ECEC-353 Chap 6 - 6.1 The system clock is updated at every...

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6.1 The system clock is updated at every clock interrupt. If interrupts were disabled—particularly for a long period of time—it is possible the system clock could easily lose the correct time. The system clock is also used for scheduling purposes. For example, the time quantum for a process is expressed as a number of clock ticks. At every clock interrupt, the scheduler determines if the time quantum for the currently running process has expired. If clock interrupts were disabled, the scheduler could not accurately assign time quantums. This effect can be minimized by disabling clock interrupts for only very short periods. 6.3 These operating systems provide different locking mechanisms depending on the application developers’ needs. Spinlocks are useful for multiprocessor systems where a thread can run in a busy-loop (for a short period of time) rather than incurring the overhead of being put in a sleep queue. Mutexes are useful for locking resources. Solaris 2 uses adaptive mutexes, meaning that the mutex is implemented with a spin lock on multiprocessor machines. Semaphores and condition variables are more appropriate tools for synchronization when a resource must be held for a long period of time, since spinning is inefficient for a long duration. 6.4 Volatile storage refers to main and cache memory and is very fast. However, volatile storage cannot survive system crashes or powering down the system. Nonvolatile storage survives system crashes and powered-down systems. Disks and tapes are examples of nonvolatile storage. Recently, USB devices using erasable program read-only memory ( EPROM ) have appeared providing nonvolatile storage. Stable storage refers to storage that technically can never be lost as there are redundant backup copies of the data (usually on disk). 6.5 A checkpoint log record indicates that a log record and its modified data has been written to stable storage and that the transaction need not to be redone in case of a system crash. Obviously, themore often checkpoints are performed, the less likely it is that redundant updates will have to be performed during the recovery process. System performance when no failure occurs—If no failures occur, the system must incur the cost of performing checkpoints that are essentially unnecessary. In this situation, performing checkpoints less often will lead to better system performance. The time it takes to recover from a system crash—The existence of a checkpoint record means that an operation will not have to be redone during system recovery. In this situation, the more often checkpoints were performed, the faster the recovery time is from a system crash. The time it takes to recover from a disk crash—The existence of a checkpoint record means that an operation will not have to be redone during system recovery. In this situation, the more often checkpoints were performed, the faster the recovery time is from a disk crash. 6.6
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This note was uploaded on 12/02/2011 for the course ECONOMY 111 taught by Professor Sm during the Spring '11 term at Hanoi University of Technology.

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ECEC-353 Chap 6 - 6.1 The system clock is updated at every...

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