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Operating Systems Chapter 9
Terms from Chapter Nine of Operating Systems Concepts by Galvin, Silberschatz, and Gagne
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Computer Science
Undergraduate 3
03/24/2013

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Term
Virtual Memory
Definition
Only putting part of the process into memory.
Term
Demand Paging
Definition
Loading a page into memory only when a process needs it.
Term
Pager
Definition
Swapping pages into memory rather than whole processes.
Term
Memory Resident
Definition
Pages that are already actually in memory.
Term
Pure Demand Paging
Definition
Move a page into memory only when it is needed. No guessing ahead of time.
Term
Locality of Reference
Definition
Generally pages are used frequently in certain sections of a programs execution, but not so much once that section is done.
Term
Swap Space
Definition
A section of disk that holds pages not present in main memory.
Term
Effective Access Time (for paged memory)
Definition
The time that it takes on average for a page reference. Equals (1-p) x ma + p x page fault time.
Term
Copy-On-Write
Definition
When a child process is made, initially both share the same pages. When one tries to write to a page, a copy is made for it.
Term
Page Replacement
Definition
Switching out pages that aren't being used for pages that a process needs. Note that if no frames are free, then two page transfers are needed.
Term
Modify Bit
Definition
A bit to indicate that this frame has been written to recently. Set by hardware. This means that if a page has been modified, the changes must be sent to the disk before it is switched out.
Term
Frame-Allocation Algorithm
Definition
Determining how many frames should be allocated to a process that is just starting.
Term
Reference String
Definition
A string of memory references that is used to evaluate a page-replacement algorithm.
Term
FIFO Page Replacement
Definition
When a new page needs to be used, the oldest page in memory is swapped out.
Term
Belady's Anomaly
Definition
The strange thing that happens when increasing the number of frame actually increases the number of page faults. This usually only occurs for bad page replacement algorithms.
Term
Stack Algorithm
Definition
Page-Replacement Algorithms that can never exhibit Belady's Anomaly.
Term
Optimal Page-Replacement Algorithm (OPT)
Definition
Replace the page that will not be used for the longest time. Best page replacement algorithm. Also not possible to implement since we can't see into the future.
Term
Least-Recently-Used Page Replacement
Definition
A close second to OPT, and is actually usable. Removes the page that was used least recently.
Term
Reference Bit
Definition
A bit that shows if the page has been looked at in this time cycle.
Term
Second-Chance-Page-Replacement Algorithm
Definition
When a page is selected to be replaces, we check if it has been referenced recently. If it has we go to the next page, and clear the reference bit. Refined FIFO.
Term
Least-Frequently-Used-Page-Replacement Algorithm
Definition
All pages keep a count of how of often it is used. The pages used the least are the ones to be brought out. Can sometimes keep pages in memory that were initially used frequently and now aren't. Can be solved by regularly shifting bits to the right.
Term
Equal Allocation
Definition
Give all processes an equal amount of frames.
Term
Proportional Allocation
Definition
Frames are allocated to a process that is proportional to the size of the process.
Term
Global Replacement
Definition
A process can select a frame to replace from all sets, even ones that it doesn't control.
Term
Local Replacement
Definition
Can only replace frames that it owns.
Term
Thrashing
Definition
High-activity paging that occurs when a process does not have enough frames, and so spends all of its time swapping out pages rather than execution.
Term
Working-Set Model
Definition
A way of reducing thrashing by looking at the number of frames a process currently has. If a page is in active use, it is in the working set.
Term
Memory-Mapped Files
Definition
Mapping a disk block to a page and the initial access to the file proceeds through ordinary demand paging. This results in a page fault. This causes a page-sized portion of the file to be read into a physical page. This helps avoid read and write system calls.
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