finalsp06solutions - Name CS 414 Final Netid This exam is...

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Name: Netid: CS 414 Final This exam is closed book. You have 120 minutes to answer all questions. Show your work for partial credit. 1. Answer TRUE or FALSE (1 point for a correct answer, -0.5 for an incorrect answer) [12] (a) Large, fragmented and sparse virtual address spaces diminish performance by reducing spatial locality and consequently lowering TLB hit ratios. False. Spatial locality refers to the tendency of many programs to reference objects that span a few pages within a segment and hence to reference virtual page numbers that are close to one-another. This is going to be common no matter how the address space as a whole happens to look. (b) If we wish to guarantee the very best possible response time for interactive applications, we will need to use a scheduling policy that might not give the shortest possible completion times for long running tasks. True: We’ll need to prioritize interactive applications over long-running tasks, so they will be delayed and won’t complete as soon. (c) If a disk driver is permitted to arbitrarily reorder disk I/O operations from a Unix filesystem to minimize seek time and a crash occurs while the disk is being updated, then upon rebooting we might find that a single disk block is shown as being in two or more files. True. For example, if a file were freed and then some block from it was reallocated from the block freelist to some other file, but the changes were written to the disk out of order, perhaps only the inode update for the very last of these operations will have been recorded on disk by the time of the crash. On restart the single block will be listed as being in two files (and the deleted file will still be shown as if it hadn’t been deleted). (d) Scheduling and synchronization between threads are more efficient from an application perspective when user level threads are used rather than kernel threads i.e., threads associated with distinct CPU contexts. Hence we should always use user level threads. False. The first part of the statement is actually true - scheduling and synchronization tend to be cheapest if implemented close to where the threads are used. But the conclusion is just false - there are many situations where we would want multiple kernel threads, notably if an application wants to do I/O operations while also doing some other user-level task like rendering an image. (e) When using the LRU policy for page replacement, increasing the number of resident pages (Δ in the policy) always increases performance of the application. False: The page miss rate could remain unchanged, for example if the working set was already in memory, or if Δ is still too small and the increase just wasn’t large enough to make any difference. (f) Internal fragmentation is the only reason for not having large page size.
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