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Process IDs

Process IDs. Process IDs are 16-bit numbers that are assigned sequentially by Linux as new processes are created. Printing the Process ID #include &lt;stdio.h&gt; #include &lt;unistd.h&gt; int main () { printf (“The process ID is %d<br>”, (int) getpid ());

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Process IDs

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  1. Process IDs • Process IDs are 16-bit numbers that are assigned sequentially by Linux as new processes are created. • Printing the Process ID #include <stdio.h> #include <unistd.h> int main () { printf (“The process ID is %d\n”, (int) getpid ()); printf (“The parent process ID is %d\n”, (int) getppid ()); return 0; }

  2. Process IDs • Viewing Active Processes % ps PID TTY TIME CMD 21693 pts/8 00:00:00 bash 21694 pts/8 00:00:00 ps • % ps -e -o pid,ppid,command The -e option instructs ps to display all processes running on the system.The -o pid,ppid,command option tells ps what information to show about each process in this case, the process ID, the parent process ID, and the command running in this process. • % ps -e -o pid,ppid,command PID PPID COMMAND 1 0 init [5] 2 1 [kflushd] 3 1 [kupdate] ... 21725 21693 xterm 21727 21725 bash 21728 21727 ps -e -o pid,ppid,command

  3. Killing a Process • The kill command works by sending the process a SIGTERM, or termination,signal. • % kill -KILL pid

  4. Creating Processes • Using system #include <stdlib.h> int main () { int return_value; return_value = system (“ls -l /”); return return_value; } • Using fork and exec

  5. Creating Processes • Using fork to Duplicate a Program’s Process #include <stdio.h> #include <sys/types.h> #include <unistd.h> int main () { pid_t child_pid; printf (“the main program process ID is %d\n”, (int) getpid ()); child_pid = fork (); if (child_pid != 0) { printf (“this is the parent process, with id %d\n”, (int) getpid ()); printf (“the child’s process ID is %d\n”, (int) child_pid); } else printf (“this is the child process, with id %d\n”, (int) getpid ()); return 0; }

  6. Process Scheduling • % nice -n 10 sort input.txt > output.txt

  7. Signals • The Linux system sends signals to processes in response to specific conditions. For instance, SIGBUS (bus error), IGSEGV (segmentation violation), and SIGFPE (floating point exception) may be sent to a process that attempts to perform an illegal peration.

  8. Signals • Using a Signal Handler #include <signal.h> #include <stdio.h> #include <string.h> #include <sys/types.h> #include <unistd.h> sig_atomic_t sigusr1_count = 0; void handler (int signal_number) { ++sigusr1_count; } int main () { struct sigaction sa; memset (&sa, 0, sizeof (sa)); sa.sa_handler = &handler; sigaction (SIGUSR1, &sa, NULL); /* Do some lengthy stuff here. */ /* ... */ printf (“SIGUSR1 was raised %d times\n”, sigusr1_count); return 0; }

  9. Waiting for Process Termination • int main () { int child_status; /* The argument list to pass to the “ls” command. */ char* arg_list[] = { “ls”, /* argv[0], the name of the program. */ “-l”, “/”, NULL /* The argument list must end with a NULL. */ }; /* Spawn a child process running the “ls” command. Ignore the returned child process ID. */ spawn (“ls”, arg_list); /* Wait for the child process to complete. */ wait (&child_status); if (WIFEXITED (child_status)) printf (“the child process exited normally, with exit code %d\n”, WEXITSTATUS (child_status)); else printf (“the child process exited abnormally\n”); return 0; }

  10. Waiting for Process Termination • waitpid function can be used to wait for a specific child process to exit instead of any child process. • wait3 function returns CPU usage statistics about the exiting child process, • wait4 function allows you to specify additional options about which processes to wait for.

  11. Zombie Processes • #include <stdlib.h> #include <sys/types.h> #include <unistd.h> int main () { pid_t child_pid; /* Create a child process. */ child_pid = fork (); if (child_pid > 0) { /* This is the parent process. Sleep for a minute. */ sleep (60); } else { /* This is the child process. Exit immediately. */ exit (0); } return 0; }

  12. Threads • Threads,like processes are a mechanism to allow a program to do more than one thing at a time.As with processes, threads appear to run concurrently; the Linux kernel schedules them asynchronously, interrupting each thread from time to time to give others a chance to execute.

