

Graduation
foreach( var d in GetLotsOfData() ){
DoSomethingReallyLong(d);
}
Parallel.ForEach( GetLotsOfData(), d=>{
DoSomethingReallyLong(d);
}
var tasks = new List<Task>()
foreach( var d in GetLotsOfData() ){
tasks.Add(DoSomethingReallyLongAsync(d));
}
Task.AwaitAll(tasks);
?

Layers of Multithreading
 Async/await two-step
 Separating task and await
 Knowin’ when to make ‘em;
And knowin’ when to tend ‘em
 Rules of Thumb
 Mostly compiler enforced/assisted
 Avoid async void (fire and forget)
 Go “floor to ceiling” when possible
 Task’s members to “stop”
 TaskCompletionSource to “create”
The scary bit:
Synchronizing (data)

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Async/Await

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