نظم تشغيل 2- المحاضرة الحادية عشرة

Thrashing
In fact, look at any process that does not have “enough” frames. If the process does not have the number of frames it needs to support pages in active use, it will quickly page-fault. At this point, it must replace some page.
However, since all its pages are in active use, it must replace a page that will be needed again right away. Consequently, it quickly faults again, and again, and again, replacing pages that it must bring back in immediately. This high paging activity is called thrashing. A process is thrashing if it is
Spending more time paging than executing.
We can limit the effects of thrashing by using a local replacement algorithm (or priority replacement algorithm). With local replacement,
? If one process starts thrashing, it cannot steel frames from another process and cause the latter to thrash as well.
? However, the problem is not entirely solved. If processes are thrashing, they will be in the queue for the paging device most of the time. The average service time for a page fault will increase because of the longer average queue for the paging device. Thus, the effective access time will increase even for a process that is not thrashing.
? To prevent thrashing, we must provide a process with as many frames as it needs. But how do we know how many frames it “needs”? There are several techniques. The working-set strategy starts by looking at how many frames a process is actually using. This approach defines the locality model of process execution.
? The locality model states that, as a process executes, it moves from locality to locality. A locality is a set of pages that are actively used together. A program is generally composed of several different localities, which may overlap.