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diff --git a/doc/assignment2.md b/doc/assignment2.md index cc571a6..9defc35 100644 --- a/doc/assignment2.md +++ b/doc/assignment2.md @@ -166,8 +166,36 @@ process gets to execute next. Processes sometimes need to access resources (chop need to be able to do this in a safe way without contention (sharing a chopstick). 1. Define the two-phase locking protocol. (6 marks) + + Locks are applied and removed in two phases: + + * Expanding/Growing phase: locks are acquired and no locks are released. + * Shrinking phase: locks are released and no new locks are taken. + + 1. Describe how an adaptive mutex functions. (6 marks) + + > An `adaptive mutex` protects access to every critical data item. + > On a multiprocessor system, an adaptive mutex starts as a standard + > semaphore implemented as a spinlock. If the data are locked and therefore + > already in use, the adaptive mutex does on of two things. If the lock + > is held by a thread that is currently running on another CPU, the thread + > spins while waiting for the lock to become available, because the thread + > holding the lock is likely to finish soon. If the thread holding the lock + > is not currently in run state, the thread blocks, going to sleep until it + > is awakended by the release of the lock. It is put to sleep so that it will + > not spin while waiting, since the lock will not be freed very soon. + 1. Describe a scenario in which the use of a reader-writer lock is more appropriate than using another synchronization tool, such as a semaphore. (6 marks) + + Reader-writer locks are most useful in the following situations: + + * In applications where it is easy to identify which processes only read shared data and which processes only write shared data. + * In applications that have more readers than writers. This is because reader-writer locks generally require more overhead to establish than semaphores or mutual-exclusion locks. + The increased concurrency of allowing multiple readers compensates for the overhead involved in setting up the reader writer lock. + 1. What is the difference between deadlock prevention and deadlock avoidance? (6 marks) + 1. Describe a wait-for graph, and explain how it detects deadlock. (6 marks) + 1. Describe how a safe state ensures that deadlock will be avoided. (6 marks) |