eleks summer school 2012: .net 06 - multithreading
DESCRIPTION
"Multithreading" lecture @ ELEKS Summer School 2012 (Jul 2012)TRANSCRIPT
Summer School 2012
Although threads seem to be a small step from sequential
computation, in fact, they represent a huge step.
They discard the most essential and appealing properties
of sequential computation: understandability,
predictability, and determinism.
Threads, as a model of computation, are wildly
nondeterministic, and the job of the programmer
becomes one of pruning that nondeterminism.
Edward A. Lee, UC Berkeley, 2006
“
”
History: so why did we need threads?
Summer School 2012
• CPU virtualization• Increased robustness• Quazi-multitasking
Process as a resource isolation unit
Summer School 2012
Concurrent executiondoes not exist
on a single logical CPU!
Summer School 2012
Context Switching
Summer School 2012
Scheduling• Processes are not scheduled. Threads are.• 32 different thread priority levels!
Summer School 2012
1. Idle2. Below
Normal3. Normal4. Above
Normal5. High6. Realtime
1. Idle2. Lowest3. Below
Normal4. Normal5. Above
Normal6. Highest7. Time-Critical
ProcessThread
Thread Overhead• Thread Kernel Object• Thread Environment Block (TEB)• User-Mode Stack [1 MB]
• Kernel-Mode Stack [12 / 24 KB]
• DLL: Attach/Detach Notifications
Summer School 2012
Moore’s Law
Summer School 2012
Threads for parallelization?
Summer School 2012
Multiple Logical CPUs
Summer School 2012
• Multi-socket motherboards• Single-core CPUs with Hyper Threading
(HT)• Multi-core CPUs• Multi-core CPUs with per-core HT
Thread Operations
Summer School 2012
I/O-boundCompute-bound
Compute-bound Operations
Summer School 2012
static void Main(string[] args) { Thread workerThread = new Thread(Factorial); workerThread.Start(10); workerThread.Join(); Console.ReadKey(true); } static void Factorial(object arg) { int n = (int)arg; int result = 1; for (int i = 2; i <= n; i++) result *= i; Console.WriteLine(result); }
Thread Lifecycle
Summer School 2012
Too many threads = too bad
Summer School 2012
• Kernel object overhead• Memory overhead• Context switch overhead• GC overhead
Use threads wisely!
Thread Pool
Summer School 2012
• Designed for background processing• Self-balancing• Avoids much overhead
Thread Pool: Compute-bound
Summer School 2012
static void Main(string[] args) { ThreadPool.QueueUserWorkItem(Factorial, 10); Console.ReadKey(true); }
static void Factorial(object n) { int x = (int)n; int result = 1; for (int i = 2; i <= x; i++) result *= i; Console.WriteLine(result); }
Self-Balancing
Summer School 2012
Too few threads:Allocate more
Too many threads:Remove excessive ones
Task-Based Threading
Summer School 2012
“Work Stealing” Principle!
