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14 Cards in this Set
- Front
- Back
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What is a race condition? |
- occurs when multiple processes orthreads read and write from the same data - final result depends on the orderof execution of instructions in the multiple processes |
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What design and management issues are raised by the existence of concurrency?
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-OS must be able to keep track of the various processes with the use of process control blocks -The OS must allocate and de-allocate various resources for each active process -multiple processes want access to the same resources: Processor time, memory, files,I/O Devices |
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What are the possible degrees of awareness for a process?
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- independent, they are unaware of each other - indirectly aware of each other through PID's - directly aware of each other and can communicate with the other processes |
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When do concurrent processes come into conflict with each other? |
-when they are competing for the same resource - one process will be allocated that resource by the OS, and the other will have to wait |
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What determines if a resource is considered a critical resource? |
when two or more processes require access to a single non-shareable resource
- a process needs to have exclusive access to the printer while it prints an entire file |
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The enforcement of mutual exclusion creates what two additional control problems?
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deadlock- where 2 (or more) processes have exclusion of a particular resource the other needs. They each hold what the other process needs and neither will let go of what they have. starvation - a resource is being left out the rotation. OS keeps selecting p1 & p3 while p2 is being starved |
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Any facility or capability that is to provide support for mutual exclusion should?
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-Only one process at a time is allowed intoits critical section at a time -halts in its noncritical section must do so without interferingwith other processes -no deadlock or starvation -process can enter critical as soon as its available -process remains inside its critical section for a finite time only |
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The semWait operation does what to the semaphore value |
decrements the semaphore value.
-If the value becomes negative the process executing the semWait is blocked -otherwise the process continues execution |
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The semSignal operation does what to the semaphore value?
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If the resulting value is less than or equal to zero-then a process blocked by a semWait operation, if any exist is unblocked
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What is a binary semaphore? |
A more restricted version of a semaphore that can only be 0 or 1.
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What is a mutex? |
A flag used to grab and release an object.
Whendata are acquired that cannot be shared or processing is started that cannot beperformed simultaneously elsewhere lock (value = 0) unlock it (value = 1) |
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What data structures do counting semaphores and binary semaphores both utilize?
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a queue is used to hold processes waiting on the semaphore |
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A semaphore whose definition includes a policy called a strong semaphore ?
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FIFO
The process that has been blocked the longest is released from the queue first |
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A semaphore whose definition includes a policy called a weak semaphore? |
Does not specify the order in which processes are removed from the queue
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