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CS 5204 Operating Systems Review Basic Concepts

Understand process management, mutual exclusion, deadlock, and implementing processes and threads. Explore threads vs. events and context switching in dual-mode operation.

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CS 5204 Operating Systems Review Basic Concepts

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  1. CS 5204Operating SystemsReview Basic Concepts Godmar Back

  2. Announcements • Send email with your presentation preference CS 5204 Fall 2012

  3. Concurrency • Review of basic concepts • Process Management as OS responsibility • process vs thread abstraction • Synchronization Issues: • mutual exclusion & race conditions • deadlock & starvation • Implementing processes & threads • Programming models for communication • threads vs events CS 5204 Fall 2012

  4. Definition: Process/Thread • Process • “program in execution” • resources: CPU context, memory maps, open files, privileges, ….; isolated • Threads • CPU context (state + stack); not isolated • “thread” is a historically recent term • Threads used to be called “processes” • Q: what primitives does an OS need to provide? CS 5204 Fall 2012

  5. Processes vs Threads P1 P2 P3 • Processes execute concurrently and share resources: • Files on disk; Open files (via inherited file descriptors); Terminal, etc. • Kernel-level concurrency • but do not (usually) share any of their memory • cannot share data easily by exchanging pointers to it • Threads are separate logical flows of control (with separate stacks!) that share memory and can refer to same data • Different models and variations exist • Application-level concurrency Kernel CS 5204 Fall 2012

  6. Context Switching • Historical motivation for processes was introduction of multi-programming: • Load multiple processes into memory, and switch to another process if current process is (momentarily) blocked • This required protection and isolation between these processes, implemented by a privileged kernel: dual-mode operation. • Time-sharing: switch to another process periodically to make sure all processes make equal progress • Switch between processes is called a context switch CS 5204 Fall 2012

  7. Dual-Mode Operation • Two fundamental modes: • “kernel mode” – privileged • aka system, supervisor or monitor mode • Intel calls its PL0, Privilege Level 0 on x86 • “user mode” – non-privileged • PL3 on x86 • Bit in CPU – controls operation of CPU • Privileged operations can only be performed in kernel mode. Example: hlt • Must carefully control transition between user & kernel mode int main() { asm(“hlt”); } CS 5204 Fall 2012

  8. Mode Switching • User  Kernel mode • For reasons external or internal to CPU • External (aka hardware) interrupt: • timer/clock chip, I/O device, network card, keyboard, mouse • asynchronous (with respect to the executing program) • Internal interrupt (aka software interrupt, trap, or exception) • are synchronous • can be intended (“trap”): for system call (process wants to enter kernel to obtain services) • or unintended (usually): (“fault/exception”) (division by zero, attempt to execute privileged instruction in user mode, memory access violation, invalid instruction, alignment error, etc.) • Kernel  User mode switch on iret instruction CS 5204 Fall 2012

  9. Timer interrupt: P1 is preempted, context switch to P2 I/O device interrupt:P2’s I/O completeswitch back to P2 user mode kernel mode System call: (trap): P2 starts I/O operation, blocks context switch to process 1 Timer interrupt: P2 still has time left, no context switch A Context Switch Scenario Process 1 Process 2 Kernel CS 5204 Fall 2012

  10. user mode kernel mode User processes access kernel services by trapping into the kernel, executing kernel code to perform the service, then returning – very much like a library call. Unless the system call cannot complete immediately, this does not involve a context switch. System Calls Process 1 Kernel Kernel’s System Call Implementation CS 5204 Fall 2012

  11. user mode kernel mode KernelThreads Most OS support kernel threads that never run in user mode – these threads typically perform book keeping or other supporting tasks. They do not service system calls or faults. Process 1 Process 2 Kernel Kernel Thread Careful: “kernel thread” not the same as kernel-level thread (KLT) – more on KLT later CS 5204 Fall 2012

  12. RUNNING Scheduler picks process Process must wait for event Process preempted BLOCKED READY Event arrived Reasoning about Processes:Process States • Only 1 process (per CPU) can be in RUNNING state CS 5204 Fall 2012

  13. Process Creation • Two common paradigms: • Cloning vs. spawning • Cloning: (Unix) • “fork()” clones current process • child process then loads new program • Spawning: (Windows) • “exec()” spawns a new process with new program • Difference is whether creation of new process also involves a change in program CS 5204 Fall 2012

  14. #include <unistd.h> #include <stdio.h> int main() { int x = 1; if (fork() == 0) { // only child executes this printf("Child, x = %d\n", ++x); } else { // only parent executes this printf("Parent, x = %d\n", --x); } // parent and child execute this printf("Exiting with x = %d\n", x); return 0; } fork() Child, x = 2 Exiting with x = 2 Parent, x = 0 Exiting with x = 0 CS 5204 Fall 2012

