Lec13_FS

Lec13_FS - Review CSE120 Principles of Operating Systems...

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1 CSE120 Principles of Operating Systems Prof Yuanyuan (YY) Zhou May 18, 2011 Review What is a replacement algorithm? Name a few replacement algorithms? Optimal algorithm What is it? Is the optimal algorithm used in practice? Why? What is the FIFO algorithm? Belady’s anomaly What is it? Which algorithm has it 5/17/2011 CSE 120 2 Review (LRU) What is it? Why is it different from FIFO? Why does it work in practice? Does Virtual Memory System implement the exact LRU? Why? What is NRU? Difference from LRU? What is CLOCK? 5/17/2011 CSE 120 3 Review (Other Algorithms) Random replacement MRU (Most Recently Used) LFU (least frequently used) LRU + LFU (considering both recency and frequency) For multiple processes Local vs. global replacement Working set algorithm 5/17/2011 CSE 120 4
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2 May 17, 2011 CSE 120 5 Review (Locality) Most paging schemes depend on locality Processes reference pages in localized patterns Temporal locality Locations referenced recently likely to be referenced again Spatial locality Locations near recently referenced locations are likely to be referenced soon Although the cost of paging is high, if it is infrequent enough it is acceptable Processes usually exhibit both kinds of locality during their execution, making paging practical May 17, 2011 CSE 120 6 Page Size Considerations Small pages Pros: Locality of reference tends to be small (256) Less fragmentation Cons require large page tables Large pages Pros Small page table I/O transfers have high seek time, so better to transfer more data per seek Cons: Internal fragmentation 5/17/2011 CSE 120 7 File Systems First we’ll discuss properties of physical disks Structure Performance Scheduling Then we’ll discuss how we build file systems on them Files Directories Sharing Protection File System Layouts File Buffer Cache Read Ahead May 17, 2011 CSE 120 8 Disks and the OS Disks are messy physical devices: Errors, bad blocks, missed seeks, etc. The job of the OS is to hide this mess from higher level software Low-level device control (initiate a disk read, etc.) Higher-level abstractions (files, databases, etc.) The OS may provide different levels of disk access to different clients Physical disk (surface, cylinder, sector) Logical disk (disk block #) Logical file (file block, record, or byte #)
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3 May 17, 2011 CSE 120 9 Physical Disk Structure Disk components Platters Surfaces Tracks Sectors Cylinders Arm Heads Arm Heads Track Platter Surface Cylinder Sector 5/17/2011 CSE 120 10 Physical Disk Structure Disk components Platters Surfaces Tracks Sectors Cylinders Arm Heads Another View of Disk 5/17/2011 CSE 120 11 May 17, 2011 CSE 120 12 Disk Interaction
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Lec13_FS - Review CSE120 Principles of Operating Systems...

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