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Operating Systems CSE 411 CPU Management Sept. 15 2006 - Lecture 5 Instructor: Bhuvan Urgaonkar Today • Quickly revisit – Timer interrupt, system clock – CPU management related data structures – Process creation and termination • Start CPU scheduling • Quiz 1 Timer Interrupt, System Clock Oscillator CLK TSC reg.++ IRQ3 IRQ2 IRQ1 CPU Slower clocks to other components (e.g., memory, PCI bus) IRQ0 tick tick (Set by OS at bootup … coming soon) jiffies++ (Linux) • System clock drives the CPU circuit, no interrupt processing • Timer interrupt handled by OS, less frequent Today • Quickly revisit Timer interrupt, system clock CPU management related data structures – Process creation and termination • Start CPU scheduling • Quiz 1 Data Structure #1: PCB Main Memory (RAM) Process id Program Counter Other registers Process state OS Processes Ptr to linked list … • Can PCBs be swapped out? – Depends on the OS design .. so sometimes YES Data Structure #2: Linked Lists based on Process States Running Ready Lock Waiting Disk Timer interrupt Running Ready Lock Waiting Disk Running Ready Lock Waiting Disk I/O call Running Ready Lock Waiting Disk Hmm .. Who should I pick to run? Running OS (scheduler) Ready Lock Waiting Disk Lets pick the second process in the ready queue Running OS (scheduler) Ready Lock Waiting Disk Today • Quickly revisit Timer interrupt, system clock CPU management related data structures Process creation and termination • Start CPU scheduling Process Creation • When would a new process be created? – When a user of the computer asks one to be created • That is, when an existing process asks for one to be created – Therefore, there is going to be a parent-child relation among processes – There must be a first process, then - the OS creates it during boot-up • Who is going to create it? The OS • What does the OS need to do? – Needs to create a PCB • Assign a unique id to the new process – Bring in the code and data for the new process into RAM – Move the new process to the ready queue, so the OS can schedule it on the CPU Process Creation fork() • Recall: Processes use system calls to request services from the OS • OS provides a system call called fork() that a process can use to request the creation of a new process • OS creates a new PCB when fork() is called with a unique id for the new process • The calling process is the parent, the new process is the child • Then adds the child’s PCB to the ready queue • The child process is ready to go! • What code is the child going to execute? Specifying the Code the Child should execute exec() • Recall: Processes use system calls to request services from the OS • OS provides a system call called exec() that a process can use to tell the OS what code it would like to execute • Once the code and data are loaded into memory, the OS sets the PC (in the PCB) to the beginning of the child’s code • What happens to the parent now??? Process Creation: What happens to the parent? • The parent could wait for the child to finish – This is what a shell does • Or it could go about its business Process Creation: Parent wants to wait • Again, the parent process has to request the OS via a system call • The wait() system call • The OS moves the parent to a waiting queue • It would be moved to ready again when the child terminates Process Creation: Parent doesn’t want to wait • Both the parent and the child would run whenever they are ready Process Creation • Why/when are processes created? 1. 2. A user wants to run a program An existing process needs another process to run the same code Why? More parallelism, make use of CPU even when it gets moved to the waiting state • So most OSes initialize the child process to have the same state as the parent • Except the process id and a few other things RAM OS Id=2000 State=ready PCB of parent Id=2001 State=ready PCB of child 1. PCB with new id created 2. Memory allocated for child Process calls fork Initialized by copying over from the parent 3. If parent had called wait, it is moved to a waiting queue 4. If child had called exec, its memory overwritten with new code & data Processes Parent’s memory Child’s memory 5. Child added to ready queue, all set to go now! • • • • • #include <iostream> #include <string> #include <sys/types.h> #include <unistd.h> using namespace std; • int globalVariable = 2; • • • • main() { string sIdentifier; int iStackVariable = 20; • • • • • • • • • • • • • • • • • • • pid_t pID = fork(); if (pID == 0) // child { // Code only executed by child process sIdentifier = "Child Process: "; globalVariable++; iStackVariable++; • • • • • • else if (pID < 0) // failed to fork { cerr << "Failed to fork" << endl; exit(1); // Throw exception } else // parent { // Code only executed by parent process sIdentifier = "Parent Process:"; } // Code executed by both parent and child. cout << sIdentifier; cout << " Global variable: " << globalVariable; cout << " Stack variable: " << iStackVariable << endl; } } Compile: g++ -o ForkTest ForkTest.cpp Run: ForkTest Parent Process: Global variable: 2 Stack variable: 20 Child Process: Global variable: 3 Stack variable: 21 Process Creation and Termination C