  15. The fork()/join() paradigm • After fork(), parent & child execute in parallel • Unlike a fork in the road, here we take both roads • Used in many contexts • In Unix, ‘join()’ is called wait() • Purpose: • Launch activity that can be done in parallel & wait for its completion • Or simply: launch another program and wait for its completion (shell does that) Parent: fork() Parent process executes Child process executes Child process exits Parent:join() OS notifies CS 5204 Fall 2012

  16. fork() #include <sys/types.h> #include <unistd.h> #include <stdio.h> int main(int ac, char *av[]) { pid_t child = fork(); if (child < 0) perror(“fork”), exit(-1); if (child != 0) { printf ("I'm the parent %d, my child is %d\n", getpid(), child); wait(NULL); /* wait for child (“join”) */ } else { printf ("I'm the child %d, my parent is %d\n", getpid(), getppid()); execl("/bin/echo", "echo", "Hello, World", NULL); } } CS 5204 Fall 2012

  17. User View of Threads • Unix/C: • fork()/wait() vs pthread_create()/pthread_join() • Java: • new Thread() • Thread.start()/join() • See also [Boehm PLDI 2005] Runnable r = new Runnable() { public void run() { /* body */ } }; Thread t = new Thread(r); t.start(); // concurrent execution starts // main t.join(); // concurrent execution ends CS 5204 Fall 2012

  18. Threading APIs • How are threads embedded in a language/environment? • POSIX Threads Standard (in C) • pthread_create(), pthread_join() • Uses function pointer to denote start of new control flow • Largely retrofitted in Unix world • Needed to define interaction of signals and threads • Java/C# • Thread.start(), Thread.join() • Java: Using “Runnable” instance • C#: Uses “ThreadStart” delegate CS 5204 Fall 2012

  19. Example pthread_create/join static void * test_single(void *arg) { // this function is executed by each thread, in parallel } pthread_t threads[NTHREADS]; int i; for (i = 0; i < NTHREADS; i++) if (pthread_create(threads + i, (const pthread_attr_t*)NULL, test_single, (void*)i) == -1) { printf("error creating pthread\n"); exit(-1); } /* Wait for threads to finish. */ for (i = 0; i < NTHREADS; i++) pthread_join(threads[i], NULL); Use Default Attributes – could set stack addr/size here 2nd arg could receive exit status of thread CS 5204 Fall 2012

  20. Java Threads Example public class JavaThreads { public static void main(String []av) throws Exception { Thread [] t = new Thread[5]; for (int i = 0; i < t.length; i++) { final int tnum = i; Runnable runnable = new Runnable() { public void run() { System.out.println("Thread #"+tnum); } }; t[i] = new Thread(runnable); t[i].start(); } for (int i = 0; i < t.length; i++) t[i].join(); System.out.println("all done"); } } Use this form of explicit threading only when knowing number & rolesof threads before hand! Threads implements Runnable – could also have subclassed Thread & overridden run() Thread.join() can throw InterruptedException – can be used to interrupt thread waiting to join via Thread.interrupt CS 5204 Fall 2012

  21. import java.util.concurrent.*; public class FixedThreadPool { public static void main(String []av) throws Exception { ExecutorService ex = Executors.newFixedThreadPool(4); final int N = 4; Future<?> f[] = new Future<?>[N]; for (inti = 0; i < N; i++) { final int j = i; f[i] = ex.submit(new Callable<String>() { public String call() { return "Future #" + j + " brought to you by “ + Thread.currentThread(); } }); } System.out.println("Main thread: " + Thread.currentThread()); for (inti = 0; i < N; i++) System.out.println(f[i].get()); ex.shutdown(); } } Java Threadpools Tasks must implement “Callable” – like Runnable except returns result. get() waits for the execution of call() to be completed (if it hasn’t already) and returns result CS 5204 Fall 2012

  22. Explicit Threads vs. Pools • Overhead: • Startup overhead per thread relatively high (between 1e4 & 1e5 cycles); pools amortize • There is no point in having more threads than there are physical cores • Compete for available CPUs • Unless some subset is blocked on I/O or other conditions • Still, sizing of pools that maximizes throughput can be challenging • “cachedThreadPool” creates thread whenever needed, but reuses existing ones that are idle • “fixedThreadPool” - # of threads fixed • Can implement custom policies CS 5204 Fall 2012

  23. Aside: Hybrid Models • The “threads share everything” + “processes share nothing” mantra does not always hold • Hybrids: • WEAVES allows groups of threads to define their own namespace, so they only share data they want • Java multitasking systems (KaffeOS, MVM): multiple “processes” may share same address space CS 5204 Fall 2012

