Operating Systems COMP 4850/CISG 5550

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Operating SystemsCOMP 4850/CISG 5550

• Threads, Part II
• Dr. James Money

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Thread Usage

• Why are they used?
• Programs block, but might have other tasks to
  perform
• Easier to create/destroy than processes
• Increase in performance
• Multiple CPU utilization

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Thread Usage
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Thread Usage
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Thread Usage

1. Dispatcher Thread
2. - Worker Thread

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Implementing Threads

• There are two basic ways to implement threads
• In User Space
• In Kernel Space
• Each has its own advantages and disadvantages

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User Space Threads

• The OS knows nothing about threads.
• Each process has its own user space thread table
  similar to the process table

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User Space Threads
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User Space Threads

• Advantages
• No traps to kernel mode better performance
• Custom scheduling algorithms
• Scales well
• Disadvantages
• Dealing with blocking system calls
• Page faults block all threads
• Requires voluntary yielding of CPU

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Kernel Space Threads

• Kernel maintains thread table
• When a system call blocks, the kernel can run
  another thread automatically
• Threads tend to get recycled not actually
  destroyed since they require a trap

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Kernel Space Threads
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Hybrid Implementations

• Research has been done on implementing kernel and
  user space threads at the same time.
• The kernel knows about its threads, but the
  process may have multiple user space threads for
  one kernel thread

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Hybrid Implementations
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Making Single Threaded Code Multithreaded

• Most programs are written for one thread
• We must convert them to multithread form to use
  them in a threaded system
• This is not easy to perform in general.

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Multithreaded systems

• Possible overwriting of global variables

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Multitheaded Systems

• Solutions
• Multiple global variables similar to separate
  stacks
• Thread wide global variable library

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Multithreaded Systems
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Private Library

• create_global(bufptr)
• set_global(bufptr,buf)
• bufptrread_global(bufptr)

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Multithreaded Systems

• Library procedures are not re-entrant usually!
• Buffered message passing may get overwritten.
• Must rewrite these functions
• Or completely block re-use of the library
  functions when one is in use.

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Multithreaded Systems

• Thread management issues
• Many times the kernel will just extend the stack
  when it overflows
• This can cause two separate thread stacks to
  collide now