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Operating Systems COMP 4850/CISG 5550

Operating Systems COMP 4850/CISG 5550. Threads, Part II Dr. James Money. 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. Thread Usage. Thread Usage.

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Operating Systems COMP 4850/CISG 5550

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  1. Operating SystemsCOMP 4850/CISG 5550 Threads, Part II Dr. James Money

  2. 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

  3. Thread Usage

  4. Thread Usage

  5. Thread Usage • – Dispatcher Thread • - Worker Thread

  6. Implementing Threads • There are two basic ways to implement threads: • In User Space • In Kernel Space • Each has it’s own advantages and disadvantages

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

  8. User Space Threads

  9. 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

  10. 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

  11. Kernel Space Threads

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

  13. Hybrid Implementations

  14. 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.

  15. Multithreaded systems • Possible overwriting of global variables:

  16. Multitheaded Systems • Solutions: • Multiple global variables similar to separate stacks • Thread wide global variable library

  17. Multithreaded Systems

  18. Private Library • create_global(“bufptr”); • set_global(“bufptr”,&buf); • bufptr=read_global(“bufptr”);

  19. 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.

  20. 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

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