  24. Implementation CS 5204 Fall 2012

  25. Implementing Threads • Issues: • Who maintains thread state/stack space? • How are threads mapped onto CPUs? • How is coordination/synchronization implemented? • How do threads interact with I/O? • How do threads interact with existing APIs such as signals? • How do threads interact with language runtimes (e.g., GCs)? • How do terminate threads safely? CS 5204 Fall 2012

  26. Cooperative Multi-threading • Special case of user-level threading • Is easy to implement using ‘swapcontext’ – see next slide • Support multiple logical execution flows • Each needs own stack  so has its own (procedure-) local (automatic) variables • But share address space  so shares heap, global vars (all kinds: global, global static, local static) • In cooperative multi-threading, a context switch can occur only if a thread voluntarily offers the CPU to (any) other thread (“yield”); later resumes and returns • Can build resource abstractions on top where threads yield if they cannot obtain the abstracted resource • This is called a “non-preemptive” model • If yield is directed (“yield to x”) this model is called “co-routines” CS 5204 Fall 2012

  27. Cooperative Multithreading via ‘swapcontext’ static char stack[2][65536]; // a stack for each coroutine static ucontext_tcoroutine_state[2]; // container to remember context // switch current coroutine (0 -> 1 -> 0 -> 1 ...) static inline void yield_to_next(void) { static int current = 0; intprev = current; int next = 1 - current; current = next; swapcontext(&coroutine_state[prev], &coroutine_state[next]); } static void coroutine(intcoroutine_number) { inti; for (i = 0; i < 5; i++) { printf("Coroutine %d counts i=%d (&i=%p)\n", coroutine_number, i, &i); yield_to_next(); } } CS 5204 Fall 2012

  28. Cooperative Multi-threading (cont’d) • Advantages: • Requires no OS support • Context switch very fast (usually involves only saving callee-saved regs + stack pointer) • Reduced potential for certain types of race conditions • E.g., i++ will never be interrupted • Used in very high-performance server designs & discrete event simulation • Disadvantages • OS sees only one thread in process, system calls block entire process (if they block) • Cannot make use of multiple CPUs/cores • Cannot preempt infinitely-looping or uncooperative threads • (though can fake preemption in just-in-time compiled languages by letting compiler insert periodic checks) CS 5204 Fall 2012

  29. Use Cases forApp-level Concurrency • Overlap I/O with computation • E.g. file sharing program downloads, checksums, and saves/repairs files simultaneously • Parallel computation • Use multiple CPUs • Retain interactivity while background activity is performed • E.g., still serve UI events while printing • Handling multiple clients in network server apps • By and large, these are best handled with a preemptive, and (typically) kernel-level multi-threading model CS 5204 Fall 2012

  30. Example Use: Threads in Servers CS 5204 Fall 2012

  31. Preemptive Multi-Threading • Don’t require the explicit, programmer-inserted “yield” call to switch between threads • “Switch” mechanism can be implemented at user-level or kernel-level • User-level threads: can be built using signal handlers (e.g. SIGVTALRM) • Requires advanced file descriptor manipulation techniques to avoid blocking entire process in system call • Kernel-level threads: natural extension for what OS already does when switching between processes • Integrated with system calls – only current thread blocks • Hybrids • Kernel-level threads is the dominant model today CS 5204 Fall 2012

  32. Kernel-level Threads 1:1 Model Threading Models Hybrid, so-called M:N model: User-level Threads 1:N Model Source: Solaris documentation (left), Stallings (right) CS 5204 Fall 2012

  33. Threading Implementations Overview * Most commonmodeltoday CS 5204 Fall 2012

  34. Threading Models • Linux, Windows, Solaris 10 or later, OSX: use 1:1 model with kernel-level threads. OS manages threads in each process • threads in Java/C#, etc. are typically mapped to kernel-level threads • Solaris (pre-10), Windows “fibers”: provide M:N model • Attempted to obtain “best of both worlds” – turned out to be difficult in practice • User-level Threads • used mainly in special/niche applications today CS 5204 Fall 2012

  35. stack1 guard stack2 guard Managing Stack Space • Stacks require continuous virtual address space • virtual address space fragmentation (example 32-bit Linux) • What size should stack have? • How to detect stack overflow? • Ignore vs. software vs. hardware • Related: how to implement • Get local thread id “pthread_self()” • Thread-local Storage (TLS) CS 5204 Fall 2012

  36. On Termination • If you terminate a thread, how will you clean up if you have to terminate it? • Strategies: • Avoid shared state where possible • Disable termination • Use cleanup handlers try/finally, pthread_cleanup Queue q1, q2; // shared thread_body() { while (true) { Packet p = q1.dequeue(); q2.enqueue(p); } } Queue q1, q2; // shared thread_body() { while (!done) { Packet p = q1.dequeue(); q2.enqueue(p); } } CS 5204 Fall 2012

