Let us now look more generally at how paging decisions are made The most

Let us now look more generally at how paging

This preview shows page 62 - 64 out of 102 pages.

Let us now look more generally at how paging decisions are made. The most important aspect of paging is that pages can still be accessed even though they are physically in secondary storage (the disk). Suppose a page fault occurs and there are no free frames into which the relevant data can be loaded. Then the OS must select a victim : it must choose a frame and free it so that the new faulted page can be read in. This is called (obviously) page replacement . The success or failure of virtual memory rest on its abililty to make page replacement decisions. Certain facts might influence these algorithms. For instance, if a process is receiving I/O from a device, it would be foolish to page it out - so it would probably I/O locked into RAM. Here are some viable alternatives for page replacement. 5.2.3.1 FIFO - first in first out Consider the figure below. Here we see the frames in the physical memory of a paging system. The memory is rather small so that we can illustrate the principles of contention for pages most clearly. Figure 5.6: Illustration of the FIFO page replacement scheme.
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The simplest way of replacing frames is to keep track of their age (by storing their age in the frame table). This could either be the date, as recorded by the system clock, or a sequential counter. When a new page fault occurs, we can load in pages until the physical memory is full - thereafter, we have to move out pages. The page which has been in memory longest is then selected as the first to go. This algorithm has the advantage of being very straightforward, but its performance can suffer if a page is in heavy use for a long period of time. Such a page would be selected even though it was still in heavy use. 5.2.3.2 Second chance A simple optimization we can add to the FIFO algorithm is the following. Suppose we keep a reference bit for each page in the page table. Every time the memory management unit accesses a page it sets that bit to . When a page fault occurs, the page replacement algorithm looks at that bit and - if it is set to - sets the bit to but jumps over it and looks for another page. The idea is that pages which are frequently use will have their bits set often and will therefore not get paged out. Of course, this testing incurs an overhead. In the extreme case that all pages are in heavy use the page algorithm must cycle through all the pages setting their bits to zero before finding the original page again. Even then, it might not find a page to replace, if the bit was set again while it was looking through the others. In such a case, the paging system simply fails. 5.2.3.3 LRU - least recently used The best possible solution to paging would be to replace the page that will not be used for the longest period of time - but unfortunately, the system has no way of knowing what that is. A kind of compromise solution is to replace the page which has not been used for the longest period (see the figure below). This does not require a crystal ball, but it does require some appropriate hardware support to make it worthwhile.
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