Concurrency: Deadlock and Starvation

Concurrency: Deadlock and Starvation Chapter 6

Revision • Describe three necessary conditions for deadlock • Which condition is the result of the three necessary conditions • Deadlock avoidance makes sure that at least one of the conditions does not exist in the system (TRUE/FALSE) • An unsafe state is a deadlocked state (T/F)

Deadlock Avoidance • Maximum resource requirement must be stated in advance • Processes under consideration must be independent; no synchronization requirements • There must be a fixed number of resources to allocate • No process may exit while holding resources

Deadlock Detection • OS can perform deadlock detection to detect and break deadlocks • Detection can be carried out on each resource request • Allocation matrix, Available vector and Request matrix are defined • Mark processes that are not deadlocked • Begin by marking zero rows in Allocation matrix

Deadlock Detection • Initialize W = A • Find an ‘i’ such that process ‘i’ is currently unmarked • Determine if ith row of Q is less than or equal to W (for all elements), if no, EXIT • If yes, mark process ‘i’ and set W = W+A for all elements in this row and go back to find next unmarked process • At the end, any unmarked process is deadlocked

Deadlock Detection P4 is marked first; P3 is taken; new W is 00011 P1 and P2 fail the test Q<W so they are deadlocked

Strategies once Deadlock Detected • Abort all deadlocked processes • Back up each deadlocked process to some previously defined checkpoint, and restart all process • original deadlock may occur • Successively abort deadlocked processes until deadlock no longer exists • Successively preempt resources until deadlock no longer exists

Selection Criteria for Deadlocked Processes(3&4) • Least amount of processor time consumed so far • Least number of lines of output produced so far • Most estimated time remaining • Least total resources allocated so far • Lowest priority

Dining Philosophers • Five philosophers are in deep thought sitting around a dining table • When they get hungry, they try to eat the spaghetti • There is only one fork to the left of each philosopher • Each philosopher must acquire two forks to eat • One philosopher can begin eating if the neighbour to the right has put down the fork • No deadlock, no snatching, no starvation

Dining Philosophers Problem

Solution With Semaphores • semaphore fork[5] = {1}; • void philosopher(int i) • { • while (true) • { • think(); • wait(fork[i]); • wait(fork[(i+1) mod 5); • eat(); • signal(fork[(i+1) mod 5); • signal(fork[i]); • } • } • All philosophers may starve to death!! Why??

UNIX Concurrency Mechanisms • Pipes • Messages • Shared memory • Semaphores • Signals

Pipes • Based on producer-consumer model • A pipe is a FIFO queue written by one process and read by another • Writing process is blocked if no room • Mutual exclusion is enforced by UNIX • Named pipes can be shared by unrelated processes as well

Messages • Each process has a “mailbox”, an associated message queue • System calls are provided for message passing • Process sending message to a full queue is suspended

Shared Memory and Semaphores • Common block of virtual memory shared by multiple processes • Fastest form of IPC • UNIX kernel handles the semaphore operations atomically • A semaphore contains • Current value • PID of last process that accessed it • Number of processes waiting • System calls are provided to handle semaphores

Signals • A signal informs a process about an event • Signals do not have priorities • Some signals in UNIX: • SIGFPT: Floating point exception • SIGALARM: Wake up after a time period • SIGBUS: Bus error

Solaris Thread Synchronization Primitives • Mutual exclusion (mutex) locks • Semaphores • Multiple readers, single writer locks • Condition variables • Implemented in KLT and ULT • Once created, either enter or release • Kernel does not enforce mutual exclusion and unlocking on abort of threads

Solaris Mutex Locks and Semaphores • If a thread has locked a mutex, only this thread will unlock it • If another thread approaches the mutex, it will be either blocked or wait in a spin wait loop • Use mutex_tryenter() to do busy waiting • Solaris provides sema_P(), sema_v() and sema_tryp() primitives for semaphores

Readers/Writer and Condition Variables • Readers/writer lock allows read-only access for an object protected by this lock • If a thread is writing, all others must wait • Condition variables are used with mutex locks • Wait until a condition is true

Windows 2000 Concurrency Mechanisms • Process • Thread • File • Console input • File change notification • Mutex • Semaphore • Event • Waitable timer

W2K Summary • Objects 5 through 9 are designed for supporting synchronization • Each of these objects can be in a signaled or unsignaled state • A thread issues wait request to W2K, using the handle of the object • When an object enters signaled state, threads waiting on it are released

W2K Example • A thread requests wait on a file that is currently open • The thread is blocked • The file is set to signaled state when the I/O operation completes • The waiting thread is released

Concurrency: Deadlock and Starvation