  13. Threads • #include <pthread.h> #include <stdio.h> void* print_xs (void* unused) { while (1) fputc (‘x’, stderr); return NULL; } int main () { pthread_t thread_id; pthread_create (&thread_id, NULL, &print_xs, NULL); /* Print o’s continuously to stderr. */ while (1) fputc (‘o’, stderr); return 0; }

  14. Passing Data to Threads #include <pthread.h> #include <stdio.h> struct char_print_parms { char character; int count; }; void* char_print (void* parameters) { struct char_print_parms* p = (struct char_print_parms*) parameters; int i; for (i = 0; i < p->count; ++i) fputc (p->character, stderr); return NULL; } int main () { pthread_t thread1_id; pthread_t thread2_id; struct char_print_parms thread1_args; struct char_print_parms thread2_args; thread1_args.character = ’x’; thread1_args.count = 30000; pthread_create (&thread1_id, NULL, &char_print, &thread1_args); thread2_args.character = ’o’; thread2_args.count = 20000; pthread_create (&thread2_id, NULL, &char_print, &thread2_args); thread2_args.character = ’o’; thread2_args.count = 20000; pthread_create (&thread2_id, NULL, &char_print, &thread2_args);

  15. Module 6: CPU Scheduling • Basic Concepts （基本概念） • Scheduling Criteria （调度准则） • Scheduling Algorithms （调度算法） • Multiple-Processor Scheduling （多处理器调度） • Real-Time Scheduling （实时调度） • Algorithm Evaluation （算法评估）

  16. Basic Concepts • Maximum CPU utilization obtained with multiprogramming （通过多道程序设计得到CPU的最高利用率） • CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait. （CPU-I/O脉冲周期 - 进程的执行包括进程在CPU上执行和等待I/O） • CPU burst distribution （CPU脉冲的分布）

  17. Alternating Sequence of CPU And I/O Bursts

  18. Histogram of CPU-burst Times

  19. CPU Scheduler • Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them.（选择内存中的就绪进程，并分配CPU给其中之一） • CPU scheduling decisions may take place when a process （CPU调度可能发生在当一个进程）: 1. Switches from running to waiting state（从运行转到等待）. 2. Switches from running to ready state（从运行转到就绪）. 3. Switches from waiting to ready（从等待转到就绪）. 4. Terminates（终止运行）. • Scheduling under 1 and 4 is nonpreemptive （发生在1、4两种情况下的调度称为非抢占式调度）. • All other scheduling is preemptive （其他情况下发生的调度称为抢占式调度）.

  20. Dispatcher • Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves（进程调度模块负责将对CPU的控制权转交给由CPU调度程序，包括）: • switching context（切换上下文） • switching to user mode（切换到用户态） • jumping to the proper location in the user program to restart that program（跳转到用户程序的适当位置并重新运行之） • Dispatch latency – time it takes for the dispatcher to stop one process and start another running（调度时间 – 调度程序终止一个进程的运行并启动另一个进程运行所花的时间）.

  21. Scheduling Criteria • CPU utilization – keep the CPU as busy as possible （CPU利用率 – 使CPU尽可能的忙碌） • Throughput – the number of processes that complete their execution per time unit（吞吐量 – 单位时间内运行完的进程数） • Turnaround time – the interval from submission to completion （周转时间 – 进程从提交到运行结束的全部时间 ） • Waiting time – amount of time a process has been waiting in the ready queue（等待时间 – 进程在就绪队列中等待调度的时间片总和 ） • 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)（响应时间 – 从进程提出请求到 首次被响应[而不是输出结果]的时间段[在分时系统环境下] ）

  22. Optimization Criteria • Max CPU utilization （最大的CPU利用率） • Max throughput （最大的吞吐量） • Min turnaround time （最短的周转时间） • Min waiting time （最短的等待时间） • Min response time （最短的响应时间）

  23. First-Come, First-Served (FCFS) Scheduling P1 P2 P3 0 24 27 30 • Example: ProcessBurst Time P1 24 P2 3 P3 3 • Suppose that the processes arrive in the order（假定进程到达顺序如下）: P1 , P2 , P3 The Gantt Chart for the schedule is（该调度的Gantt图为）: • Waiting time（等待时间）for P1 = 0; P2 = 24; P3 = 27 • Average waiting time（平均等待时间）: (0 + 24 + 27)/3 = 17

  24. FCFS Scheduling (Cont.) Suppose that the processes arrive in the order （假定进程到达顺序如下）P2 , P3 , P1 . • The Gantt chart for the schedule is （该调度的Gantt图为） : • Waiting time （等待时间）for P1 = 6;P2 = 0; P3 = 3 • Average waiting time （平均等待时间） : (6 + 0 + 3)/3 = 3 • Much better than previous case（比前例好得多）. • Convoy effect short process behind long process （此种结果产生是由于长进程先于短进程到达） P2 P3 P1 0 3 6 30

