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 1for(int d=2; d<c-1; d++)…

Only even prime is 2for(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<dSo 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”


Getting Real (Data)

Entity Framework 6 is TAP enabled

Secret: using System.Data.Entity;

All|AnyAsync

Count|Average|Sum|Min|MaxAsync

ContainsAsync

First|Single[OrDefault]Async

ForEachAsync

LoadAsync

ToArray|List|DictionaryAsync


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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Software

Async/Await