Operating Systems CMPSC 473

1. Operating SystemsCMPSC 473 Processes (4) September 19 2008 - Lecture 10 Instructor: Bhuvan Urgaonkar

2. Overview of Process-related Topics • How a process is born • Parent/child relationship • fork, clone, … • How it leads its life • Loaded: Later in the course • Executed • CPU scheduling • Context switching • Where a process “lives”: Address space • OS maintains some info. for each process: PCB • Process = Address Space + PCB • How processes request services from the OS • System calls • How processes communicate • Some variants of processes: LWPs and threads • How processes die Done Partially done

3. Kernel Mode Stack PCB (task_struct) • Since KM stacks make little use of the stack, only a few thousand bytes suffice • An example of “Design for the most common case”, we’ll see more • Linux: 8KB , thread_info 52 bytes Stack esp task thread_info structure curent thread_info

4. Kernel Mode Stack PCB (task_struct) • Since KM stacks make little use of the stack, only a few thousand bytes suffice • An example of “Design for the most common case”, we’ll see more • Linux: 8KB • Why combine KM stack and thread_info into a union? Stack esp task thread_info structure curent thread_info • union thread_union { • struct thread_info thread_info; • unsigned long stack[2048]; • };

5. Kernel Mode Stack PCB (task_struct) • Since KM stacks make little use of the stack, only a few thousand bytes suffice • An example of “Design for the most common case”, we’ll see more • Linux: KM Stack 8KB, thread_info 52 bytes • Why combine KM stack and thread_info into a union? • You might think spatial locality • The kernel can easily obtain the address of the thread_info structure of the process currently running on the CPU from the value of the esp register • task field is at offset 0 • Other benefits apply to multi-processors: makes it easy to efficiently find the current process on each processor • Earlier approach: Have an array of current pointers Stack esp task thread_info structure curent thread_info • union thread_union { • struct thread_info thread_info; • unsigned long stack[2048]; • };

6. Resource Limits • Kernel imposes limits on the amount of resources a process may have • E.g., address space size, open files • Linux: For each resource, for each process struct rlimit { unsigned long rlim_cur; unsigned long rlim_max; } • System calls: getrlimit (), setrlimit () to increase upto some max allowed by kernel

7. Relationships among processes • Several relatives of P recorded in its PCB • real_parent • Process that called fork to create P • Or init (process 1) • parent • Usually real_parent • Kernel signals this parent process when child exits • Or process that issues ptrace () to monitor P • Pop quiz: If you run a background process and exit the shell, who is the parent of the process? • children • siblings • Why maintain these?

8. Process Switch • Suspend the current process and resume a previously suspended process • Also called context switch or task switch • What does the kernel need to save when suspending a process? • Hint: The entire address space is already saved (either in memory or on swap space). What else would the process need when it has to be resumed?

9. Process Switch • Suspend the current process and resume a previously suspended process • Also called context switch or task switch • What does the kernel need to save when suspending a process? • Hint: The entire address space is already saved (either in memory or on swap space). What else would the process need when it has to be resumed? • CPU registers • This is called the hardware context of the process • Linux: Part of h/w context saved within PCB, rest on kernel mode stack (why?)

10. Process Switch • So process switch involves • Saving h/w context of currently running process • Restoring h/w context of process to resume • Who decides which process to resume?

11. Process Switch • So process switch involves • Saving h/w context of currently running process • Restoring h/w context of process to resume • Who decides which process to resume? • The CPU scheduler • Generic code for a process switch next = schedule (prev); switch_to (next, prev); ------> GORY! SKIPPING!! blah blah blah • Note: next starts executing not blah. How? • Project 1 will give you a taste of this :)

12. Creating Processes • fork () • Take 1: Used in older kernels • Create a copy of the entire address space of the parent • Create a new PCB for the new process • Update parent, children, sibling pointers • Place the new process in the ready queue • S . L . O . W .

13. Creating Processes • fork () • Problems with Take 1 • Child rarely needs to read/modify ALL the resources inherited from the parent • Often, it immediately issues an execve() rendering all the effort that went into copying the address space useless!

14. Creating Processes • fork () • What modern kernels do to avoid this waste of precious CPU time Something in the way she moos … COW!!

15. Creating Processes: COW • fork () • What modern kernels do to avoid this waste of precious CPU time • Use COW! • Copy-On-Write • Basic idea: Postpone work till the last minute • Sounds familiar? Think assignments, quizzes, …