Tasks: Compute-bound
Summer School 2012
static void Main(string[] args) { Task<Int32> task = new Task<Int32>(Factorial, 10); task.Start(); task.Wait(); Console.WriteLine("The Sum is: " + task.Result); Console.ReadKey(true); }
static int Factorial(object n) { int x = (int)n; int result = 1; for (int i = 2; i <= x; i++) result *= i; return result; }
Tasks 101
Summer School 2012
• Each task can have child tasks…• …and, therefore, throw multiple exceptions• “Task” does not mean “separate thread”!• Task templating via TaskFactory • Custom scheduling via TaskScheduler • Cooperative cancellation via
CancellationToken• Continuations, WaitAny, WaitAll, …
Tasks: Continuations
Summer School 2012
static void Main(string[] args)
{
Task<Int32> task = new Task<Int32>(Factorial, 10);
task.ContinueWith(t => Console.WriteLine("Completed"),
TaskContinuationOptions.OnlyOnRanToCompletion);
task.ContinueWith(t => Console.WriteLine("Canceled"),
TaskContinuationOptions.OnlyOnCanceled);
task.ContinueWith(t => Console.WriteLine("Error"),
TaskContinuationOptions.OnlyOnFaulted);
task.Start();
}
Parallel LINQ (PLINQ)
Summer School 2012
enumerable.AsParallel()
Parallel Computations via PLINQ
Summer School 2012
static void Main(string[] args) { Parallel.For(1, 10, n => Console.WriteLine("{0}: {1}", n, Factorial(n))); Enumerable.Range(1, 9).AsParallel().ForAll(n => /* Console... */); new int [] { 1, 2, 3 }.AsParallel().ForAll(n => /* Console... */); }
Summer School 2012
.AsParallel()
does not mean faster execution
• Possible GC pressure ( i => new { ... } )
• Item acquisition and delegate execution are implemented in virtual methods
• You have to benchmark every particular situation
Synchronous I/O
Summer School 2012
Async I/O
Summer School 2012
Async I/O Benefits
Summer School 2012
• Less threads• Less GC• Faster debugging• Faster I/O if made parallel• Responsive UI (a must for WinPhone &
Silverlight)
Asynchronous Programming Model (APM)
Summer School 2012
Implemented by:
• Streams• Sockets• Serial Ports• SQL Commands• DNS Requests• Web Services …etc.
The Async Model Soup
Summer School 2012
Which one to follow? Stop the madness!
New APM in .NET 4.5 / C# vNext
Summer School 2012
private byte[] GetURLContents(string url) { var content = new MemoryStream(); var webReq = (HttpWebRequest)WebRequest.Create(url); using (var response = webReq.GetResponse()) using (Stream responseStream = response.GetResponseStream()) responseStream.CopyTo(content); return content.ToArray(); } private async Task<byte[]> GetURLContentsAsync(string url) { var content = new MemoryStream(); var webReq = (HttpWebRequest)WebRequest.Create(url); using (WebResponse response = await webReq.GetResponseAsync()) using (Stream responseStream = response.GetResponseStream()) await responseStream.CopyToAsync(content); return content.ToArray(); }
Threads and Shared State
Summer School 2012
Threads and Race Conditions
Summer School 2012
Synchronization Constructs
Summer School 2012
User-Mode• Volatile Read/Write• Interlocked
Kernel-Mode• Events• Semaphores• …and everything derived from them
Hybrid• Double-checked locking• Many others…
To make things even worse
Summer School 2012
• Language Compiler optimizations• JIT Compiler optimizations• CPU optimizations – Out-of-order execution – Branch prediction – Memory barriers
Atomic (Interlocked) Operations
Summer School 2012
• Atomic Swap• Test-and-Set• Compare-and-Swap• Fetch-and-Add• Load-Link / Store-Conditional
Kernel-mode Constructs
Summer School 2012
• WaitHandle (base class)• AutoResetEvent• ManualResetEvent• CountdownEvent• Semaphore• Mutex
Hybrid constructs: double-checked locking
Summer School 2012
internal sealed class Singleton { private static readonly Object s_lock = new Object(); private static Singleton s_value = null;
private Singleton() { }
public static Singleton GetSingleton() { if (s_value != null) return s_value; Monitor.Enter(s_lock); if (s_value == null) { Singleton temp = new Singleton(); Interlocked.Exchange(ref s_value, temp); } Monitor.Exit(s_lock); return s_value; }}
Concurrent Collections
Summer School 2012
• BlockingCollection<T>
• ConcurrentBag<T>
• ConcurrentDictionary<K, V>
• ConcurrentQueue<T>
• ConcurrentStack<T>
Building Scalable Applications
Summer School 2012
• Avoid thread blocking• Avoid shared state• Avoid statics• Avoid mutable structures
Thank you!
Summer School 2012