Multithreaded programming with NSPR

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Multi-threaded programming with NSPR

• Larry Hardiman

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Overview

• What is NSPR?
• Why program using threads?
• What you need to program with threads
• NSPRs thread abstraction
• Thread synchronization using locks
• Thread synch using condition variables
• Techniques for high performance threads

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What is NSPR?

• Original NSPR 1.0 was NS Java base
• NSPR 2.0 began in early 1996
• NSPR 2 is platform abstraction
• No GUI
• Threading, thread sync
• File and Socket I/O
• Time, IPC, ...

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Why program using threads?

• Simplify program design
• Improve performance when calling blocking
  functions
• Utilize all processors on multi-processor systems

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Thread Programming Basics

• Thread Management
• Thread private data
• Thread synchronization

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Thread Management

• PR_CreateThread()
• PR_Interrupt()
• PR_JoinThread()
• PR_SetThreadPriority()
• PR_GetThreadPriority()

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Creating a thread

• 1 include prthread.h
• 2 PRThread
• 3 PR_CreateThread(
• 4 PRThreadType type, / user or system /
• 5 (void)(start)(void arg), / function /
• 6 void arg /argument to start function /
• 7 PRThreadPriority priority, / low, norm /
• 8 PRThreadScope scope, / local, global /
• 9 PRThreadState state, / joinable? /
• 10 PRInt32 stacksize )

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Thread Private Data

• Think of thread private data as global
  variables, but for exclusive use for a single
  thread.
• Void (PR_ThreadPrivateDTOR)(void)
• PRStatus PR_NewThreadPrivateIndex( int ndx,
  (dtor))
• PRStatus PR_SetThreadPrivate( ndx, priv )
• void PR_GetThreadPrivate( ndx )
• See mozilla/nsprpub/pr/tests/tpd.c

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Thread sync using PRLock

• 1 include prlock.h
• 2 PRLock lock
• 3 lock PR_NewLock()
• 4 PR_Lock(lock)
• 5 / operate on your data /
• 6 PR_Unlock(lock)
• 7 PR_DestroyLock(lock)

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Using Condition Variables

• 1 PRIntn data 0 / Thread A /
• 2 PRLock lock PR_NewLock()
• 3 PRCondVar cvar PR_NewCondVar(lock)
• 4 PR_Lock(lock)
• 5 while( data 0 )
• 6 PR_WaitCondVar( cvar, timeout )
• / lock released while blocked. /
• / re-locked at fall through /
• 7 data 0 / satisfy Thread Bs condition /
• 8 PR_NotifyCondVar( cvar )
• 9 PR_Unlock( lock )

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Using Condition Variables

• / Thread B /
• 4 PR_Lock(lock)
• 5 while( data 1 )
• 6 PR_WaitCondVar( cvar, timeout )
• / lock released while blocked. /
• / re-locked at fall through /
• 7 data 1
• 8 PR_NotifyCondVar( cvar )
• 9 PR_Unlock( lock )

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What Not to Do

• 1 PRLock lock
• 2 PRCondVar cv
• 3 PR_Lock( lock )
• 4 PR_WaitCondVar( cv, PR_INTERVAL_NO_TIMEOUT )
• / implies state in cv /
• 5 / do something /
• 6 PR_Unlock( lock )
• There is no state in a PRCondVar.

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More What Not to Do

• 1 PRBool dataIsAvailable PR_FALSE
• 2 PR_Lock( lock )
• 3 if ( !dataIsAvailable )
• 4 PR_WaitCondVar( cv, PR_INTERVAL_NO_TIMEOUT
  )/ Bad! dataIsAvailable may change during wait
  /
• 5 / work on the available data /
• 6 PR_Unlock( lock )
• The lock is released during PR_WaitCondvar().
  Other threads can change dataIsAvailable. Use the
  while() looptest, wait, test again.

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Condvar Example

• A hacked cvar.c goes here.

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High Performance Threads

• you are protecting data, not code
• partition assets so that you protect the smallest
  piece possible
• hold exclusive control as short a time as possible

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Bibliography

• David R. Butenhof, "Programming with POSIX
  Threads", Addison-Wesley, 1997
• Steve Kleiman, et.al, Programming with Threads,
  Prentice Hall, 1996
• NSPR group, NSPR Reference, Ch. 1, 3, 5, 6.
  http//www.mozilla.org/projects/nspr/reference/htm
  l/index.html
• UseNet newsgroup comp.programming.threads.