Page 196 - DCAP103_Principle of operating system

Unit 5: Memory Management

5.12.1 Cause of Thrashing Notes

Thrashing results in severe performance problems. Consider the following scenario, which
is based on the actual behaviour of early paging systems. The operating system monitors
CPU utilization. If CPU utilization is too low, we increase the degree of multiprogramming
by introducing a new process to the system. A global page-replacement algorithm is used;
it replaces pages with no regard to the process to which they belong. Now suppose that a
process enters a new phase in its execution and needs more frames. It starts faulting and
taking frames away from other processes. These processes need those pages, however, and
so they also fault, taking frames from other processes. These faulting processes must use
the paging device to swap pages in and out. As they queue up for the paging device, the
ready queue empties. As processes wait for the paging device, CPU utilization decreases.
The CPU scheduler sees the decreasing CPU utilization, and increases the degree of
multiprogramming as a result. The new process tries to get started by taking frames from
running processes, causing more page faults, and a longer queue for the paging device. As
a result, CPU utilization drops even further, and the CPU scheduler tries to increase the
degree of multiprogramming even more. Thrashing has occurred and system throughput
plunges. The page fault rate increases tremendously. As a result, the effective memory
access time increases. No work is getting done, because the processes are spending all their
time paging. This phenomenon is illustrated in the Figure 5.36, in which CPU utilization
is plotted against the degree of multiprogramming. As the degree of multiprogramming
increases, CPU utilization also increases, although more slowly, until a maximum is reached.
If the degree of multiprogramming is increased even further, thrashing sets in and CPU
utilization drops sharply. At this point, to increase CPU utilization and stop thrashing, we
must decrease the degree of multiprogramming. 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 steal frames from another process and cause the
latter to thrash also. Pages are replaced with regard to the process of which they are a part.
However, 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, due to 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 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.

Figure 5.36: Thrashing

thrashing
utilization

CPU

Degree of multiprogramming

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