Recitation 8 (Nov. 1)

1. Outline Lab 5 hints Virtual Memory Reminder Shell Lab: Due THIS Thursday Recitation 8 (Nov. 1) TA: Kun Gao Modified from Minglong Shao’s Recitation, Fall 2004

2. Lab 5 • A tiny shell (tsh) with job control & I/O redirection • Key points: • Understand process tree • Executing programs • Reap all child processes • Handle SIGCHLD, SIGTSTP, SIGINT • Avoid race hazard • I/O redirection

3. Process tree for shell Shell pid=10 pgid=10 Fore- ground job Back- ground job #1 Back- ground job #2 pid=20 pgid=20 pid=32 pgid=32 pid=40 pgid=40 Background process group 32 Backgroud process group 40 Child Child pid=21 pgid=20 pid=22 pgid=20 Each job has a unique process group id int setpgid(pid_t pid, pid_t pgid); setpgid(0, 0); Foreground process group 20

4. Process tree for shell (part 2) UNIX shell pid=5 pgid=5 Foreground job receives SIGINT, SIGTSTP, when you type ctrl-c, ctrl-z pid=10 pgid=10 tsh Forward signals Fore- ground job Back- ground job #1 Back- ground job #2 pid=20 pgid=20 pid=32 pgid=32 pid=40 pgid=40 Background process group 32 Backgroud process group 40 Child Child int kill(pid_t pid, int sig) pid > 0: send sig to specified process pid = 0: send sig to all processes in same group pid < -1: send sig to group -pid pid=21 pgid=20 pid=22 pgid=20 Foreground process group 20

5. Execute program int execve(const char *fname, char *const argv[], char *const envp[]); Examples: execve(“/bin/ls”, NULL, NULL); execve(“./mytest”, argv, envp);

6. Reaping child process waitpid(pid_t pid, int *status, int options) pid: wait for child process with pid (process id or group id) -1: wait for any child process status: tell why child terminated options: WNOHANG:return immediately if no children zombied WUNTRACED: report status of terminated or stopped children In Lab 5, use waitpid(-1, &status, WNOHANG|WUNTRACED) In sigchld_handler(): while ((c_pid = waitpid(…)) > 0) { … }

7. Reaping child process (part 2) • int status; waitpid(pid, &status, WNOHANG|WUNTRACED) What to check in sigchld_handler: • WIFEXITED(status):child exited normally (the child finished, and quit normally on its own) • WEXITSTATUS(status): returns code when child exits • WIFSIGNALED(status): child exited because a signal is not caught (SIGINT, or typing CTRL-C) • WTERMSIG(status):gives the terminating signal number • WIFSTOPPED(status):child is stopped by the receipt of a signal (SIGSTOP, or typing CTRL-Z) • WSTOPSIG(status): gives the stop signal number

8. Reaping child process (part 3) • Where to put waitpid(…) • In sigchld_handler(): for both • tsh should still wait for fg job to complete, how?

9. Busy wait for foreground job In eval(): if(fork() != 0) { /* parent */ addjob(…); while(fg process still alive){ /* do nothing */ } }

10. Sleep In eval(): if(fork() != 0) { /* parent */ addjob(…); while(fg process still alive){ sleep(1); } }

11. Race hazard • A data structure is shared by two pieces of code that can run concurrently • Different behaviors of program depending upon how the schedule interleaves the execution of code.

12. An example of race hazard sigchld_handler() { while ((pid = waitpid(…)) > 0){ deletejob(pid); } } eval() { pid = fork(); if(pid == 0) { /* child */ execve(…); } /* parent */ /* signal handler may run BEFORE addjob()*/ addjob(…); }

13. An okay schedule Signal Handler Child time Shell fork() addjob() execve() exit() sigchld_handler() deletejobs()

14. A problematic schedule time Signal Handler Child Shell fork() execve() exit() sigchld_handler() deletejobs() addjob() Job added to job list after the signal handler tried to delete it!

