Understanding Threads and Processes in Operating Systems

1. Threads Today Why threads Thread model & usage Implementing threads Scheduler activations Making single-threaded code multithreaded Next time CPU Scheduling

2. EECS 343 Operating Systems Northwestern University 2 The problem with processes A process consists of (at least): An address space The code for the running program The data for the running program An execution stack and stack pointer (SP) Traces state of procedure calls made The program counter (PC), indicating the next instruction A set of general-purpose processor registers and their values A set of OS resources open files, network connections, sound channels, A lot of concepts bundled together!

3. EECS 343 Operating Systems Northwestern University 3 The problem with processes Many programs need to perform mostly independent tasks that do not need to be serialized e.g. web server, text editor, database server, In each examples Everybody wants to run the same code wants to access the same data has the same privileges uses the same resources (open files, net connections, etc.) But youd like to have multiple HW execution states: An execution stack & SP PC indicating the next instruction A set of general-purpose processor registers & their values

4. How can we get this? Given the process abstraction as we know it fork several processes cause each to map to the same address space to share data see the shmget() system call for one way to do this (kind of) Not very efficient Space: PCB, page tables, etc. Time: creating OS structures, fork and copy addr space, etc. Some equally bad alternatives for some of the cases: Entirely separate web servers Finite-state machine or event-driven a single process and asynchronous programming (non-blocking I/O) 4 EECS 343 Operating Systems Northwestern University

5. 5 The thread model Traditionally Process = 1 address space + 1 thread of execution Process = resource grouping + execution stream Resources: program text, data, open files, child processes, pending alarms, accounting info, Key idea with threads Separate the concept of a process (address space, etc.) From that of a minimal thread of control (execution state) EECS 343 Operating Systems Northwestern University Kernel space User space Kernel space User space Three traditional single-threaded processes One multithreaded process

6. 6 The thread model Concurrency & parallelism Concurrency whats possible with infinite processors Provided at the System level: Kernel recognizes multiple threads of control within a process & schedules them independently Application level: Through user-level thread library; a good structuring tool Parallelism your actual degree of parallel exec. Threads states ~ processes states One stack per thread w/ one frame per procedure called but not yet returned from Common calls thread_create() thread_exit() thread_wait() thread_yield() (why would you need this?) EECS 343 Operating Systems Northwestern University

7. 7 The thread model Share and private items No protection bet/ threads (Should they be?) Per process Address space Global variables Open files Child processes Pending alarms Signals and signal handlers Accounting information Per thread Program counter Registers Stack State EECS 343 Operating Systems Northwestern University

8. Old and new process address space 8 0x00000000 0xFFFFFFFF address space code (text segment) static data (data segment) heap (dynamic allocated mem) stack (dynamic allocated mem) PC SP code (text segment) static data (data segment) heap (dynamic allocated mem) thread 1 stack PC (T2) SP (T2) thread 2 stack SP (T1) PC (T1) Old one without threads New one with threads EECS 343 Operating Systems Northwestern University

9. 9 A simple example void do_wrap_up(int one, int another) { int total; total = one + another; printf(wrap up: one %d, another %d and total %d\n, one, another, total); } int main (int argc, char *argv[]) { do_one_thing(&r1); do_another_thing(&r2); do_wrap_up(r1,r2); return 0; } /* main! */ int r1 = 0, r2 = 0; void do_one_thing(int *ptimes) { int i, j, k; for (i = 0; i < 4; i++) { printf(doing one\n); for (j = 0; j < 1000; j++) x = x + i; (*ptimes)++; } /* do_one_thing! */ void do_another_thing(int *ptimes) { int i, j, k; for (i = 0; i < 4; i++) { printf(doing another\n); for (j = 0; j < 1000; j++) x = x + i; (*ptimes)++; } /* do_another_thing! */ EECS 343 Operating Systems Northwestern University

10. 10 Registers SP PC GP0 GP1 Layout in memory & threading Identity Resources PID UID GID Open Files Locks Sockets do_one_thing() i, j, k _______________________ main() main() -- -- do_one_thing() -- -- do_another_thing() -- -- Lowest address Stack Text Data Heap Highest address r1 r2 Virtual Address Space Thread 1 Registers SP PC GP0 GP1 do_another_thing() i, j, k Thread 2 Stack EECS 343 Operating Systems Northwestern University

11. 11 Benefits of threads A web server Single-threaded: no parallelism, blocking system calls Event-driven: parallelism, non-blocking system calls, interrupts Multithreaded: parallelism, blocking system calls Reasons for threads Simpler programming model when application has multiple, concurrent activities Easy/cheaper to create/destroy than processes since they have no resources attached to them With good mix of CPU and I/O bound activities, better performance Even better if you have multiple CPUs User space Kernel space Dispatcher thread Web server process Worker threads Network connection Cache EECS 343 Operating Systems Northwestern University

