CS Advanced Operating Systems Structures and Implementation Lecture 8. Synchronization Continued. Goals for Today. Synchronization Scheduling

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1 Goals for Today CS Advanced Operating Systems Structures and Implementation Lecture 8 Synchronization Continued Synchronization Scheduling Interactive is important! Ask Questions! February 25 th, 2013 Prof. John Kubiatowicz Note: Some slides and/or pictures in the following are adapted from slides 2013 Lec 8.2 Recall: Implementing Locks with test&set: Spin Lock The Test&Test&Set lock: int value = 0; // Free Acquire() { while (true) { while(value); // Locked, spin with reads if (!test&set(value)) break; // Success! Release() { value = 0; Significant problems with Test&Test&Set? Multiple processors spinning on same memory location» Release/Reacquire causes lots of cache invalidation traffic» No guarantees of fairness potential livelock Scales poorly with number of processors» Because of bus traffic, average time until some processor acquires lock grows with number of processors Busy-Waiting: thread consumes cycles while waiting Lec 8.3 Better option: Exponential backoff The Test&Test&Set lock with backoff: int value = 0; // Free Acquire() { int nfail = 0 while (true) { while(value); // Locked, spin with reads if (!test&set(value)) break; // Success! else exponentialbackoff(nfail++) Release() { value = 0; If someone grabs lock between when we see it free and attempt to grab it, must be contention Backoff exponentially: Choose backoff time randomly with some distribution Double mean time with each iteration Why does this work? Lec 8.4

2 Recall: Using of Compare&Swap for queues compare&swap (&address, reg1, reg2) { /* */ if (reg1 == M[address]) { M[address] = reg2; return success; else { return failure; Here is an atomic add to linked-list function: addtoqueue(&object) { do { // repeat until no conflict ld r1, M[root] // Get ptr to current head st r1, M[object] // Save link in new object until (compare&swap(&root,r1,object)); root next next next New Object Lec 8.5 Mellor-Crummey-Scott Lock Another lock-free option Mellor-Crummey Scott (MCS): public void lock() { QNode qnode = mynode.get(); qnode.next = null; QNode pred = swap(tail,qnode); if (pred!= null) { qnode.locked = true; pred.next = qnode; while (qnode.locked); // wait for predecessor pred 1 pred 2 pred 3 tail next 1 next 2 next 3 Lec 8.6 Mellor-Crummey-Scott Lock (con t) public void unlock() { QNode qnode = mynode.get(); if (qnode.next == null) { if (compare&swap(tail,qnode,null)) return; // wait until predecessor fills in my next field while (qnode.next == null); qnode.next.locked = false; pred 1 tail next 1 pred 2 next 2 pred 3 next 3 Mellor-Crummey-Scott Lock (Con t) Nice properties of MCS Lock Never more than 2 processors spinning on one address Completely fair once on queue, are guaranteed to get your turn in FIFO order» Alternate release procedure doesn t use compare&swap but doesn t guarantee FIFO order Bad properties of MCS Lock Takes longer (more instructions) than T&T&S if no contention Releaser may be forced to spin in rare circumstances Hardware support? Some proposed hardware queueing primitives such as QOLB (Queue on Lock Bit) Not broadly available Lec 8.7 Lec 8.8