15. Solution: blocking signals sigchld_handler() { pid = waitpid(…); deletejob(pid); } eval() { sigprocmask(SIG_BLOCK, …) pid = fork(); if(pid == 0) { /* child */ sigprocmask(SIG_UNBLOCK, …) execve(…); } /* parent */ /* signal handler might run BEFORE addjob() */ addjob(…); sigprocmask(SIG_UNBLOCK, …) }

16. Solution: blocking signals (part 2) • What should our block set be? • SIGSTP • SIGINT • SIGCHLD

17. I/O redirection • Do it before call execve() in child process eval() { pid = fork(); if(pid == 0) { /* child */ /* Redirect I/O */ if (redirect_input) dup2(…);/* redirect STDIN */ if (redirect_output) dup2(…);/* redirect STDOUT */ execve(…); } addjob(…); }

18. dup2(int oldfd, int newfd) • Covered in Chapter 11 oldfd: old file descriptor newfd: new file descriptor • Dup2 makes newfd be a copy of the oldfd (i.e. operations on newfd is done on oldfd instead) Some examples: Get input from in_fd instead of standard input dup2(in_fd, STDIN_FILENO); Print output to out_fd instead of standard output dup2(out_fd, STDOUT_FILENO);

19. Reminders • Some important system calls: • fork(), execve(), waitpid(), sigprocmask(), setpgid(), kill() … • Check man pages for details about system calls • man 2 kill • Check return values of all system calls • STEP by STEP • Test your shell by typing commands first • Each trace worth the same (work on easy ones first!) • Start now!

20. Virtual memory • One of the most important concepts in CS • Why use virtual memory (VM) • Use RAM as a cache for disk • Easier memory management • Access protection • Enable “partial swapping” • Share memory efficiently

21. 0: 1: CPU N-1: Virtual memory Memory • Page • Page table • Page hits, Page faults • Demand Paging Per Process: Page Table Virtual Addresses Physical Addresses 0: 1: P-1: Disk

22. Conceptual address translation • Higher bits of address (page number) mapped from virtual addr. to physical addr. • Lower bits (page offset) stay the same. p p–1 0 n–1 virtual address virtual page number (VPN) page offset address translation m–1 p p–1 0 physical page number (PPN) page offset physical address Virtual Memory: up to 2n -1 bytes Physical Memory: up to 2m -1 bytes

23. Address translation through page table • Translation • Separate (set of) page table(s) per process • VPN forms index into page table (points to a page table entry)

24. Address translation with TLB n–1 p p–1 0 virtual address virtual page number page offset valid tag physical page number TLB . . . = TLB hit physical address tag byte offset index valid tag data Cache = data cache hit

25. Integrating VM and cache • Most caches are physically addressed • Why? • Perform address translation before cache lookup, could potentially go to memory =*( • TLB keeps things manageable 1. VA 4. PA CPU MMU/ Translation Cache/Memory 2. VPN 3. PTE TLB 5. Data

26. Integrating VM and cache (TLB hit) • Most caches are physically addressed • Why? • Perform address translation before cache lookup, could potentially go to memory =*( • TLB keeps things manageable 1. VA 4. PA CPU MMU/ Translation Cache/Memory 2. VPN 3. PTE TLB 5. Data

27. Integrating VM and cache (TLB miss) • TLB miss, but page table is in cache/memory 3. PTEA (PTBP + VPN) 1. VA Cache/Memory CPU MMU/ Translation 5. PA 4. PTE 2. VPN TLB 6. Data

28. What is the worst case mem read (most delay)? • TLB miss, page fault on page table, then page fault on memory read 3. PTEA (PTBP + VPN) 1. VA Cache/Memory CPU MMU/ Translation 5. PA 4. PTE 2. VPN TLB Page Fault/ Page Table Page Fault/ Mem Read 6. Data DISK

29. Example • Understand end-to-end addr. translation • 20-bit virtual addresses • 18-bit physical addresses • Page size is 1024 bytes • TLB is 2-way associative with 16 total entries

30. 0 16 14 13 12 11 10 9 2 1 6 5 4 3 17 18 7 8 2 15 10 0 1 16 15 14 17 12 19 13 9 8 7 6 5 4 3 11 Part 1 • A. Virtual address • B. Physical address TLBT TLBI VPN VPO PPN PPO

31. Example: TLB and page table

32. Part 2 Virtual address 0x04AA4 • A. 04AA4 = 0000 0100 1010 1010 0100 • B. Address translation • C. Physical address 01 1010 0010 1010 0100

33. Part 2 Virtual address 0x78E6 • A. 078E6 = • B. Address translation • C. Physical address 0000 0111 1000 1110 0110 01 0101 1100 1110 0110

34. End-to-end address translation • Section 10.6.4: a concrete example • Read carefully and solve practice problem 10.4