  37. Synchronization CS 5204 Fall 2012

  38. Synchronization • Access to resources must be protected • Race Condition problem • Definition • Approaches for detecting them • Static vs dynamic CS 5204 Fall 2012

  39. Critical Section Problem • Many algorithms known • purely software-based (Dekker’s, Peterson’s algorithm) vs. hardware-assisted (disable irqs, test-and-set instructions) • Criteria for good algorithm: • mutual exclusion • progress • bounded waiting while (application hasn’t exited) { enter critical section inside critical section exit critical section in remainder section } CS 5204 Fall 2012

  40. Synchronization Abstractions • Atomic primitives • e.g. Linux kernel “atomic_inc()” • Dijkstra’s semaphores • P(s) := atomic { while (s<=0) /* no op */; s--; } • V(s) := atomic { s++; } • Q: what’s wrong with this implementation? • Binary semaphores, locks, mutexes • Difference between mutex & semaphore CS 5204 Fall 2012

  41. Expressing Critical Sections pthread_mutex_t m; … pthread_mutex_lock(&m); /* in critical section */ if (*) { pthread_mutex_unlock(&m); return; } pthread_mutex_unlock(&m); synchronized (object) { /* in critical section */ if (*) { return; } } Pthreads/C vs Java CS 5204 Fall 2012

  42. Expressing Critical Sections pthread_mutex_t m; … pthread_mutex_lock(&m); /* in critical section */ if (*) { pthread_mutex_unlock(&m); return; } pthread_mutex_unlock(&m); synchronized (object) { /* in critical section */ if (*) { return; } } Pthreads/C vs JavaNote benefits of language support CS 5204 Fall 2012

  43. Monitors (Hoare) Enter • Data Type: • internal, private data • public methods wrapped by Enter/Exit • wait/signal methods • “Monitor Invariant” Region of mutual exclusion Wait Signal Wait Signal Exit CS 5204 Fall 2012

  44. Expressing Monitors pthread_mutex_t m; pthread_cond_t c; … pthread_mutex_lock(&m); /* in critical section */ while (somecond != true) pthread_cond_wait(&c, &m); pthread_mutex_unlock(&m); synchronized (object) { /* in critical section */ while (somecond != true) { object.wait(); } } pthread_mutex_lock(&m); /* in critical section */ pthread_cond_signal(&c, &m); pthread_mutex_unlock(&m); synchronized (object) { /* in critical section */ object.notify(); } See also Java’s insecure parallelism [Per Brinch Hansen 1999] CS 5204 Fall 2012

  45. A B Thread 1 Thread 2 Deadlock pthread_mutex_t A; pthread_mutex_t B; … pthread_mutex_lock(&A); pthread_mutex_lock(&B); … pthread_mutex_unlock(&B); pthread_mutex_unlock(&A); pthread_mutex_lock(&B); pthread_mutex_lock(&A); … pthread_mutex_unlock(&A); pthread_mutex_unlock(&B); CS 5204 Fall 2012

  46. Reusable vs. Consumable Resources • Distinguish two types of resources when discussing deadlock • A resource: • “anything a process needs to make progress” • (Serially) Reusable resources (static, concrete, finite) • CPU, memory, locks • Can be a single unit (CPU on uniprocessor, lock), or multiple units (e.g. memory, semaphore initialized with N) • Consumable resources (dynamic, abstract, infinite) • Can be created & consumed: messages, signals • Deadlock may involve reusable resources or consumable resources CS 5204 Fall 2012

  47. Resource Allocation Graph R → P Assignment P → R Request R1 R3 P1 P2 P3 P4 R2 R4 Deadlocks, more formally • 4 necessary conditions • Mutual Exclusion • Hold and Wait • No Preemption • Circular Wait • Q.: what are strategies to detect/break/avoid deadlocks? CS 5204 Fall 2012

  48. Strategies for dealing with Deadlock • Deadlock Prevention • Remove a necessary condition • Deadlock Avoidance • Can’t remove necessary condition, so avoid occurrence of deadlock – maybe by clever resource scheduling strategy • Deadlock Recovery • Deadlock vs. Starvation CS 5204 Fall 2012

  49. Example: x86 • Nonpreemptive = C calling conventions: • Caller-saved: eax, ecx, edx + floating point • Callee-saved: ebx, esi, edi, esp • ebp, eip for a jmpbuf size of 6*4 = 24 bytes • Preemptive = save entire state • All registers + 108 bytes for floating point context • Note: context switch cost = save/restore state cost + scheduling overhead + lost locality cost CS 5204 Fall 2012

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