3 Comparison between options Reactive Locks Lots of threads trying to: acquire(); wait for bit(); release(); Metric: Average time in critical section Total # critical sections/total time Lesson: need something to handle high contention situations Exponential backoff Queue lock Try to determine if you are in a high-contention or lowcontention domain Choose a protocol accordingly Dynamic choice Why is this useful? Because algorithms go through phases Because different machines have different characteristics Lec 8.9 Lec 8.10 Higher-level Primitives than Locks What is the right abstraction for synchronizing threads that share memory? Want as high a level primitive as possible Good primitives and practices important! Since execution is not entirely sequential, really hard to find bugs, since they happen rarely UNIX is pretty stable now, but up until about mid-80s (10 years after started), systems running UNIX would crash every week or so concurrency bugs Synchronization is a way of coordinating multiple concurrent activities that are using shared state This lecture and the next presents a couple of ways of structuring the sharing Recall: Semaphores Semaphores are a kind of generalized lock First defined by Dijkstra in late 60s Main synchronization primitive used in original UNIX Definition: a Semaphore has a non-negative integer value and supports the following two operations: P(): an atomic operation that waits for semaphore to become positive, then decrements it by 1» Think of this as the wait() operation V(): an atomic operation that increments the semaphore by 1, waking up a waiting P, if any» This of this as the signal() operation Note that P() stands for proberen (to test) and V() stands for verhogen (to increment) in Dutch Lec 8.11 Lec 8.12

4 Semaphores Like Integers Except Semaphores are like integers, except No negative values Only operations allowed are P and V can t read or write value, except to set it initially Operations must be atomic» Two P s together can t decrement value below zero» Similarly, thread going to sleep in P won t miss wakeup from V even if they both happen at same time Semaphore from railway analogy Here is a semaphore initialized to 2 for resource control: Value=1 Value=0 Value=2 Two Uses of Semaphores Mutual Exclusion (initial value = 1) Also called Binary Semaphore. Can be used for mutual exclusion: semaphore.p(); // Critical section goes here semaphore.v(); Scheduling Constraints (initial value = 0) Locks are fine for mutual exclusion, but what if you want a thread to wait for something? Example: suppose you had to implement ThreadJoin which must wait for thread to terminiate: Initial value of semaphore = 0 ThreadJoin { semaphore.p(); ThreadFinish { semaphore.v(); Lec 8.13 Lec 8.14 Producer-Consumer with a Bounded Buffer Problem Definition Producer puts things into a shared buffer (wait if full) Consumer takes them out (wait if empty) Use a fixed-size buffer between them to avoid lockstep» Need to synchronize access to this buffer Correctness Constraints: Consumer must wait for producer to fill buffers, if none full (scheduling constraint) Producer must wait for consumer to empty buffers, if all full (scheduling constraint) Only one thread can manipulate buffer queue at a time (mutual exclusion) Remember why we need mutual exclusion Because computers are stupid General rule of thumb: Use a separate semaphore for each constraint Semaphore fullbuffers; // consumer s constraint Semaphore emptybuffers;// producer s constraint Semaphore mutex; // mutual exclusion Lec 8.15 Full Solution to Bounded Buffer Semaphore fullbuffer = 0; // Initially, no coke Semaphore emptybuffers = numbuffers; // Initially, num empty slots Semaphore mutex = 1; // No one using machine Producer(item) { emptybuffers.p(); mutex.p(); Enqueue(item); mutex.v(); fullbuffers.v(); Consumer() { fullbuffers.p(); mutex.p(); item = Dequeue(); mutex.v(); emptybuffers.v(); return item; // Wait until space // Wait until buffer free // Tell consumers there is // more coke // Check if there s a coke // Wait until machine free // tell producer need more Lec 8.16

