Threads

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Threads
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Threads Increase Responsiveness and Efficiency

• Responsiveness
• Created and switched quicker than processes
• Efficiency
• Blocked threads can be switched with one that can
  make progress

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Multi-Thread Task Tree
Single Threaded Process Task Tree
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Reasons to Use Threads

• Creation and switching speed vs. processes
• Simultaneous Operations
• Concurrency
• Illusion of multitasking
• Not actually more efficient
• Multiprocessing
• Actually Multitasking

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Concurrency vs Multiprocessing
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Reasons to Use Threads cont.

• Resource Sharing
• Static Variables
• Memory image in physical memory
• Modularity
• Code in modules that work independantly of each
  other
• Good for problem solving
• Good for customizing general programs

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Common Implementations

• Operating Systems
• Introduced to the Microsoft users with Win32
• Increase program flexibility
• Helpful in event driven programming
• Web Servers
• Handle Client requests
• Master/Slave architecture
• One main process, many slave threads
• Pool Architecture
• Set Number of re-usable threads
• Must be tweaked

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PROBLEMS

• And Solutions

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Philosophers (Threads)
Rice Bowl (Processor)
Chopsticks (Resources)
Eating (Performing Task)
DEADLOCK!!!!!!
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Prevent Deadlocks

• Do not hold any resources until all are free
• Time-Out feature

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Philosophers (Threads)
Rice Bowl (Processor)
Chopsticks (Resources)
Eating (Performing Task)
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Other Problems
And Solutions

• Excessive Threading
• Program spends more time in context switches than
  executing useful code
• Solution is Common Sense
• Use threads only when needed
• Impose thread creation limits

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Other Problems
And Solutions

• Asynchronous Threading
• Race Conditions
• Value of object is determined by which thread
  modifies it first
• Solution is Semaphores
• Create Critical sections
• Set up flag system to only allow one thread in at
  a time.

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Other Problems
And Solutions

• Debugging Issues
• Non Sequential Program Execution
• Can no longer debug line for line
• Decreased debugging speed may not detect
  deadlocks / race conditions
• Solution is Time stamped Logs
• Every thread creates an execution log with a time
  stamp followed by thread ID
• Use consolidation program to create master log