An Insider’s View to Concurrency at MicrosoftStephen Toub (stoub@microsoft.com)Parallel Computing PlatformMicrosoft Corporation

Agenda• Why We (I) Care• What We’ve Built for Developers• What We’re Building for Developers

Moore’s Law: Alive and Well

http://upload.wikimedia.org/wikipedia/commons/2/25/Transistor_Count_and_Moore%27s_Law_-_2008_1024.png

MS Apps Using ParallelismExample: Visual Studio• Background

compilation• Regex-based file

search• Graph layout• Reference

highlighting• IntelliSense sorting• Project build system

• Code analysis• Unit testing• …

But it’s not just about core count…• Increasingly connected applications

• More latency• e.g. everything as a service

• More UI responsiveness problems• e.g. the toilet bowl of death

• More scalability issues• Server, Cloud

• e.g. streaming data sources, data distribution• Client

• e.g. modeling interacting biological entities• Async-only APIs

• e.g. Silverlight

The Challenges of Concurrency• A different way of thinking

• Forcing the square peg (concurrency) into the round hole (sequential) is a common (inadequate) mistake

• Parallel patterns are not prevalent, well known, nor easy to implement• CS curriculum is still largely “sequential”: Cormen, et. al; Knuth; etc.

• A different way of writing software• Indeterminate program behavior

• No longer: “A happens, then B happens, then C happens”• It’s just: “A, B, and C happen”

• With distinct costs, too• Deadlocks, livelocks, latent race conditions, priority inversions,

hardware-specific memory model reordering, …

• Businesses have little desire to go deep• Best devs should focus on business value, not concurrency• Need simple ways to allow all devs to write concurrent code

Example: Searching and Sorting

IEnumerable<RaceCarDriver> drivers = ...;var results = new List<RaceCarDriver>();foreach(var driver in drivers){ if (driver.Name == queryName && driver.Wins.Count >= queryWinCount) { results.Add(driver); }}results.Sort((b1, b2) => b1.Age.CompareTo(b2.Age));

Manual Parallel SolutionIEnumerable<RaceCarDriver> drivers = …;var results = new List<RaceCarDriver>();int partitionsCount = Environment.ProcessorCount;int remainingCount = partitionsCount;var enumerator = drivers.GetEnumerator();try { using (var done = new ManualResetEvent(false)) { for(int i = 0; i < partitionsCount; i++) { ThreadPool.QueueUserWorkItem(delegate { while(true) { RaceCarDriver driver; lock (enumerator) { if (!enumerator.MoveNext()) break; driver = enumerator.Current; } if (driver.Name == queryName && driver.Wins.Count >= queryWinCount) { lock(results) results.Add(driver); } } if (Interlocked.Decrement(ref remainingCount) == 0) done.Set(); }); } done.WaitOne(); results.Sort((b1, b2) => b1.Age.CompareTo(b2.Age)); }}finally { if (enumerator is IDisposable) ((IDisposable)enumerator).Dispose(); }

LINQ Solution

IEnumerable<RaceCarDriver> drivers = ...; var results = from driver in drivers where driver.Name == queryName && driver.Wins.Count >= queryWinCount orderby driver.Age ascending select driver;

.AsParallel()

P

DemoPLINQ

Visual Studio 2010Tools, Programming Models, Runtimes

Parallel Pattern Library

Resource Manager

Task Scheduler

Task Parallel Library

Parallel LINQ

Managed NativeKey:

ThreadsOperating System

Concurrency Runtime

Programming Models

ThreadPoolTask Scheduler

Resource Manager

Data Structures D

ata

Stru

ctur

es

Tools

Tooling

ParallelDebugge

r Tool Windows

ProfilerConcurren

cy Visualizer

AsyncAgentsLibrary

UMS Threads

.NET Framework 4 Visual C++ 2010Visual Studio2010

Windows

Investigations for the future…

This is somesynchronous code with .NET 4…public void CopyStreamToStream(Stream source, Stream destination){    byte[] buffer = new byte[0x1000];    int numRead;    while ((numRead = source.Read(buffer, 0, buffer.Length)) != 0)    {        destination.Write(buffer, 0, numRead);    }}

This is an expert’s asynchronous code with .NET 4…public void CopyStreamToStream(Stream source, Stream destination){    byte[] buffer = new byte[0x1000];    int numRead;    while ((numRead = source.Read(buffer, 0, buffer.Length)) != 0)    {        destination.Write(buffer, 0, numRead);    }}

public IAsyncResult BeginCopyStreamToStream( Stream source, Stream destination){    var tcs = new TaskCompletionSource<object>();    byte[] buffer = new byte[0x1000];

    Action<IAsyncResult> readWriteLoop = null;    readWriteLoop = iar =>    {        try        {            for (bool isRead = iar == null; ; isRead = !isRead)            {                switch (isRead)                {                    case true:                        iar = source.BeginRead(buffer, 0, buffer.Length,  readResult =>                        {                            if (readResult.CompletedSynchronously) return;                            readWriteLoop(readResult);                        }, null);                        if (!iar.CompletedSynchronously) return;                        break;

                    case false:                        int numRead = source.EndRead(iar);                        if (numRead == 0)                        {                            tcs.TrySetResult(null);                            return;                        }                        iar = destination.BeginWrite(buffer, 0, numRead, writeResult =>                        {                            if (writeResult.CompletedSynchronously)  return;                            destination.EndWrite(writeResult);                            readWriteLoop(null);                        }, null);                        if (!iar.CompletedSynchronously) return;                        destination.EndWrite(iar);                        break;                }            }        }        catch (Exception e) { tcs.TrySetException(e); }    };    readWriteLoop(null);

    return tcs.Task;}

public void EndCopyStreamToStream(IAsyncResult asyncResult){    ((Task)asyncResult).Wait();}

A compiler could do the work for us…

public void CopyStreamToStream(Stream source, Stream destination){    byte[] buffer = new byte[0x1000];    int numRead;    while ((numRead = source.Read(buffer, 0, buffer.Length)) != 0)    {        destination.Write(buffer, 0, numRead);    }}

public Task CopyStreamToStream(Stream source, Stream destination){    byte[] buffer = new byte[0x1000];    int numRead;    while ((numRead = await source.ReadAsync(buffer, 0, buffer.Length)) != 0)    {         await destination.WriteAsync(buffer, 0, numRead);    }}

public Task CopyStreamToStream(Stream source, Stream destination){    byte[] buffer = new byte[0x1000];    int numRead;    while ((numRead = await source.ReadAsync(buffer, 0, buffer.Length)) != 0)    {         await destination.WriteAsync(buffer, 0, numRead);    }}

Q&A

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