5 Administrivia How did the first design document go? How did design reviews go? Recall: Midterm I: Two ½ weeks from today! Wednesday 3/13 Intention is a 1.5 hour exam over 3 hours No class on day of exam! Midterm Timing: Listed as 6:00-9:00PM Could also be: 4:00-7:00pm Preferences? (I need to get a room for the exam) Topics: everything up to the previous Monday OS Structure, BDD, Process support, Synchronization, Scheduling, Memory Management, I/O Review: Definition of Monitor Semaphores are confusing because dual purpose: Both mutual exclusion and scheduling constraints Cleaner idea: Use locks for mutual exclusion and condition variables for scheduling constraints Monitor: a lock and zero or more condition variables for managing concurrent access to shared data Use of Monitors is a programming paradigm Lock: provides mutual exclusion to shared data: Always acquire before accessing shared data structure Always release after finishing with shared data Condition Variable: a queue of threads waiting for something inside a critical section Key idea: allow sleeping inside critical section by atomically releasing lock at time we go to sleep Contrast to semaphores: Can t wait inside critical section Lec 8.17 Lec 8.18 Review: Programming with Monitors Monitors represent the logic of the program Wait if necessary Signal when change something so any waiting threads can proceed Basic structure of monitor-based program: lock while (need to wait) { Check and/or update condvar.wait(); state variables Wait if necessary unlock do something so no need to wait lock condvar.signal(); unlock Check and/or update state variables Readers/Writers Problem W R Motivation: Consider a shared database Two classes of users:» Readers never modify database» Writers read and modify database Is using a single lock on the whole database sufficient?» Like to have many readers at the same time» Only one writer at a time R R Lec 8.19 Lec 8.20

6 Basic Readers/Writers Solution Correctness Constraints: Readers can access database when no writers Writers can access database when no readers or writers Only one thread manipulates state variables at a time Basic structure of a solution: Reader() Wait until no writers Access data base Check out wake up a waiting writer Writer() Wait until no active readers or writers Access database Check out wake up waiting readers or writer State variables (Protected by a lock called lock ):» int AR: Number of active readers; initially = 0» int WR: Number of waiting readers; initially = 0» int AW: Number of active writers; initially = 0» int WW: Number of waiting writers; initially = 0» Condition oktoread = NIL» Conditioin oktowrite = NIL Code for a Reader Reader() { // First check self into system lock.acquire(); while ((AW + WW) > 0) { // Is it safe to read? WR++; // No. Writers exist oktoread.wait(&lock); // Sleep on cond var WR--; // No longer waiting AR++; // Now we are active! // Perform actual read-only access AccessDatabase(ReadOnly); // Now, check out of system lock.acquire(); AR--; // No longer active if (AR == 0 && WW > 0) // No other active readers oktowrite.signal(); // Wake up one writer lock.release(); Lec 8.21 Lec 8.22 Code for a Writer Writer() { // First check self into system lock.acquire(); while ((AW + AR) > 0) { // Is it safe to write? WW++; // No. Active users exist oktowrite.wait(&lock); // Sleep on cond var WW--; // No longer waiting AW++; // Now we are active! // Perform actual read/write access AccessDatabase(ReadWrite); // Now, check out of system lock.acquire(); AW--; // No longer active if (WW > 0){ // Give priority to writers oktowrite.signal(); // Wake up one writer else if (WR > 0) { // Otherwise, wake reader oktoread.broadcast(); // Wake all readers lock.release(); Lec 8.23 C-Language Support for Synchronization C language: Pretty straightforward synchronization Just make sure you know all the code paths out of a critical section int Rtn() { lock.acquire(); if (exception) { return errreturncode; return OK; Watch out for setjmp/longjmp!» Can cause a non-local jump out of procedure Proc A Proc B Calls setjmp Proc C lock.acquire Proc D Proc E Calls longjmp» In example, procedure E calls longjmp, poping stack back to procedure B» If Procedure C had lock.acquire, problem! Stack growth Lec 8.24