  25. 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.（关联到每个进程下次运行的CPU脉冲长度，调度最短的进程） • Two schemes: • nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst（非抢占式调度 – 一旦进程拥有CPU，它的使用权限只能在该CPU 脉冲结束后让出). • Preemptive – if a new process arrives with CPU burst length less than remaining time of current 法executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF).（抢占式调度 – 发生在有比当前进程剩余时间片更短的进程到达时，也称为最短剩余时间优先调度） • SJF is optimal – gives minimum average waiting time for a given set of processes.（SJF是最优的 – 对一组指定的进程而言，它给出了最短的平均等待时间）

  26. Example of Non-Preemptive SJF Process Arrival TimeBurst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • SJF (non-preemptive) • Average waiting time = (0 + 6 + 3 + 7)/4 - 4 P1 P3 P2 P4 0 3 7 8 12 16

  27. Example of Preemptive SJF Process Arrival TimeBurst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • SJF (preemptive) • Average waiting time = (9 + 1 + 0 +2)/4 - 3 P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7

  28. Determining Length of Next CPU Burst • Can only estimate the length（其长度只能估计）. • Can be done by using the length of previous CPU bursts, using exponential averaging（可以通过先前的CPU脉冲长度及计算指 数均值进行）.

  29. Examples of Exponential Averaging •  =0 • n+1 = n • Recent history does not count. •  =1 • n+1 = tn • Only the actual last CPU burst counts. • If we expand the formula, we get: n+1 =  tn+(1 - )  tn -1 + … +(1 -  )j  tn -1 + … +(1 -  )n=1 tn 0 • Since both  and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor.

  30. Prediction of the Length of the Next CPU Burst

  31. Priority Scheduling • A priority number (integer) is associated with each process（每个进程都有自己的优先数[整数]） • The CPU is allocated to the process with the highest priority (smallest integer  highest priority)（CPU分配给最高优先级的进程[假定最小的整数  最高的优先级]）. • Preemptive（抢占式） • Nonpreemptive （非抢占式） • SJF is a priority scheduling where priority is the predicted next CPU burst time（SJF是以下一次CPU脉冲长度为优先数的优先级调度）. • Problem  Starvation – low priority processes may never execute （问题  饥饿 – 低优先级的可能永远得不到运行）. • Solution  Aging – as time progresses increase the priority of the process （解决方法  老化 – 视进程等待时间的延长提高其优先数）.

  32. Round Robin (RR) • Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue （每个进程将得到小单位的CPU时间[时间片]，通常为10-100毫 秒。时间片用完后，该进程将被抢占并插入就绪队列末尾）. • If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units（假定就绪队列中有n个进程、时间量为q, 则每个进程每次得到1/n的、不超过q单位的成块CPU时间，没有任何一个进程的等待时间会超过(n-1) q单位）. • Performance（特性） • q large  FIFO • q small  q must be large with respect to context switch, otherwise overhead is too high（q相对于切换上下文的时间而言不许足够长，否则将导致系统开销过大）.

  33. Example: RR with Time Quantum = 20 P1 P2 P3 P4 P1 P3 P4 P1 P3 P3 ProcessBurst Time P1 53 P2 17 P3 68 P4 24 • The Gantt chart is: • Typically, higher average turnaround than SJF, but better response（典型的说，RR的平均周转时间比SJF长，但响应时间要短一些）. 0 20 37 57 77 97 117 121 134 154 162

  34. How a Smaller Time Quantum Increases Context Switches

  35. Turnaround Time Varies With The Time Quantum

  36. Multilevel Queue • Ready queue is partitioned into separate queues（就绪队列分为）:foreground (interactive)（前台）[交互式]background (batch) （后台） [批处理] • Each queue has its own scheduling algorithm （每个队列有自己的调度算法）foreground – RRbackground – FCFS • Scheduling must be done between the queues（调度须在队列间进行）. • Fixed priority scheduling; i.e., serve all from foreground then from background. Possibility of starvation（固定优先级调度，即前台运行完后再运行后台。有可能产生饥饿）. • Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; e.g.,80% to foreground in RR 20% to background in FCFS （给定时间片调度，即每个队列得到一定的CPU时间，进程在给定时间内执行；如，80%的时间执行前台的RR调度，20%的时间执行后台的FCFS调度）