Program Forking Separate Process int main( ) { pid_t pid; /* fork another process */ pid = fork( ); if (pid < 0) { /* error occurred */ fprintf(stderr, "Fork Failed"); exit(-1); } else if (pid == 0) { /* child process */ execlp("/bin/ls", "ls", NULL); } else { /* parent process */ /* parent will wait for the child to complete */ wait (NULL); printf ("Child Complete"); exit(0); } } Process Termination • Process executes last statement and asks the operating system to delete it (exit) – Output data from child to parent (via wait) – Process’ resources are deallocated by operating system • Parent may terminate execution of children processes (abort) – Child has exceeded allocated resources – Task assigned to child is no longer required – If parent is exiting • Some operating system do not allow child to continue if its parent terminates – All children terminated - cascading termination Today Quickly revisit Timer interrupt, system clock CPU management related data structures Process creation and termination Start CPU scheduling • Quiz 1 Hmm .. Who should I pick to run? Running OS (scheduler) Ready Lock Waiting Disk First-Come, First-Served Scheduling (FCFS) Process Run Time P1 24 P2 3 P3 3 • Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: P1 0 P2 24 • Waiting time for P1 = 0; P2 = 24; P3 = 27 • Average waiting time: (0 + 24 + 27)/3 = 17 P3 27 30 FCFS Scheduling (Cont.) Suppose that the processes arrive in the order P2 , P3 , P1 • The Gantt chart for the schedule is: P2 0 • • • • P3 3 P1 6 Waiting time for P1 = 6; P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case Convoy effect short process behind long process 30 Shortest-Job-First (SJF) Scheduling • Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time • SJF is optimal for avg. waiting time – gives minimum average waiting time for a given set of processes – In class: Compute average waiting time for the previous example with SJF – Prove it (Homework 1, Will be handed out next Friday) • Two schemes: – nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst – preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF) Choosing the Right Scheduling Algorithm/Scheduling Criteria • CPU utilization – keep the CPU as busy as possible • Throughput – # of processes that complete their execution per time unit • Turnaround time – amount of time to execute a particular process • Waiting time – amount of time a process has been waiting in the ready queue • Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment) • Fairness When is the scheduler invoked? • CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state 2. Switches from running to ready state 3. Switches from waiting to ready 4. Terminates • Scheduling only under 1 and 4: nonpreemptive scheduling • All other scheduling is preemptive Dispatcher • Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: – switching context – switching to user mode – jumping to the proper location in the user program to restart that program • Dispatch latency – time it takes for the dispatcher to stop one process and start another running Example from Linux 2.6.x asmlinkage void __sched schedule(void) { [...] prepare_arch_switch(rq, next); prev = context_switch(rq, prev, next); barrier(); finish_task_switch(prev); [...] } task_t * context_switch(runqueue_t *rq, task_t *prev, task_t *next) { struct mm_struct *mm = next->mm; struct mm_struct *oldmm = prev->active_mm; /* Here we just switch the register state and the stack. */ switch_to(prev, next, prev); return prev; } #define switch_to(prev,next,last) \ asm volatile(SAVE_CONTEXT \ "movq %%rsp,%P[threadrsp](%[prev])\n\t" /* saveRSP */ \ "movq %P[threadrsp](%[next]),%%rsp\n\t" /* restore RSP */ \ "call __switch_to\n\t" \ ".globl thread_return\n" \ "thread_return:\n\t" \ "movq %%gs:%P[pda_pcurrent],%%rsi\n\t" \ "movq %P[thread_info](%%rsi),%%r8\n\t" \ LOCK "btr %[tif_fork],%P[ti_flags](%%r8)\n\t" \ "movq %%rax,%%rdi\n\t" \ "jc ret_from_fork\n\t" \ RESTORE_CONTEXT \ : "=a" (last) : [next] "S" (next), [prev] "D" (prev), \ \ [threadrsp] "i" (offsetof(struct task_struct, thread.rsp)), \ [ti_flags] "i" (offsetof(struct thread_info, flags)),\ [tif_fork] "i" (TIF_FORK), \ [thread_info] "i" (offsetof(struct task_struct, thread_info)), \ [pda_pcurrent] "i" (offsetof(struct x8664_pda, pcurrent)) \ : "memory", "cc" __EXTRA_CLOBBER) Shortest Remaining Time First (SRTF) Priority-based Scheduling • UNIX Deadline-based Algorithms Proportional-Share Schedulers Lottery Scheduling • Project 1 Work Conservation Reservation-based Schedulers Rate Regulation Hierarchical Schedulers Problem introduced by I/O-bound processes Scheduler Considerations: Context-Switch Overhead Scheduler Considerations: Quantum Length Scheduler Considerations: Setting Parameters Scheduler Considerations: CPU Accounting Scheduler Considerations: Time and Space Requirements Algorithm Evaluation A Look at the Linux CPU Scheduler • Show a timer interrupt, blocking due to I/O etc. Threads Thread Libraries Thread or Process? Event-driven Programming • Synchronous vs NS I/O Multi-processor Scheduling CPUs with Hyper-threading Process Synchronization Inter-process Communication Deadlocks System Boot-up Booting a Linux Kernel • Next: Memory Management