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

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.

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

• 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).