7 C++ Language Support for Synchronization (con t) Must catch all exceptions in critical sections Catch exceptions, release lock, and re-throw exception: void Rtn() { lock.acquire(); try { DoFoo(); catch () { // catch exception // release lock throw; // re-throw the exception void DoFoo() { if (exception) throw errexception; Even Better: auto_ptr<t> facility. See C++ Spec.» Can deallocate/free lock regardless of exit method Lec 8.25 Problem: Busy-Waiting for Lock Positives for this solution Machine can receive interrupts User code can use this lock Works on a multiprocessor Negatives This is very inefficient because the busy-waiting thread will consume cycles waiting Waiting thread may take cycles away from thread holding lock (no one wins!) Priority Inversion: If busy-waiting thread has higher priority than thread holding lock no progress! Priority Inversion problem with original Martian rover For semaphores and monitors, waiting thread may wait for an arbitrary length of time! Thus even if busy-waiting was OK for locks, definitely not ok for other primitives Exam solutions should not have busy-waiting! Lec 8.26 Busy-wait vs Blocking Busy-wait: I.e. spin lock Keep trying to acquire lock until read Very low latency/processor overhead! Very high system overhead!» Causing stress on network while spinning» Processor is not doing anything else useful Blocking: If can t acquire lock, deschedule process (I.e. unload state) Higher latency/processor overhead (1000s of cycles?)» Takes time to unload/restart task» Notification mechanism needed Low system overheadd» No stress on network» Processor does something useful Hybrid: Spin for a while, then block 2-competitive: spin until have waited blocking time What about barriers? Barrier global (/coordinated) synchronization simple use of barriers -- all threads hit the same one work_on_my_subgrid(); barrier(); read_neighboring_values(); barrier(); barriers are not provided in all thread libraries How to implement barrier? Global counter representing number of threads still waiting to arrive and parity representing phase» Initialize counter to zero, set parity variable to even» Each thread that enters saves parity variable and Atomically increments counter if even Atomically decrements counter if odd» If counter not at extreme value spin until parity changes i.e. Num threads if even or zero if odd» Else, flip parity, exit barrier Better for large numbers of processors implement atomic counter via combining tree Lec 8.27 Lec 8.28

8 Review: CPU Scheduling Earlier, we talked about the life-cycle of a thread Active threads work their way from Ready queue to Running to various waiting queues. Question: How is the OS to decide which of several tasks to take off a queue? Obvious queue to worry about is ready queue Others can be scheduled as well, however Scheduling: deciding which threads are given access to resources from moment to moment Scheduling Assumptions CPU scheduling big area of research in early 70 s Many implicit assumptions for CPU scheduling: One program per user One thread per program Programs are independent Clearly, these are unrealistic but they simplify the problem so it can be solved For instance: is fair about fairness among users or programs?» If I run one compilation job and you run five, you get five times as much CPU on many operating systems The high-level goal: Dole out CPU time to optimize some desired parameters of system USER1 USER2 USER3 USER1 USER2 Time Lec 8.29 Lec 8.30 Assumption: CPU Bursts Weighted toward small bursts Execution model: programs alternate between bursts of CPU and I/O Program typically uses the CPU for some period of time, then does I/O, then uses CPU again Each scheduling decision is about which job to give to the CPU for use by its next CPU burst With timeslicing, thread may be forced to give up CPU before finishing current CPU burst Lec 8.31 Scheduling Policy Goals/Criteria Minimize Response Time Minimize elapsed time to do an operation (or job) Response time is what the user sees:» Time to echo a keystroke in editor» Time to compile a program» Real-time Tasks: Must meet deadlines imposed by World Maximize Throughput Maximize operations (or jobs) per second Throughput related to response time, but not identical:» Minimizing response time will lead to more context switching than if you only maximized throughput Two parts to maximizing throughput» Minimize overhead (for example, context-switching)» Efficient use of resources (CPU, disk, memory, etc) Fairness Share CPU among users in some equitable way Fairness is not minimizing average response time:» Better average response time by making system less fair Lec 8.32

9 Summary (Synchronization) Important concept: Atomic Operations An operation that runs to completion or not at all These are the primitives on which to construct various synchronization primitives Talked about hardware atomicity primitives: Disabling of Interrupts, test&set, swap, comp&swap, load-linked/store conditional Showed several constructions of Locks Must be very careful not to waste/tie up machine resources» Shouldn t disable interrupts for long» Shouldn t spin wait for long Key idea: Separate lock variable, use hardware mechanisms to protect modifications of that variable Talked about Semaphores, Monitors, and Condition Variables Higher level constructs that are harder to screw up Lec 8.33