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CMU CS 15213 - Concurrent Programming

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Concurrent Programming November 20, 2003Concurrent Programming is Hard!Iterative ServersFundamental Flaw of Iterative ServersConcurrent Servers: Multiple ProcessesThree Basic Mechanisms for Creating Concurrent FlowsProcess-Based Concurrent ServerProcess-Based Concurrent Server (cont)Slide 9Implementation Issues With Process-Based DesignsPros and Cons of Process-Based DesignsEvent-Based Concurrent Servers Using I/O MultiplexingThe select FunctionMacros for Manipulating Set Descriptorsselect Exampleselect Example (cont)Slide 17Event-based Concurrent Echo ServerEvent-based Concurrent Server (cont)Slide 20Slide 21Slide 22Pro and Cons of Event-Based DesignsTraditional View of a ProcessAlternate View of a ProcessA Process With Multiple ThreadsLogical View of ThreadsConcurrent Thread ExecutionThreads vs. ProcessesPosix Threads (Pthreads) InterfaceThe Pthreads "hello, world" ProgramExecution of Threaded“hello, world”Thread-Based Concurrent Echo ServerThread-Based Concurrent Server (cont)Issues With Thread-Based ServersPros and Cons of Thread-Based DesignsConcurrent ProgrammingNovember 20, 2003Concurrent ProgrammingNovember 20, 2003TopicsTopicsLimitations of iterative serversProcess-based concurrent serversEvent-based concurrent serversThreads-based concurrent servers15-213“The course that gives CMU its Zip!”class26.ppt– 2 –15-213, F’03Concurrent Programming is Hard!Concurrent Programming is Hard!The human mind tends to be sequentialThe human mind tends to be sequentialThe notion of time is often misleadingThe notion of time is often misleadingThinking about all possible sequences of events in a computer Thinking about all possible sequences of events in a computer system is at least error prone and frequently impossiblesystem is at least error prone and frequently impossibleClassical problem classes of concurrent programs:Classical problem classes of concurrent programs:Races: outcome depends on arbitrary scheduling decisions elsewhere in the systemDeadlock: improper resource allocation prevents forward progressLifelock / Starvation / Fairness: external events and/or system scheduling decisions can prevent sub-task progressMany aspects of concurrent programming are beyond the Many aspects of concurrent programming are beyond the scope of 15-213scope of 15-213– 3 –15-213, F’03Iterative ServersIterative ServersIterative servers process one request at a time.Iterative servers process one request at a time.client 1 server client 2call connectcall acceptret connectret acceptcall connectcall readwriteret readcloseclosecall acceptret connectcall readret readclosewriteret acceptclose– 4 –15-213, F’03Fundamental Flaw of Iterative ServersFundamental Flaw of Iterative ServersSolution: use Solution: use concurrent servers concurrent servers instead.instead.Concurrent servers use multiple concurrent flows to serve multiple clients at the same time.client 1 server client 2call connectcall acceptcall readret connectret acceptcall connectcall fgetsUser goesout to lunchClient 1 blockswaiting for userto type in dataClient 2 blockswaiting to completeits connection request until afterlunch!Server blockswaiting fordata fromClient 1– 5 –15-213, F’03Concurrent Servers:Multiple ProcessesConcurrent Servers:Multiple ProcessesConcurrent servers handle multiple requests concurrently.Concurrent servers handle multiple requests concurrently.client 1 server client 2call connectcall acceptcall readret connectret acceptcall connectcall fgetsforkchild 1User goesout to lunchClient 1 blockswaiting for user to type in datacall acceptret connectret acceptcall fgetswriteforkcall readchild 2writecall readend readcloseclose...– 6 –15-213, F’03Three Basic Mechanisms for Creating Concurrent FlowsThree Basic Mechanisms for Creating Concurrent Flows1. Processes1. ProcessesKernel automatically interleaves multiple logical flows.Each flow has its own private address space.2. I/O multiplexing with 2. I/O multiplexing with select()select()User manually interleaves multiple logical flows.Each flow shares the same address space.Popular for high-performance server designs.3. Threads3. ThreadsKernel automatically interleaves multiple logical flows.Each flow shares the same address space.Hybrid of processes and I/O multiplexing!– 7 –15-213, F’03Process-Based Concurrent ServerProcess-Based Concurrent Server/* * echoserverp.c - A concurrent echo server based on processes * Usage: echoserverp <port> */#include <ics.h>#define BUFSIZE 1024void echo(int connfd);void handler(int sig);int main(int argc, char **argv) { int listenfd, connfd; int portno; struct sockaddr_in clientaddr; int clientlen = sizeof(struct sockaddr_in); if (argc != 2) { fprintf(stderr, "usage: %s <port>\n", argv[0]); exit(0); } portno = atoi(argv[1]); listenfd = open_listenfd(portno);– 8 –15-213, F’03Process-Based Concurrent Server (cont)Process-Based Concurrent Server (cont) Signal(SIGCHLD, handler); /* parent must reap children! */ /* main server loop */ while (1) { connfd = Accept(listenfd, (struct sockaddr *) &clientaddr, &clientlen)); if (Fork() == 0) { Close(listenfd); /* child closes its listening socket */ echo(connfd); /* child reads and echoes input line */ Close(connfd); /* child is done with this client */ exit(0); /* child exits */ } Close(connfd); /* parent must close connected socket! */ }}– 9 –15-213, F’03Process-Based Concurrent Server(cont)Process-Based Concurrent Server(cont)/* handler - reaps children as they terminate */void handler(int sig) { pid_t pid; int stat; while ((pid = waitpid(-1, &stat, WNOHANG)) > 0) ; return;}– 10 –15-213, F’03Implementation Issues With Process-Based DesignsImplementation Issues With Process-Based DesignsServer should restart Server should restart acceptaccept call if it is interrupted by call if it is interrupted by a transfer of control to the SIGCHLD handlera transfer of control to the SIGCHLD handlerNot necessary for systems with POSIX signal handling.Our Signal wrapper tells kernel to automatically restart accept Required for portability on some older Unix systems.Server must reap zombie childrenServer must reap zombie childrento avoid fatal memory leak.Server must Server must closeclose its copy of its copy of connfdconnfd..Kernel keeps reference for each socket.After fork,


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CMU CS 15213 - Concurrent Programming

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