1. Debugging on Shared Memory Introduction to Valgrind Multithreads Tools & Principles 林孟潇 14302010043

2. Shared Memory Parallel Programming ▪All processes/threads share a common address space. ▪Some of them may be running at the same time on different processors. ▪Has shown better efficiency in Web Server and some other fields.

3. Data Racing ▪One or more threads access the same memory location without sufficient locking. ▪If one subtask is reading a variable while the other is writing it, the result is unpredictable. ▪The safe way is to use a mutex or lock to protect the critical section.

4. Data Racing Thread 1 my_rank=1 racer=0

5. Data Racing Thread 1 my_rank=1 tmp=0 racer=0

6. Data Racing Thread 1 my_rank=1 tmp=0 racer=0 Thread 2 my_rank=2

7. Data Racing Thread 1 my_rank=1 tmp=0 racer=0 Thread 2 my_rank=2 tmp=0

8. Data Racing Thread 1 my_rank=1 tmp=0 racer=0 Thread 2 my_rank=2 tmp=0

9. Data Racing Thread 1 my_rank=1 tmp=0 racer=0 Thread 2 my_rank=2 tmp=0

10. Data Racing Thread 1 my_rank=1 tmp=0 racer=1 Thread 2 my_rank=2 tmp=0

11. Data Racing Thread 1 my_rank=1 tmp=0 racer=1 Thread 2 my_rank=2 tmp=0

12. Data Racing Thread 1 my_rank=1 tmp=0 racer=1 Thread 2 my_rank=2 tmp=0

13. Data Racing Thread 1 my_rank=1 tmp=0 racer=1 Thread 2 my_rank=2 tmp=0

14. Valgrind Helgrind ▪Helgrind is a thread debugger which finds data races in multithreaded programs. ▪It looks for memory locations which are accessed by more than one thread, but for which no consistently used lock can be found. ▪Working with POSIX pthreads.

15. Valgrind DRD ▪The tool works for any program that uses the POSIX threading primitives or that uses threading concepts built on top of the POSIX threading primitives. ▪For most programs DRD needs less memory to perform its analysis. ▪DRD has support for detached threads. ▪Newer than Helgrind

16. Happens-before Relation Parent ThreadChild Thread int var; pthread_create(…) var=20var=10 exit pthread_join(…) printf("%d\n", var); Which instruction will be executed first?

17. Happens-before Relation Parent ThreadChild Thread int var; pthread_create(…) var=20 send message to childwait message var=10 exit pthread_join(…) printf("%d\n", var); Now the order is clear!

18. Happens-before Relation

19. Run Valgrind Tools ▪valgrind --tool=drd ▪valgrind --tool=helgrind

20. Q&A