Linux

The most ubiquitous, widely used Operating System. Created by Linus Torvalds.

Linux Scheduler

• Market
  • Interactive Desktop, Server
• Goals
  • Efficiency, interactivity, real-time, no Starvation
• 3 Classes of tasks
  • Real-time First Come First Serve, Real-time Round Robin, Timeshared
  • 140 Priority; basically has 140 Ready Queues
    • 0-99 for real-time; remaining for timeshared
    • Carrot and Stick approach
      • Reward Good behavior
      • Punish bad behavior
      • Reward interactive I/O by increasing priority
      • Punish compute bound tasks by decreisng priority
    • Starvation Threshold
      • Set a time interval (starvation goal) for the threshold waiting time that determines a process is being starved by higher priority processes
      • When starvation occurs, give the process a time slice at the highest interactive priority
• Winner is the first task in the highest priority list in the active array
• If the task blocks (due to I/O) put it aside and pick the next highest one to run
• If the time quantum runs out (doesn’t apply to real time tasks) for the current task, place it in the expired array
• On I/O completion, place the relevant task in the active array at the right priority level, having adjusted its remaining time quantum
• When the active array is empty, flip the active and expired array pointers and continue with the scheduling algorithm (i.e. the expired array becomes the active array and vice versa).
• This scheduler is O(1)

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Linux Syscalls

• The system call is the fundamental interface between a Program and the Linux Kernel
• Generally not invoked directly, but rather via glibc wrapper functions

Linux Syscall Table

Syscall	Function	Man Page
open	Request the kernel to open a file, and return its File Descriptor	man
read	Request the kernel to read data from a File Descriptor and place that data into a buffer supplied by the caller	man
write	Request the kernel to write data from a caller specified buffer to the file (or output device) identified by a File Descriptor	man
fork	Request the kernel to duplicate (fork) the provided Process, including it s open File Descriptors	man
exit	Request the kernel to terminate a process	man

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