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lecture13 - Still VM: Global/Local Allocation, Page Table...

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Still VM: Global/Local Allocation, Page Table Management and Opti- mizations Local and Global Allocation Strategies Up until nowwehav e tacitly assumed that only one program has been faulting pages in and out of memory; this is not usually the case. One question that the OS has to answer on a page fault is which pro- cess will lose a page. •l ocal allocation assigns each process a fixed number of pages and a process can only evict its ownpages. This is the same as each process having a fixed size memory allocation. •g lobal allocation allows processes to fight it out among each other. Even global allocations need to control howmanypages are allocated to a process. Processes need a minimum number of pages to makeany forward progress (a single instruction might require 6 pages to be present!). Furthermore, taking steps to keep processes’ page allocations close to the size of their working set prevents memory thrashing, which is the memory analog to CPU thrashing we talked about earlier. Some ways to do this: •A ssign equal shares of memory (with a fewleft ove rtospeed page faults). 1 This is not very effective ifprocess working sets vary widely. •U se the wsclock algorithm to estimate working set size. This fares poorly if working sets change quickly. se page fault frequency (PFF) to determine the allocations. If a process page rate is higher than some high-water mark, increase its allocation, if it gets too low, the process is a candidate to have it’sallocation reduced.
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This note was uploaded on 02/28/2008 for the course CS 402 taught by Professor Tedfaber during the Spring '05 term at USC.

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lecture13 - Still VM: Global/Local Allocation, Page Table...

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