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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 256
SEC. 3.5
DESIGN ISSUES FOR PAGING SYSTEMS
225
On the other hand, for other page replacement algorithms, only a local strategy
makes sense.
In particular, the working set and WSClock algorithms refer to some
specific process and must be applied in that context. There really is no working set
for the machine as a whole, and trying to use the union of all the working sets
would lose the locality property and not work well.
3.5.2 Load Control
Even with the best page replacement algorithm and optimal global allocation
of page frames to processes, it can happen that the system thrashes.
In fact, when-
ever the combined working sets of all processes exceed the capacity of memory,
thrashing can be expected. One symptom of this situation is that the PFF algorithm
indicates that some processes need more memory but no processes need less mem-
ory.
In this case, there is no way to give more memory to those processes needing
it without hurting some other processes. The only real solution is to temporarily
get rid of some processes.
A good way to reduce the number of processes competing for memory is to
swap some of them to the disk and free up all the pages they are holding. For ex-
ample, one process can be swapped to disk and its page frames divided up among
other processes that are thrashing.
If the thrashing stops, the system can run for a
while this way.
If it does not stop, another process has to be swapped out, and so
on, until the thrashing stops. Thus even with paging, swapping may still be needed,
only now swapping is used to reduce potential demand for memory, rather than to
reclaim pages.
Swapping processes out to relieve the load on memory is reminiscent of two-
level scheduling, in which some processes are put on disk and a short-term sched-
uler is used to schedule the remaining processes. Clearly, the two ideas can be
combined, with just enough processes swapped out to make the page-fault rate ac-
ceptable. Periodically, some processes are brought in from disk and other ones are
swapped out.
However, another factor to consider is the degree of multiprogramming.
When
the number of processes in main memory is too low, the CPU may be idle for sub-
stantial periods of time. This consideration argues for considering not only process
size and paging rate when deciding which process to swap out, but also its charac-
teristics, such as whether it is CPU bound or I/O bound, and what characteristics
the remaining processes have.
3.5.3 Page Size
The page size is a parameter that can be chosen by the operating system. Even
if the hardware has been designed with, for example, 4096-byte pages, the operat-
ing system can easily regard page pairs 0 and 1, 2 and 3, 4 and 5, and so on, as
8-KB pages by always allocating two consecutive 8192-byte page frames for them.
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