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os-xv6

os projects - modify xv6 OS (https://github.com/mit-pdos/xv6-public)

Project 01: Implement simple scheduler on xv6

The xv6 scheduler uses "Round-Robin" policy by default. This project modifies the scheduler to "3-level MLFQ".

  • 3-level consists of L0, L1, L2.
    • L0 queue gets the highest priority.
    • Each level's time quantum: Ln = 2n+4 ticks.
  • To prevent starvation, priority boosting occurs once every 100 global ticks.
    • All processes are rescheduled to the L0 queue.
  • There may be processes that should always be prioritized over those scheduled by the MLFQ scheduler.
    • 2 system calls schedulerLock(), schedulerUnlock() are implemented.
    • If there is a process that locks the scheduler, the MLFQ scheduler will not work, and the process will be scheduled first.

Project 02: Implement process manager and threading (LWP)

The xv6 os does not provide threading and process management. This project improves xv6 by adding these features.

  • process manager

    • The xv6 allocates one stack page when exec called. System call exec2 in this project allocates various size of stack (1 to 100 stack pages).
    • System call setmemorylimit limits the maximum amount of memory that can be allocated for a particular process.
    • User program "pmanager" supports process managing functions
      • list - Outputs information about currently running processes
      • kill - Kill a process with the pid
      • execute - Run the program with specified stacksize
      • memlim - set the program's memory limit
      • exit

  • Threading (LWP, light-weight process)

    • Like pthread in Unix, this project provides threading APIs.
    • thread_create, thread_exit, thread_join

Project 03: Improve file system on xv6

  • multi indirect
    • The xv6 stores file data through direct and single indirect method, which limits file write to 140 blocks (70KB) max.
    • This project adds multi indirect, so that created file has 10 direct (10 blocks), 1 single indirect (128 blocks), 1 double direct (128 * 128 blocks), and 1 triple indirect (128 * 128 * 128 blocks).
    • Resulting maximum file size will be 2,113,674 blocks.
  • symbolic link
    • The xv6 supports hard link, not symbolic link.
    • This project implements symbolic link on xv6, so you can create shortcuts that are not dependent to the original file.
    • ln [-s] creates symbolic link.
  • sync
    • The xv6 performs group flush for write operation. This approach degrades performance when a particular process generates a large number of write operations.
    • Instead, modified xv6 performs "Buffered I/O", which does not flush data by default.
    • Only flushes data when the system call sync called, and when the buffer is full (current buffer size = 10 * 3 * 512B).