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21 Cards in this Set
- Front
- Back
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Virtual Memory
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Involves the separation of logical memory as perceived by users from physical memory. Makes the task of programming much easier because the programmer does not need to worry about how much physical memory is available.
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virtual address space
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this refers to the logical (or virtual) view of how a process is stored in memory.
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Demand paging
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commonly used in virtual memory systems. Pages are loaded only when they are demanded during program execution. Pages that are never accessed are thus never loaded into physical memory.
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Pure Demand paging
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never brings a page into memory until it is required.
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page fault
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A trap to the software raised by the hardware when a program accesses a page that is mapped in the virtual address space, but not loaded in physical memory.
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Copy-on-write
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Allows the parent and child processes initially to share the same pages. If either process writes on the shared page, a copy of the shared page is created.
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effective address time
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As long as there are no page faults, the effective access time is equal to the memory-access time. Otherwise, first read the relevant page from disk and then access the desired word.
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effective access time
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EAT = (1-p) * ma + p * page fault time
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Page replacement
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If no frame is free, we find one that is not currently being used and free it. We can free the frame by writing its contents to swap space and changing the page table to indicate that the page is no longer in memory.
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FIFO Replacement
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The simplest page-replacement algorithm. Associates with each page the time when that page was brought into memory. When a page must be replaced, the oldest page is chosen.
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Optimal Page-replacement algortithm
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The algorithm that has the lowest page-fault rate of all algorithms and will never suffer from Belady's anomoly. "Replace the page that will not be used for the longest period of time.
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Belady's Anomoly
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For some page-replacement algorithms, the page-fault rate may increase as the number of allocated frames increases.
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least recently used (LRU) algorithm
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Associates with each page the time of that page's last use. When a page must be replaced, it chooses the page that has not been used for the longest period of time.
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Second-chance replacement algorithm
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The reference bit to a page is cleared, and the arrival time is reset to the current time. Makes updating faster.
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least frequently used (LFU)
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requires that the page with the smallest count be replaced. The reason for this selection is that an actively used page should have a large reference count.
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most frequently used (MFU)
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page-replacement algorithm is based on the argument that the page with the smallest count was probably brought in and has yet to be used.
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equal allocation
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an equal share of m frames among n processes . Leftover frames can be used as a free-frame buffer pool.
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proportional allocation
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allocate available memory to each processes according to its size.
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global replacement
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allows a process to select a replacement frame from the set of all frames, even if that frame is currently allocated to some other process; one process can take from another.
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local replacement
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requires that each process can select from only it's own set of allocated frames.
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thrashing
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When a process is spending more time paging than executing. Pages fault quickly over and over, replacing pages that must be brought back in immediately.
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