1 / 21

Lecture 10: Synchronization (Chapter 6)

Lecture 10: Synchronization (Chapter 6). #include <pthread.h> #include <stdio.h> int sum; /* this data is shared by the thread(s) */ void *runner(void *param); /* the thread */ main(int argc, char *argv[]) { pthread_t tidl, tid; /* the thread identifier */

trent
Download Presentation

Lecture 10: Synchronization (Chapter 6)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lecture 10: Synchronization(Chapter 6)

  2. #include <pthread.h> #include <stdio.h> int sum; /* this data is shared by the thread(s) */ void *runner(void *param); /* the thread */ main(int argc, char *argv[]) { pthread_t tidl, tid; /* the thread identifier */ pthread_attr_t attr; /* set of thread attributes */ if (argc != 2) { fprintf(stderr, ’’usage: a.out<integer value>\n’’); exit(); } if (atoi(argv[1] < 0) { fprintf(stderr, ’’%d must be >= 0\n’’, atoi(argv[1])); exit(); } pthread_attr_init(&attr); /* create the threads*/ pthread_create(&tid,&attr,runner,argv[1]); pthread_create(&tidl,&attr,runner,argv[1]); pthread_join(tid,NULL); pthread_join(tidl,NULL); printf(sum = %d\n,sum); } /* The thread will begin control in this function */ void *runner(void *param) { int upper = atoi(param); int i; sum = 0; if (upper > 0) { for (i = 1; i <= upper; i++) sum += i; } pthread_exit(0); } Name the Problem:

  3. Producer-Consumer Problem • Paradigm for cooperating processes • producer process produces information that is consumed by a consumer process • unbounded-buffer places no practical limit on the size of the buffer • bounded-buffer assumes that there is a fixed buffer size

  4. Bounded-Buffer: Shared-Memory Solution • Shared data #define BUFFER_SIZE 10 typedef struct { . . . } item; item buffer[BUFFER_SIZE]; int in = 0; //next free position in the buffer int out = 0; //next full position in the buffer

  5. Producer/Consumer with BUFFER_SIZE items PRODUCER: while (true) { /* produce an item and put in nextProduced */ while (count == BUFFER_SIZE) ; // do nothing buffer [in] = nextProduced; in = (in + 1) % BUFFER_SIZE; count++; } CONSUMER: while (true) { while (count == 0) ; // do nothing nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; count--; /* consume the item in nextConsumed */ }

  6. Race Condition count++ could be implemented asregister1 = count register1 = register1 + 1 count = register1 count-- could be implemented asregister2 = count register2 = register2 - 1 count = register2 Consider this execution interleaving with “count = 5” initially: S0: producer execute register1 = count {register1 = 5}S1: producer execute register1 = register1 + 1 {register1 = 6} S2: consumer execute register2 = count {register2 = 5} S3: consumer execute register2 = register2 - 1 {register2 = 4} S4: producer execute count = register1 {count = 6 } S5: consumer execute count = register2 {count = 4}

  7. Definitions • Race Condition: two or more processes are reading and writing on shared data and the final result depends on who runs precisely when • Mutual exclusion: making sure that if one process is accessing a shared memory, the other will be excluded from doing the same thing • Critical region: the part of the program where shared variables are accessed

  8. Solution to Critical-Section Problem 1. Mutual Exclusion - If a process is executing in its critical section, then no other processes can be executing in their critical sections 2. Progress - If no process is executing in its critical section and there exist some processes that wish to enter their critical section, then the selection of the processes that will enter the critical section next cannot be postponed indefinitely Bounded Waiting - A bound must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted Two assumptions: Assume that each process executes at a nonzero speed No assumption concerning relative speed of the N processes

  9. Peterson’s Solution Two-process solution Assume that the LOAD and STORE instructions are atomic The two processes share two variables: int turn; Boolean interested[2] The variable turn indicates whose turn it is to enter the critical section. The interested array is used to indicate if a process is ready to enter the critical section. interested[i] = true implies that process Pi is ready.

  10. do { some work here interested[i] = TRUE; turn = j; while (interested[j] && turn == j) ; critical section interested[i] = FALSE; remainder section } while (TRUE); Algorithm for Process Pi Entry section Exit section

  11. Synchronization Hardware Many systems provide hardware support for critical section code Uniprocessors – could disable interrupts Currently running code would execute without preemption Generally too inefficient on multiprocessor systems Modern machines provide special atomic hardware instructions Atomic = non-interruptable Either test memory word and set value (test-and-set) Or swap contents of two memory words (swap)

  12. Solution to Critical-section Problem Using Locks do { acquire lock critical section release lock remainder section } while (TRUE);

  13. TestAndSet Instruction boolean TestAndSet (boolean *target) { boolean rv = *target; *target = TRUE; return rv: }

  14. Solution using TestAndSet Shared boolean variable lock, initialized to false. Solution: do { while (TestAndSet (&lock )) ; // do nothing // critical section lock = FALSE; // remainder section } while (TRUE); Does it implementthe conditions of critical section?

  15. Swap Instruction void Swap (boolean *a, boolean *b) { boolean temp = *a; *a = *b; *b = temp: }

  16. Solution using Swap Shared Boolean variable lock initialized to FALSE; Each process has a local Boolean variable key Solution: do { key = TRUE; while ( key == TRUE) Swap (&lock, &key); // critical section lock = FALSE; // remainder section } while (TRUE); Does it implementthe conditions of critical section?

  17. Bounded-waiting Mutual Exclusion with TestandSet() do { waiting[i] = TRUE; key = TRUE; while (waiting[i] && key) key = TestAndSet(&lock); waiting[i] = FALSE; // critical section j = (i + 1) % n; while ((j != i) && !waiting[j]) j = (j + 1) % n; if (j == i) lock = FALSE; else waiting[j] = FALSE; // remainder section } while (TRUE);

  18. memory ICN cache cache cache . . . . . Caches and Consistency in Multiprocessors • cache with each processor to hide latency for memory accesses • miss in cache => go to memory to fetch data • multiple copies of same address in caches • caches need to be kept consistent

  19. Cache Coherence • write-invalidate memory invalidate --> ICN X -> X’ X -> Inv X -> Inv . . . . . Before a write, an “invalidate” command goes out on the bus. All other processors are “snooping” and they invalidate their caches. Invalid entries cause read faults and new values will be fetched from main memory.

  20. Cache Coherence As soon as the cache line is written, it is broadcast on the bus, so all processors update their caches. • write-update (also called distributed write) memory update --> ICN X -> X’ X -> X’ X -> X’ . . . . .

More Related