Swimming, not drowning in the 'NET Thread Pool

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Swimming, not drowning in the .NET Thread Pool

• .NET Threads and the Thread Pool
• Nick Wienholt
• Nick_at_DotNetPerformance.com

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Nick Wienholt

• Windows and .NET systems software engineer/
  architect
• Microsoft Certified Solution Developer
• Lead software engineer (Microsoft Technologies)
  Softbuild
• Author of upcoming book of .NET performance (Q1
  2003)

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Threading before .NET a reflection on the past
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Agenda

• Threading why, how and when
• Thread Pool
• Monitoring Threads
• The Synch Generator a VS.NET add-in

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Basic Terminology

• Thread path of code execution. Has a call
  stack where functions and data are placed, and is
  the base unit for which processor time is
  scheduled.
• Thread safe a piece of code is thread safe
  when it has inbuilt protection against data
  corruption that can be caused by simultaneous
  modification from multiple threads.

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Threading Before .NET

• Threading was a bit of a mess.
• Different languages had different levels of
  support for threading.
• Some language runtimes where not thread-safe.
• Different libraries had different levels of
  thread safety.
• C had many competing libraries with overlapping
  functionality (Win32, C runtime, MFC, ATL\ COM).
• Operating system dependant Win9x and WinNT
  based systems had different feature sets.

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Threading in .NET

• Writing multi-threaded code possible in all
  languages.
• One unified API for dealing with threads.
• Improved documentation that indicates the
  thread-safety of the Base Class Library (BCL)
  types.
• Everything designed for the ground-up with
  awareness of threading issues.

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New Threading Features

• Thread pool.
• Clear and consistent methods for terminating
  threads (Thread.Abort).
• Language level support for data locking (C lock
  statement, VB.NET SyncLock statement).
• Threading patterns in various sections of the BCL
  to support light-weight synchronization.
• ReaderWriter lock provides in-built support for
  allowing multiple readers or a single writer on a
  data structure.

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Multiple threads when to use them

• Support asynchronous operations.
• Read a large file from disk and keep the UI
  responsive.
• Service multiple client requests simultaneously.
• Conduct operations at a higher priority
• Thread priority is specified when a thread is
  created. A thread with higher priority gets a
  large slice of processor time.

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Thread Synchronization

• System.Threading.Monitor class can be used to
  prevent multiple threads from modifying data
  structures simultaneously.
• The lock\ SyncLock statements in C\ VB.NET use a
  Monitor lock in a try block and release the
  Monitor in finally block.

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Writing a thread-safe class

• public class ThreadSafe
• private long _data
• public long Data
• get
• lock(this)return _data
• set
• lock(this)_data value

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Thread safety

• The vast majority .NET BCL is NOT thread safe.
• Paths to thread safety
• Immutable types (e.g. System.String)
• Synchronization wrapper (e.g. System.Collections.A
  rrayList) (demo)
• Synchronized contexts very similar in concept
  to COM apartments. Heavy-weight and slow.
• Hand-coded wrappers.
• Dont share objects between threads.

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Using Threads

• Code demo manually creating a new thread.

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Thread Pool

• Motivation threads are expensive to create and
  cheap to recycle.
• Too many threads in a process hurts performance
  context thrashing
• The need for a common thread management tool that
  all components and classes used the end of
  hand-code hacks

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Thread Pool the internals

• Defaults to 25 threads per process per processor.
• Can be configured by the runtime host and the
  managed code. (DEMO)
• Machine.config and web.config can specify default.

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Thread Pool when to use

• Servicing requests for multiple clients the web
  server model
• Waiting on OS handles to become signalled Win32
  events
• Code demos

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Thead Pool when not to use

• Need to set thread priority
• Long running requests.
• Thread security token and call-context issues gt
  the big thread pool gotcha.
• System.Threading.Timer does NOT propagate call
  content or security permissions
• New threads and queued worker items inherit
  security but NOT call context.

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Tools for monitoring threads

• Task Manager
• Perfmon
• Process Explorer (www.sysinternals.com)
• VS.NET be sure to enable unmanaged debugging.
• DeadlockDetection utility from Debugging
  Applications (MSPRESS) by John Robbins

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Thread Synchronization Made Easy

• Demo
• When not to use
• Transactional semantics.
• ReaderWriter lock optimizations.
• Manual lock granularity optimizations.

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References

• MSDN documentation Jan 2002 MSDN is much more
  complete than the VS.NET RTM docs
• Mike Woodrings web site www.BearCanyon.com
• Applied .NET Framework Programming, Jeffrey
  Richter, Microsoft Press
• Developing Applications with Visual Studio .NET,
  Richard Grimes, DevelopMentor series
• Essential .NET, Don Box, DevelopMentor series

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www.DotNetPerformance.com All code and slides