AdministriviaProcessesA process's view of the worldSpeedProcesses in the real worldInter-Process CommunicationRest of lectureUNIX files I/OError returnsOperations on file descriptorsFile descriptor numbers exttt {type.c}Creating processesDeleting processesRunning programs exttt {minish.c} (simplified)Manipulating file descriptors exttt {redirsh.c}Pipes exttt {pipesh.c} (simplified)Why fork?Spawning process w/o forkImplementing processesProcess statesSchedulingScheduling policyPreemptionContext switchContext switch detailsThreadsWhy threads?Thread package APIKernel threadsWhy kernel threads suckUser threadsImplementing user-level threadsBackground: calling conventionsBackground: procedure callsThreads vs. proceduresExample user threads implementationi386 thread_md_switchWhy user threads suckUthreads on kthreadsProblemsLessonsAdministrivia• Lab 1 will be up by tomorrow, Due Oct. 11- Due at start of lecture – 4:15pm- Free extension to midnight if you come to lecture- Or for SCPD students only if you w atch lecture live•No credit for late assignments w/o extension- Ask course staff for an extension if you can’t finish on time- We are nice people- . . . but can’t help you if you miss deadlines and don’t talk to us• Section tomorrow, 4:15pm, this room• Pass/fail allowed, but not advisable- You must tell your team members if you are enrolled pass/fail- You will probably work almost as hard anyway– p.1/45Process e s• A process is an instance of a program running• Modern OSes run multiple processes simultaneously• Examples (can all run simultaneously):-g file_A.– compiler running on file A-g file_B.– compiler running on file B-emas– text editor-firefox– web browser• Non-examples (implemented as one process):- Multiple firefox windows or emacs frames (still one process)• Why processes?- Simplicity of programming- Higher throughput (better CPU utilization), lower latency– p.2/45A process’s view of the world• Each proc. Pihas own view of machine- Its own address space- Its own open files- Its own virtual CPU (through preeptivemultitasking)•*(har *)0x000different in P1& P2• Greatly simplifies programming model-gdoes not care thatfirefoxis running• Sometimes want interaction between processes- Simplest is through files:emasedits file,gcompiles it- More complicated: Shell/command, Window manager/app.– p.3/45Speed• Multiple processes can increase CPU utilization- Overlap one process’s computation with another’s w ait• Multiple processes can reduce latency- Running A then B requires 100 sec for B to complete- Running A and B concurrently makes B finish faster- A slightly slower, but less than 100 sec unless A and B bothcompletely CPU-bound– p.4/45Process e s in the re al world• Processes, parallelism fact of life much longer thanOSes have been around- E.g., say takes 1 worker 10 months to make 1 widget- Company may hire 100 workers to make 10,000 widgets- Latency for first w idget >> 1/10 month- Throughput may be < 10 widgets per month(if can’t perfectly parallelize task)- Or > 10 widgets per month if better utilization (e.g., 100workers on 10,000 widgets never idly waiting for paint to dry)• You will see this with Pintos- Don’t expect labs to take 1/3 time with three people– p.5/45Inter-Process Communication• How can processes interact in real tim e?(a) By passing messages through the kernel(b) By sharing a region of physical memory(c) Through asynchronous sig nals or alerts– p.6/45Rest o f lecture• User view of processes- Crash course in basic Unix/Linux system call interface- How to create, kill, and communicate between processes• Kernel v ie w of process e s- Implementing processes in the kernel• Threads• How to implement threads– p.7/45UNIX files I/O• Applications “open” files (or devices) by name- I/O happens through open files•int open(har *path, int flags, /*mode*/...);-flags:O_RDONLY,O_WRONLY,O_RDWR-O_CREAT: create the file if non-existent-O_EXCL: (w.O_CREAT) create if file exists already-O_TRUNC: Truncate the file-O_APPEND: Start writing from end of file-mode: final argument withO_CREAT• Returns file descriptor—used for a ll I/O to file– p.8/45Error returns• What ifopenfails? Returns -1 (invalid fd)• Most s y s te m ca lls return -1 on failure- Specific kind of error in global interrno•#inlude <sys/errno.h>for possible values- 2 =ENOENT“No such file or directory”- 13 =EACCES“Permission Denied”•perrorfunction prints human-readable message-perror ("initfile");→ “initfile: No suh file or diretory”– p.9/45Operations o n file descriptors•int read (int fd, void *buf, int nbytes);- Returns number of bytes read- Returns 0 bytes at end of file, or -1 on error•int write (int fd, void *buf, int nbytes);- Returns number of bytes written, -1 on error•off_t lseek (int fd, off_t pos, int whene);-whene: 0 – start, 1 – current, 2 – end- Returns previous file offset, or -1 on error•int lose (int fd);– p.10/45File des criptor numbers• File descriptors a re inherited by proces s e s- When one process spawns another, same fds by default• Descriptors 0, 1, and 2 have special meaning- 0 – “standard input” (stdinin ANSI C)- 1 – “standard output” (stdout, printfin ANSI C)- 2 – “standard error” (stderr, perrorin ANSI C)- Normally all three attached to terminal• Example:type.- Prints the contents of a file tostdout– p.11/45type.voidtypefile (char *filename){int fd, nread;char buf[1024];fd = open (filename, O_RDONLY);if (fd == -1) {perror (filename);return;}while ((nread = read (fd, buf, sizeof (buf))) > 0)write (1, buf, nread);close (fd);}– p.12/45Creating processes•int fork (void);- Create new process that is exact copy of current one- Returns process ID of new proc. in “parent”- Returns 0 in “child”•int waitpid (int pid, int *stat, int opt);-pid– process to wait for, or -1 for any-stat– will contain exit value, or signal-opt– usually 0 orWNOHANG- Returns process ID or -1 on error– p.13/45Deleting proc esses•void exit (int status);- Current process ceases to exist-statusshows up inwaitpid(shifted)- By convention,statusof 0 is success, non-zero error•int kill (int pid, int sig);- Sends signalsigto processpid-SIGTERMmost common value, kills process by default(but application can catch it for “cleanup”)-SIGKILLstronger, kills process always–
View Full Document
Unlocking...