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 19, 2013

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 only divisible by itself c 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

First Attempt
var nums = Enumerable.Range(1, Math.Sqrt(c));
var query =nums.AsParallel().Where(n => IsPrime(n));
var primes = query.ToArray();
Problems:
 Doesn’t scale (parallel “never” does)
 Doesn’t improve perf on a loaded system
 May not improve perf “anyway”

Which Would You Prefer?
(100s)
(100s)
(100s)
(100s)
(100s)
Synchronous
500s elapsed
1 thread
(100s)
(100s)
(100s)
(100s)
(100s)
Parallel
300s elapsed
2 threads
Asynchronous
100.1s elapsed
1 thread
(100s)
(100s)
(100s)
(100s)
(100s)
20ms

Simple Code
public async void Click(){
var client = new AsyncSample();
int answer = await client.LongAsync();
txtResult.Text = "Life the universe..."
}
class AsyncSample{
public async Task<int> LongAsync(){
var client = new FrameworkClass();
Task task = client.LongAsync();
…
int result = await task;
…
}
}

CPU Bound IO Bound
 Needs “backing” thread
 Parallel.ForEach and
Task.Run
 Unless writing scalable
(server-side) code
 Special advice to “library”
writers (don’t lie/chatty)
 Threads don’t increase
throughput under load
 Doesn’t need/want
 Always use await
rather than another
(background) thread
Ying  Yang

Misconceptions about async/await
 async modifier:
 method is async
 await keyword:
 call the async method
and wait until it returns
 suspends the thread
 async modifier:
 method may call async methods/
use await
 await keyword:
 call the async method and return
immediately (assuming it does);
when method completes,
continue from here
 suspends the method (IP)

Basic ROT
 Using await forces signature to: async Task[<T>]
 Warning if async method without using await
 await converts Task<T> to <T> (Task to void)
 Method returns Task<T>, but you return T;
 If you have used await
 Async is cheap—but does allocate; the GC costs
 State machine for method’s local variables
 A delegate
 Task object

Rules
 Can’t await in catch/finally (C#6/VS15) or lock
 Can’t make properties async
 “Never” call async void (event handlers only)
 Or if you must “fire and forget”

How Sweet the Syntactic Sugar Is
async Task<T> MyMethodAsync(«args»){
var client = …
var r = await client.WhateverAsync(…);
… use r, as needed
return «some expression <T>, i.e., r»;
}
async Task<T> MyMethodAsync(«args»){
var tcs = TaskCompletionSource<T>();
var client = …
client.WhateverAsync(…).ContinueWith(task -> {
var r = task.Result;
… use r, as needed
tcs.SetResult(«some expression <T>, i.e., r»)
});
return tcs.Task;
}

Task Class – a Promise…
 ctors – not typically used (takes Action<T>)
 Properties
 Factory, CreationOptions
 IsCanceled, IsCompleted, IsFaulted and Status
 Methods
 Task.Delay – replaces Thread.Sleep
 Task.Run – new in 4.5 (simple sugar)
 FromResult<T>
 ConfigureAwait(bool continueOnCapturedContext)
 ContinueWith…, Wait…, WaitAll…, WaitAny…, Task.Yield

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.
 Or “eat” the 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”

Async Programming Model
IAsyncResult BeginDoIt(path,…,
DoitCallback, doItState);
public void DoItCallback(IAsyncResult result){
…
var i = int EndDoIt(result);
}
int DoIt(string path,…); (inputs)
(added—optional)
Options:
1. Block
2. Wait Handle
3. Poll
4. Callback
Interface with
• AsyncState
• AsyncWaitHandle
• IsCompleted
• CompletedSynchronously

Event-based Asynchronous Pattern
delegate void DoItCompletedEventHandler(
object sender, DoItCompletedArgs args);
class DoItCompletedArgs :AsyncCompletedEventArgs{
int Result{ get; set; }
}
class MyClass{
void DoItAsync(string path,…);
Event DoItCompletedEventHandler DoItCompleted;
}

Task-based Asynchronous Pattern
class MyClass{
async Task<int> DoItAsync(string path,…);
}

Transitioning from APM
static Task<int> ToDoItAsync(this MyClass mc
, string path,…)
{
if( mc==null ) throw new ArgumentNullException(…
return Task<int> task = Task.Factory.FromAsync<int>(
mc.BeginDoIt, mc.EndDoIt, path,…);
}

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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