Least Recently Used
Define Least Recently Used(LRU)?
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Least Recently Used (LRU): use the past to predict the future.
Replace the page which hasn't been referenced for the longest time
Strange but true: for some placement policies, like FIFO, adding up more memory can sometimes cause paging performance to be worse. It is called "Belady's Anomaly" .
Implementing LRU: need hardware support to keep track of which pages have been utilized recently .
+ Keep a register for each of the page, store system clock in that register onto each memory reference.
+ To select page for placement, scan through all pages to search the one with the oldest clock.
+ Hardware costs would have been unaffordable in the early days of paging; also, costly to scan all pages throughout replacement.
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