  37. Multilevel Queue Scheduling

  38. Multilevel Feedback Queue • A process can move between the various queues; aging can be implemented this way（进程能在不同的队列间移动；可实现老化）. • Multilevel-feedback-queue scheduler defined by the following parameters（多级反馈队列调度程序由以下参数定义）: • number of queues（队列数） • scheduling algorithms for each queue（每一队列的调度算法） • method used to determine when to upgrade a process（决定进程升级的方法） • method used to determine when to demote a process（决定进程降级的方法） • method used to determine which queue a process will enter when that process needs service（决定需要服务的进程将进入哪个队列的方法）

  39. Multilevel Feedback Queues

  40. Example of Multilevel Feedback Queue • Three queues: • Q0 – time quantum 8 milliseconds • Q1 – time quantum 16 milliseconds • Q2 – FCFS • Scheduling • A new job enters queue Q0which is servedFCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1（新的作业进入FCFS的Q0队列，它得到CPU时能使用8毫秒，如果它不能在8毫秒内完成，将移动到队列Q1 ） . • At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2（作业在Q1仍将作为FCFS调度，能使用附加的16毫秒，如果它还不能完成，将被抢占，移至队列Q2 ）.

  41. Multiple-Processor Scheduling • CPU scheduling more complex when multiple CPUs are available（多个CPU可用时，CPU调度将更为复杂）. • Homogeneous processors within a multiprocessor （多处理器内的同类处理器）. • Load sharing（负载共享） • Symmetric Multiprocessing (SMP) – each processor makes its own scheduling decisions（对称多处理器 – 每个处理器决定自己的调度方案）. • Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing. （非对称多处理器 – 仅一个处理器能处理系统数据结构，这就减轻了对数据的共享需求）

  42. Real-Time Scheduling • Hard real-time systems – required to complete a critical task within a guaranteed amount of time. （硬件实现的实时系统 – 用于实现要求在给定的时间内执行完临界进程的场合） 比如航天中的宇宙飞船的控制等就是现实中这样的系统. • Soft real-time computing – requires that critical processes receive priority over less fortunate ones. （软件实现的实时计算 – 当要求临界进程得到比其他进程更高的优先级时）

  43. Linux内核在实时性方面的不足 • 1）Linux分为用户态和内核态两种模式，进程运行在用户态时，实时进程具有高的优先级，能进行进程抢占，故可以较好的完成任务；运行在内核态时，如系统调用，实时进程不能抢占该进程。因此，从实质上来说，Linux内核是非抢占式的。

  44. 内核kernel 将系统的一些关键性程序分离出来构成操作系统内核 .内核必须完成下面的一些任务： • 管理对文件系统的读写，把对文件系统的操作映射成对磁盘或其他块设备的操作 • 管理程序的运行，为程序分配资源，并且处理程序之间的通讯 • 管理存储器，为程序分配内存，并且管理虚拟内存 • 管理输入输出，将设备映射成设备文件 管理网络

  45. LKMs就是可卸载的内核模块(Loadable Kernel Modules)。这些模块本来是Linux系统用于扩展他的功能的。

  46. 2）在定时器方面，有下列几方面缺陷：第一，Linux的周期模式定时器频率仅为100Hz，远不能满足多种实时应用的要求。第二，软定时由时钟定时器完成，当软定时器较多时，势必引起共享时钟定时器的冲突。第三，Linux中断句柄不可调度，但在实时系统中，期望能在一个可调度整体内处理这些中断句柄，从而能更有效的区分不同实时任务的紧迫度，分配不同的优先级。因此，单纯缩短时间片在对实时性能严格要求的场合是不受欢迎的。

  47. （3）Linux进程采用多级轮转调度算法，该调度算法，仅能获得秒级响应时间，一个实时进程在一个时间片内未完成，其优先级将降低，从而可能造成到截止时间无法完成。（3）Linux进程采用多级轮转调度算法，该调度算法，仅能获得秒级响应时间，一个实时进程在一个时间片内未完成，其优先级将降低，从而可能造成到截止时间无法完成。

  48. 4）Linux虽然给实时进程提供了较高的优先级，但是，并没有加入时间限制。例如完成的最后期限、应在多长时间内完成、执行周期等等。同时，其它大量的非实时进程也可能对实时进程造成阻塞，无法确保实时进程的响应时间。另外，当有多个实时进程互斥请求共享资源时，由于其使用的同步原语不支持优先级继承协议（PIP），易产生优先级倒置。

  49. 当前Linux在实时内核方面的发展 • 1)兼容内核方法。即含有一个兼容Linux内核，符合POSIX的API层的实时内核 • 2)双内核方法。双内核系统在硬件平台上增加一个实时内核，建立双内核，如现在流行的RT-Linux • 3)内核修改方法。通过修改Linux内核源码来实现。如RED-Linux就采用该方法。在内核代码中增加抢占点，从而减少内核抢占延迟。

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