12. 12 Implementing threads in user space Kernel unaware of threads no modification required (many-to-one model) Run-time system: a collection of procedures Each process needs its own thread table Pros Thread switch is very fast No need for kernel support Customized scheduler Each process ~ virtual processor Cons - real world factors Multiprogramming, I/O, Page faults Blocking system calls? Can you check? EECS 343 Operating Systems Northwestern University

13. 13 Implementing threads in the kernel One-to-one model No need for runtime system No wrapper for system calls But creating threads is more expensive recycle And system calls are expensive EECS 343 Operating Systems Northwestern University

14. 14 Hybrid thread implementations Trying to get the best of both worlds Multiplexing user-level threads onto kernel- level threads (many-to-many model) One popular variation two-level model (you can bound a user-level thread to a kernel one) EECS 343 Operating Systems Northwestern University

15. 15 Costs of threads (creation) Creation time User-level threads LWP/Kernel- level threads Processes SPARCstation 2, Solaris 52 sec 350 sec 1700 sec 700MHz Pentium, Linux 2.2.* 4.5 sec create/join 94 sec create/join 251 sec fork/exit EECS 343 Operating Systems Northwestern University

16. 16 Scheduler activations* Goal Functionality of kernel threads & Performance of user-level threads Without special non-blocking system calls Problem : needed control & scheduling information distributed bet/ kernel & each apps address space Basic idea When kernel finds out a thread is about to block, upcalls the runtime system (activates it at a known starting address) When kernel finds out a thread can run again, upcalls again Run-time system can now decide what to do Pros fast & smart Cons upcalls violate layering approach *Anderson et al., Scheduler Activations: effective Kernel Support for the User-level Management of Parallelism, SOSP, Oct. 1991. EECS 343 Operating Systems Northwestern University

17. 17 Thread libraries Pthreads POSIX standard (IEEE 1003.1c) API for thread creation & synchronization API specifies behavior of the thread library, implementation is up to the developers of the library Common in UNIX OSs (Solaris, Linux, Mac OS X) Win32 threads slightly different (more complex API) Java threads Managed by the JVM May be created by Extending Thread class Implementing the Runnable interface Implementation model depends on OS (1-to-1 in Windows but many-to-many in early Solaris) EECS 343 Operating Systems Northwestern University

18. 18 Multithreaded C/POSIX /* shared by thread(s) */ int sum; /* runner: the thread */ void *runner(void *param) { int i, upper = atoi(param); sum = 0; for (i = 1; i < upper; i++) sum += 1; pthread_exit(0); } /* runner! */ int main (int argc, char *argv[]) { pthread_t tid; /* thread id */ /* set of thread attrs */ pthread_attr_t attr; if (argc != 2 || atoi(argv[1]) < 0) { fprintf (stderr, "usage: %s \n", argv[0]); exit(1); } /* get default attrs */ pthread_attr_init(&attr); pthread_create(&tid, &attr, runner, argv[1]); /* wait to exit */ pthread_join(tid, NULL); printf("sum = %d\n", sum); exit(0); } /* main! */ EECS 343 Operating Systems Northwestern University

19. 19 Complications with threads Semantics of fork() & exec() system calls Duplicate all threads or single-threaded child? Are you planning to invoke exec()? Other system calls (closing a file, lseek, ?) Signal handling, handlers and masking 1. Send signal to each thread too expensive 2. A master thread per process asymmetric threads 3. Send signal to an arbitrary thread ( control C ?) 4. Use heuristics to pick thread (SIGSEGV & SIGILL caused by thread, SIGTSTP & SIGINT caused by external events) 5. Create a thread to handle each signal situation specific Visibility of threads Stack growth EECS 343 Operating Systems Northwestern University

20. 20 Threads and global variables An example problem Prohibit global variables? Legacy code? Assign each thread its own global variables Allocate a chunk of memory and pass it around Create new library calls to create/set/destroy global variables Single-threaded to multithreaded EECS 343 Operating Systems Northwestern University

21. 21 Single-threaded to multithreaded Many library procedures are not reentrant Re-entrant: able to handle a second call while not done with previous one e.g. assemble msg in a buffer before sending it Solutions Rewrite library? Wrappers for each call? Signal handling EECS 343 Operating Systems Northwestern University

22. 22 Summary You really want multiple threads per address space Kernel threads are more efficient than processes, but theyre still not cheap all operations require a kernel call and parameter verification User-level threads are: Really fast Great for common-case operations, but Can suffer in uncommon cases due to kernel obliviousness Scheduler activations are a good answer Next time Multiple processes in the ready queue, but only one processor which you should you pick next? EECS 343 Operating Systems Northwestern University